Photo: Aspen Airport at Night, 90-second exposure (Courtesy Tom Cuccio)
I was in Aspen about fifteen years ago, sitting in the FBO with at least ten other crews all doing the same thing: looking at the overcast. The Obstacle Departure Procedure for the Aspen-Pitkin County/Sardy Field Airport (KASE) simply says, "use SARDD DEPARTURE." That departure procedure requires the weather be at least 400-1 and mandates a climb of at least 460 feet per nautical mile all the way up to 14,000 feet.
"If we can't see the obstacles," I explained to our lead passenger, "we have to out-climb them. Our Gulfstream is too heavy to do that so we have to wait until the weather improves to VFR." Just then all heads in the FBO turned to the runway to see another Gulfstream take off and disappear into the clouds. Was that crew operating foolishly or was I being overly cautious? I made it my highest priority to figure this out: what is the best strategy when dealing with departure obstacles?
If I asked five pilots how to best deal with departure obstacles, I got five different answers. Even the supposed experts at training centers and commercial vendors couldn't agree on the basics, much less the solution. The situation isn't much better today, and no wonder, this is complicated!
There are many strategies when it comes to dealing with this problem, some better than others. I'll group them all into three major categories, look at the pros and cons, and then recommend a strategy.
Recommendations — You can use any of these solutions and probably be okay. Of course if you hit something you will not be okay and the regulatory agencies will find a way to blame you. You can improve your odds against the obstacles (and the system), here is my recommendation.
Gray Area — There is one last source of confusion when operating daily without losing an engine but always being mindful of losing one when obstacles are a factor. If you lose an engine at V1 you climb out at V2 until the obstacle is beat. Easy. If you don't lose an engine you might be tempted to accelerate to 250 knots, cleaning up the flaps as quickly as you can. You are on two engines and you've made sure you can beat the obstacles with a normal climb. Or have you? And what happens if you lose that engine after you brought the flaps up? Now V2 has gone out the window. You need to think about this. Here's my answer.
See and Avoid — Left unsaid here is that if the weather is good, why can't we just "see and avoid" the obstacles. The U.S. Air Force used to teach this and has had a long list of crashes to show for it. There are at least two notable problems: Even with good visibility it may be impossible to see the obstacles at night or with the sun in your eyes. Secondly, even if you can see the obstacles, you may not have as the needed directional control with the adverse yaw of an engine failure. Is it legal to do so? Maybe.
This can be a very complex subject because there are so many rules from such a variety of sources. I've tried to simplify things by quoting only the relevant regulatory passages but I link to more complete coverage of those passages at the bottom of this page under Source Extracts.
Everything here is from the references shown below, with a few comments in an alternate color.
Photo: American Airlines MD-82 engine fire, September 2007, St. Louis, from the Associated Press.
(I am told this was Photoshopped.)
These criteria are predicated on normal aircraft operations for considering obstacle clearance requirements.
The design of procedures in accordance with this section assumes normal operations and that all engines are operating.
The airplane must be accelerated on the ground to VEF, at which point the critical engine must be made inoperative and remain inoperative for the rest of the takeoff.
The take-off path shall comprise the ground or water run, initial climb and climb-out, assuming the critical engine to fail suddenly during the take-off.
Some pilots dismiss 14 CFR 25 and ICAO Annex 8 as not really applicable to flight operations since they have more to do with aircraft certification than pilot procedures. True or not, these rules cause aircraft manufacturers to focus only on OEI data so most aircraft certified under these rules present only OEI takeoff performance data. This greatly impacts turbine aircraft performance planning, especially when dealing with takeoff obstacles. This is why the classic, "old school" method of dealing with departure obstacles is to use engine-out data. (In most aircraft that is all you have.)
Photo: Aeroflot IL-96-300 departing Salzburg (LOWS), from Hajdufi Gabor (with photographer's permission).
For an airplane certificated after September 30, 1958 (SR422A, 422B), that allows a net takeoff flight path that clears all obstacles either by a height of at least 35 feet vertically, or by at least 200 feet horizontally within the airport boundaries and by at least 300 feet horizontally after passing the boundaries.
(Full extract: 14 CFR 91, §91.13.)
No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.
The net takeoff flight path data must be determined so that they represent the actual takeoff flight paths reduced at each point by a gradient of climb equal to—
(1) 0.8 percent for two-engine airplanes;
(2) 0.9 percent for three-engine airplanes; and
(3) 1.0 percent for four-engine airplanes.
There are those who argue that you don't really need to follow anything in a departure procedure if you are flying under Part 91. The rules, after all, are for the commercial guys. The shortest regulation in the book tells us otherwise . . .
[15 CFR 97, §97.1]
(a) This part prescribes standard instrument approach procedures to civil airports in the United States and the weather minimums that apply to landings under IFR at those airports.
(b) This part also prescribes obstacle departure procedures (ODPs) for certain civil airports in the United States and the weather minimums that apply to takeoffs under IFR at civil airports in the United States.
In the United States, every instrument approach, arrival, and departure procedure is regulatory. If you are flying under instrument flight rules, you have to obey everything on that plate.
If you are flying an instrument procedure in the United States, chances are it was designed under the criteria set down by the United States Standard for Terminal Instrument Procedures, also known as "TERPS" or Federal Aviation Administration 8260.3B. TERPS has other names with various branches of the U.S. military and it is also used by various foreign governments wtihout their own airspace design authorities. If you are flying an obstacle departue procedure designed under TERPS, there are a few things you need to know about departure obstacle avoidance . . .
Illustration: Climb Segment, from TERPS, ¶202b, figure 1-3.
For TERPS purposes, the MINIMUM climb gradient that will provide adequate ROC in the climb segment is 200 ft/NM.
The vertical distance between the climbing flight path and the OCS is ROC. ROC for a climbing segment is defined as ROC = 0.24 CG . This concept is often called the 24 percent rule.
Where an obstruction penetrates the OCS, a nonstandard climb gradient (greater than 200 ft/NM) is required to provide adequate ROC.
The nonstandard ROC expressed in ft/NM can be calculated using the formula: (0.24 h) ÷ (0.76d) where "h" is the height of the obstacle above the altitude from which the climb is initiated, and "d" is the distance in NM from the initiation of climb to the obstacle.
Illustration: Initial Climb Area, from TERPS, Vol 4, figure 1-5.
(Full extract: TERPS, Volume 4, ¶1.6.)
The Initial Climb Area Baseline (ICAB) is a line extending perpendicular to the runway centerline 500 at DER. The splay of 15° and length of the ICA determine its width. The ICA length is normally 2 NM. The ICA may be extended beyond 2 NM to maximum length of 10 NM.
(Full extract: AC 120-91, ¶ 10, 11, 12.)
The Area Analysis Method defines an obstacle accountability area (OAA) within which all obstacles must be cleared vertically. The OAA is centered on the intended flight track and is acceptable for use without accounting for factors that may affect the actual flight track relative to the intended track, such as wind and available course guidance.
During straight-out departures or when the intended track or airplane heading is within 15 degrees of the extended runway centerline heading, the minimum width of the OAA is 200 feet on each side of the intended track within the airport boundaries, and 300 feet on each side of the intended track outside the airport boundaries. During departures involving turns of the intended track or when the airplane heading is more than 15 degrees from the extended runway centerline heading, the maximum width of the OAA is 3,000 feet on each side of the intended track.
(Full extract: ICAO Annex 6, Part I, ATT C.)
The term “net take-off flight path”, as relating to the aeroplane, has its meaning defined in the airworthiness requirements under which the aeroplane was certificated. If this definition is found inadequate, then a definition specified by the State of the Operator should be used.
The net take-off flight path is the one-engine-inoperative flight path which starts at a height of 10.7 m (35 ft) at the end of the take-off distance required and extends to a height of at least 450 m (1,500 ft) calculated in accordance with the conditions of 2.9, the expected gradient of climb being diminished at each point by a gradient equal to: 0.5 per cent, for aeroplanes with two engines, 0.8 per cent, for aeroplanes with four engines.
Illustration: Procedure Design Gradient, from ICAO Doc 8168 Vol II, figure I-3-2-2.
(Full extract: ICAO Doc 8168 Vol II, ¶2.)
The standard procedure design gradient (PDG) is 3.3 per cent. The PDG begins at a point 5 m (16 ft) above the departure end of the runway (DER).
The standard PDG provides an additional clearance of 0.8 per cent of the distance flown from the DER, above an obstacle identification surface (OIS). The OIS has a gradient of 2.5 per cent.
Where an obstacle penetrates the OIS, a steeper PDG may be promulgated to provide obstacle clearance of 0.8 per cent of the distance flown from the DER.
The minimum obstacle clearance (MOC) in the primary area is 0.8 per cent of the distance flown from the DER. The MOC is zero at the DER.
Illustration: Straight departure area without track guidance, from ICAO Doc 8168 Vol II, figure I-3-3-1.
(Full extract: ICAO Doc 8168 Vol II, ¶188.8.131.52.)
The area begins at the DER and has an initial width of 300 m (Cat H, 90 m).
The departure procedure ends at the point where the PDG reaches the minimum altitude/height authorized for the next phase of flight.
(Full extract: ICAO Annex 6, Part I, ATT C.)
No aeroplane should commence a take-off at a mass in excess of that shown in the flight manual to correspond with a net take-off flight path which clears all obstacles either by at least a height of 10.7 m (35 ft) vertically or at least 90 m (300 ft) plus 0.125D laterally, where D is the horizontal distance the aeroplane has travelled from the end of take-off distance available.
Where the intended track does not include any change of heading greater than 15°, for operations conducted in VMC by day, or for operations conducted with navigation aids such that the pilot can maintain the aeroplane on the intended track with the same precision as for operations specified in 5.1.1 a), obstacles at a distance greater than 300 m (1,000 ft) on either side of the intended track need not be cleared.
Where the intended track does not include any change of heading greater than 15° for operations conducted in IMC, or in VMC by night, except as provided in 5.1.1 b); and where the intended track includes changes of heading greater than 15° for operations conducted in VMC by day, obstacles at a distance greater than 600 m (2,000 ft) on either side of the intended track need not be cleared.
Where the intended track includes changes of heading greater than 15° for operations conducted in IMC, or in VMC by night, obstacles at a distance greater than 900 m (3,000 ft) on either side of the intended track need not be cleared.
Figure: Climb gradient requirement comparison, from Eddie's notes.
There are a few differences in the way the ICAO and the U.S. FAA (through TERPS) address obstacles that will impact what you can do to avoid them. You cannot address the vertical component of obstacle avoidance without also considering the lateral component. If the obstacle is right below you, of course, you must out-climb it. What if it is 100 feet to your right? How about a mile? For now let's table the lateral discussion and simply look at the vertical.
The minimum climb gradient. Both ICAO and TERPS specify a minimum climb gradient for all departure procedures. The ICAO calls this the Minimum Procedure Design Gradient (PDG) and says it can never be less than 3.3 percent. In the U.S., TERPS calls this the Minimum Climb Gradient (CG) and says it can never be less than 200 feet per nautical mile. These values are about the same, since 200/6076 = 0.033 and that is another way of writing 3.3 percent.
The obstacle clearance / identification surface. Both ICAO and TERPS specify a surface below the aircraft's path that identifies a zone where obstacles cannot penetrate without having to change the climb gradient. There is an exception to this for Low, Close-In Obstacles, but more on that later. The ICAO calls this the Obstacle Identification Surface (OIS) and defines it as a surface starting at the Departure End of Runway (DER) inclined upward by 2.5 percent. In the U.S., TERPS calls this the Obstacle Clearance Surface (OCS) and defines it as a surface that starts at the DER inclined upward by 152 feet per nautical mile. The values are about the same, since 152/6076 = 0.025 and that is another way of writing 2.5 percent.
Minimum / Required Obstacle Clearance. If you take the minimum climb gradient and subtract the obstacle surface you get the safety margin between the two. If you simply look at the minimum climb gradient where no obstacles penetrate the identification / clearance surface, both ICAO and TERPS provide for about the same margin. Under ICAO you have 3.3 - 2.5 = 0.8 percent. Under TERPS you have 200 - 152 = 48 feet per nautical mile, and that comes to (48 / 6076) = 0.0079, or about 0.8 percent. But as the climb gradient increases, the TERPS value increases. Here TERPS jumps from using feet per nautical mile to percentages, but the TERPS percentage is different.
Adjusting climb gradient for obstacles. If an obstacle, other than a Low, Close-In Obstacle (more on that later) penetrates the OIS / OCS, the procedure's climb gradient must be raised to preserve the MOC / ROC.
You can compute the resulting ROC = (0.24) (CG), which is why they call this the "24 percent rule." Yes, the ROC is larger than the MOC, but the numbers 24 and 0.8 exaggerate the difference because they are percentages of different things.
Let's say we have an obstacle that is 1500 feet above and 5 nm (30,380 feet) away from the DER.
Under ICAO, the obstacle has a gradient of (1500 / 30380) = 0.0494, or 4.94 percent. The MOC is always 0.8 percent so our PDG is 4.94 + 0.8 = 5.74 percent. Our height above the obstacle would be (0.0574)(30380) - 1500 = 244 feet.
Under TERPS, the climb gradient is h / (0.76 d), or 1500 / (0.76 x 5) = 395 feet per nautical mile. (That's 6.5 percent, much higher than the ICAO PDG.) So our ROC = (0.24) (395) = 95 feet per nautical mile. At 5 nm, our height above the obstacle will be (5)(95) = 475 feet. You can also derive this by figuring your altitude (5)(395) = 1,975, subtracting the obstacle height (1,500 feet) to arrive at the same answer, 475 feet.
Is 244 feet (ICAO) or 475 feet (TERPS) a comfortable margin? Keep in mind that if your multi-engine turbine aircraft was certified under 14 CFR 25, you will also have the net takeoff flight path margin. A twin-engine aircraft, for example, will be (0.008) (5) (6076) = 243 feet higher than the AFM states, unless there is a tailwind or temperature inversion. Only you can decide this, but you will need to give it some thought. Chipping away at this vertical margin is a fundamental step in the techniques to follow.
Figure: Obstacle area lateral comparison, from Eddie's notes.
Obstacle departure procedures are designed with very wide lateral tolerances under both ICAO and TERPS, which means those minimum climb gradients could be unnecessarily high because they are considering obstacles miles away from course centerline. Of course this was necessary back when an aircraft climbing into a cloud deck was lucky to be within a mile of course. But these days? If you have an airplane with an instantaneous readout of "position uncertainty" you very seldom see your airplane more than 0.05 nautical miles off course. Three hundred feet! While departure procedures continue to be built off these wide lateral areas, we as pilots are allowed to narrow our gaze if we have a plan to do that.
Original ODP Designs. TERPS procedure construction can be very complicated; the lateral margins vary with distance from the departure end of the runway, relationship to the airport boundary, any turns, and available track guidance. The lateral margin starts at 200 feet either side of runway centerline and quickly expands by thousands of feet, to as much as 3 miles. ICAO procedure construction mimics TERPS in many ways and becomes almost as wide. Unless the procedure says otherwise, the climb gradient on these procedures could be based on obstacles that you will have no chance of seeing.
Tighter Lateral Margins. ICAO Annex 6 narrows the lateral margin for large (more than 5,700 kg, about 12,500 lbs.) turbine aircraft. The margin can be as tight as 1,000 feet but will be no more than 3,000 feet (about a half nautical mile), depending on course guidance, turns, and distance from the runway. U.S. Advisory Circular 120-91 provides a method of applying a obstacle clearance area that is much more narrow than TERPS and is almost as narrow as the tightest ICAO margin. If an aircraft can maintain course within 3,000 feet (about a half nautical mile), the result can be a significant increase in payload.
Figure: 3-D Terrain Model, TERPS, Aft View, from Eddie's notes.
Let's say you are departing in a two-engine aircraft from an airport that leads into a valley with what looks to be a challenging obstacle departure procedure. The SID says you need to climb at 400 ft/nm to an altitude that is 4,000 ft above the departure end of the runway. Looking at the chart you see a number of mountains and it appears the greatest problem will be around 10 nm after takeoff about 3 nm to the right. The departure takes you right down the middle of the valley, so can you improve your situation by keeping to the course centerline better than 3 nautical miles? How high above the obstacle will you really be?
Figure: 3-D Terrain Model, AC 120-91, Aft View, from Eddie's notes.
In our example, if you lose an engine at V1 and manage to keep the aircraft on the departure procedure's course centerline, you will be 1,481 feet above (vertically) and 3 nautical miles away (laterally). If you don't lose an engine, you will of course be much higher.
You have several vertical margins at work: the 35 feet required by 14 CFR 135.379 (which you cannot give up), the net takeoff flight path margin (0.8 percent for a two-engine aircraft), and the 24 percent Required Obstacle Clearance afforded by TERPS. Giving up the ROC would still leave you a margin of 35 + (10)(0.008)(6076) = 521 feet above and abeam the obstacle.
We will shortly examine a method that addresses the vertical margins and another that addresses the vertical and lateral margins. But first, there is a more immediate problem with all obstacle departure procedures, regardless of the number of engines operating . . .
Photo: Burbank Runway 33, from Kurt Preisler.
There is a catch when it comes to low, close-in obstacles under both ICAO and U.S. TERPS departure procedures. You are expected to avoid them using "see and avoid" or procedural techniques, but you aren't given precise location information. This information is detailed in Volume 4 of TERPS.
(Full extract: TERPS Vol 4, ¶ 1.4.6)
Where low, close-in obstacles result in a climb gradient to an altitude 200 feet or less above DER elevation . . . publish a note identifying the obstacle(s) type, location relative to DER, AGL height, and MSL elevation.
The 200 feet or less implies any obstacles below 200 feet above DER within 1 nm of the DER because the standard climb gradient is 200 ft/nm. Higher and farther obstacles require a change to published climb gradients and possibly to takeoff minimums. But those low, close-in obstacles require only a note.
(Full extract: TERPS Vol 4, ¶ 1.3.1)
Do not publish a CG to a height of 200 feet or less above the DER elevation. Annotate the location and height of any obstacles that cause such climb gradients.
The same "catch" exists under ICAO.
(Full extract: ICAO Doc 8168 Vol II, ¶2.)
An increased gradient that is required to a height of 60 m (200 ft) or less, (normally due to low, close-in obstacles) shall not be promulgated. The position and elevation/height of close-in obstacles penetrating the OIS shall be promulgated
Where it all falls apart with this low, close-in obstacle methodology is the "publish a note identifying the obstacle(s)" proviso. To see this in action, consider Runway 33 at Bob Hope Airport (KBUR) at Burbank, California.
Figure: Burbank Airport Takeoff Minimums, (Obstacle) Departure Procedures.
At first reading the published notes seem ridiculous. Can there possibly be a 100' obstacle 33' from the DER just 30' right of centerline? I would have noticed! The word "beginning" explains the rationale behind the poorly worded sentence but does little to help the pilot. Where are these obstacles, exactly? They are not drawn precisely (if at all) on the airport diagrams and a sectional is useless at this level of detail. The FAA does offer a digital obstacle file for the United States at http://www.faa.gov/air_traffic/flight_info/aeronav/digital_products/dof/ but these are very large, cumbersome, and would take a good computer to really digest. If you wanted to try, you will see that the file covering KBUR is 790 pages long and includes these two gems:
06-030661 O US CA BURBANK 34 12 56.17N 118 21 50.28W POLE 1,00050 00846 R 2 C U A 2013106
06-001786 O US CA BURBANK 34 12 52.00N 118 21 41.00W POLE 1,00048 00831 L 1 A U C 2014152
So if you were able to find these two obstacles out of the thousands given, and if you plotted them, you would see where exactly your low, close-in obstacles are:
Figure: Burbank Airport Low, Close-In Obstacles, from Eddie's notes.
Case solved, right? Not so fast. These were just the two obstacles I found after combing through the digital obstacle file for several hours. Even if I managed to plot all of them, the file only includes man-made objects. Can you really be expected to see and avoid these low, close-in obstacles even if the location data was more precise?
Keep in mind this problem exists even if you load your aircraft to meet TERPS and ICAO climb gradients because those climb gradients do not consider low, close-in obstacles.
Is this a problem? Almost never. Almost. In our example, let's say it is raining, the weather is above standard and you are permitted to leave with the minimum climb gradient of 300 ft/nm to 5,000 feet. That comes to 300 / 6076 = 4.94 percent. If the maximum weight for this climb gradient requires a ground run following an engine failure that equals the runway available, you can expect to cross the DER at 15 feet. A 4.94 percent gradient across a distance of 812 feet results in a climb of (0.0494) (812) = 40 feet. If you cross the DER at 15 feet that means you are at 40 + 15 = 55 feet when you cross the pole marked as DOF 06-001786, which is 53 feet above the DER. You have a clearance of 2 feet!
Figure: How to out-climb low, close-in obstacles, from Eddie's notes.
If you are departing an airport with ambiguous notes about low, close-in obstacles the only thing you can be sure of is that they are out there. You can examine the area with your own eyes, comb through digital obstacle files, or simply out-climb them. If you cross the DER at 200 feet you know you will be above them all. To do this, you will need to know how much runway you must have left over when you lift off. You can figure this out by multiplying 6076 by 200, and dividing all that by the climb gradient in feet per nautical miles. If, for example, the AFM says you will be climbing at 400 feet per nautical mile, you will need to have (6076)(200) / (400) = 3038 feet of the runway remaining when you lift off.
Of course this is a ridiculous solution to what should be a simple problem. If you are going to be using all of that runway and there is any doubt about the low, close-in obstacles, the airport manager should be able to help with a more specific idea of where these obstacles are. There is, fortunately, an even easier solution. (See below, A High Tech Strategy.)
Now that we've covered most of the issues involved with departure obstacle avoidance, let's look at three possible strategies.
Not too many years ago the vast majority of pilots would tell you that the only way to legally and safely depart a mountainous airport was to look at the departure procedure, look at your Airplane Flight Manual (AFM) performance charts, and make everything agree. So let's try the Aspen Obstacle Departure Procedure (ODP) with my favorite example airplane, the Gulfstream G450.
Figure: G450 Net Gradient Takeoff Second Segment Chart, KASE, 20 Flaps, 20°C, from Eddie's notes.
Chasing through the charts it looks like an impossible task. We find the appropriate charts in G450 AFM, §05-06-00, Figure 3. Our first task is to compute the climb gradient. The departure tells us we need 460 feet per nautical mile. We know a nautical mile is 6,076 feet, therefore:
But the Gulfstream manual tells us this is a "gross climb gradient" and that we should subtract 0.8 percent so that it becomes a net takeoff flight path climb gradient. It seems the manual wants us to give up this safety margin when flying an obstacle departure procedure with no further explanation. Since we know there are other safety margins out there, we dutifully enter the chart with a value of:
Chasing up the chart we find we need a "Gross Level-off Height" which the AFM says we can find by subtract the elevation of the Departure End of Runway (DER) from the MSL level off:
We come up with a transfer scale number of 8.4. Entering the next chart at 20°C and 7600' pressure altitude we find our maximum grossweight will be around 48,000 lbs. Since our airplane has a BOW of around 43,000 lbs, we won't have enough gas to make it to Denver with any kind of reserve. Since we are trying to make it back to the east coast, that only leaves us with one option: wait for the weather to improve so we no longer have to comply with the climb gradient.
Photo: G450 MCDU Takeoff Data Page 1/3, KASE SARDD Obstacle Limited Page, from Eddie's aircraft.
Meanwhile your copilot remembers that the aircraft FMS has a performance computer that can automate those silly spaghetti charts and enters all the appropriate data. With this FMS you enter climb gradient in feet per nautical mile to a stated elevation (MSL) in feet.
Photo: G450 MCDU Takeoff Data Page 1/3, KASE SARDD Obstacle Limited Page, from Eddie's aircraft.
The answer, unfortunately, comes up very close to those spaghetti charts.
Remember that this number assumes you need to avoid all obstacles within very large vertical and lateral margins. Though the Gulfstream solution immediately gives up the net takeoff flight path 0.8 percent performance margin, you still have the 24 percent Required Obstacle Clearance that isn't really required at all if you lose an engine. There is room for improvement . . .
There is a strategy in use by many operators and provided for by several commercial vendors to simply subtract either the ICAO 0.8 percent MOC or the TERPS 24 percent ROC from the Obstacle Departure Procedure (ODP) climb gradient. Since very few aircraft manufacturers supply All-Engine-Operative (AEO) takeoff climb performance data, we've normally ensured we meet the ODP climb gradient with One-Engine-Inoperative (OEI). We know that if we can make the climb gradient with an engine failed, we'll have no problems with all engines operating:
Figure: Meeting the ODP climb gradient with OEI, from Eddie's notes.
You can do this, but there are three caveats:
So let's say you've figure a new, higher gross weight by reducing the climb gradient by 24 percent on a TERPS departure procedure. Instead of a 400 feet per nautical mile gradient, for example, you enter your performance computer with (1 - 0.24) 400 = 304 feet per nautical mile. If you lose an engine, you will clear the obstacles and don't have to worry about the procedure's climb gradient. But if you don't lose the engine, will you still make the required climb gradient? You are in uncharted territory:
Figure: Meeting the ODP obstacle clearance gradients with OEI, from Eddie's notes.
If your AFM does not include AEO takeoff climb data you might have a problem, but you might find something that gives you something that is a conservative analog. In the G450, for example, we have a chart that produces all engine climb data with the gear down and flaps fully extended to 39°. If the airplane can make the climb gradient in this configuration, it can surely do so with the gear up and a smaller angle of flaps:
Figure: G450 All Engine Climb, from G450 AOM, §13-02-10, figure 2.
Even if you don't have any conservative chart that gives you the necessary reassurance, you might be okay. Let's say, as with the Aspen example, you have a ODP climb gradient of 7.6 percent and elect to reduce that by the TERPS 24 percent ROC, lowering your OEI climb gradient to (1 - 0.24) (7.6) = 5.8 percent. You know you will clear the obstacles because the climb gradient minus the ROC based on that. Now if you don't lose an engine what is your AEO climb gradient? What follows is my personal theory.
Should? There might be something I haven't thought of here. I've tested this in the simulator in a GIV, GV, G450, and CL-604. See the G450 results here: Departure Obstacle Avoidance / Takeoff Climb Performance AEO Versus OEI. I encourage you to do the same. Our results have been very good. If, for example, the OEI climb gradient was 8 percent our AEO gradient was easily 20 percent. To do this, have the simulator operator record the aircraft's flight track and depart on an ODP twice, once with an engine failed at V1 and once with an all-engine aircraft.
We can examine the SARDD obstacle departure procedure from Aspen to better understand these margins. Recall that we were unable to depart Runway 33 at Aspen under Instrument Flight Rules because we could not meet the 460 feet per nautical mile climb gradient all the way to 14,000 feet.
Using terrain mapping software, such as Google Earth, you can draw the obstacle departure procedure course line from the Departure End of Runway (DER) all the way to the completion of the procedure. You can also diagram the borders of the obstacle clearance area and discover the most challenging obstacles are about a mile right of course. This is a laborious process and no pilot should be expected to do this. But this examination will help illustrate what exactly is going on when you choose to reduce your vertical margins to make an obstacle departure procedure climb gradient.
From this we see h = (0.76) (d) (CG) = (0.76) (13.74) (460) = 4804 feet
(Pretty close to the theoretical gradient.)
This is less than the 2.5 percent ICAO OIS and the TERPS 152 ft/nm OCS, since (0.0233)(6076) = 142 ft/nm. If you could remain on course you would only need the minimum 200 ft/nm (TERPS) 3.3 percent (ICAO) climb gradient!
Figure: Aspen SARDD obstacle departure gradients, from Eddie's notes.
Since this departure procedure was designed using TERPS, one could argue that the 24 percent ROC can be removed in the event of an engine failure and the airplane would still have the net takeoff flight path safety pad.
Just 5 feet vertical clearance! So will you cross the obstacle right at that altitude? No, remember your net takeoff flight path factor. A two-engine aircraft will actually be (.008) (4.5) (6076) = 219 feet above the obstacle. This is precisely the solution favored by some commercial vendors.
Another option would be to apply the smaller ICAO 0.8 percent MOC to the TERPS procedure. In our example, the aircraft would be loaded to provide for a climb gradient of 7.57 - 0.8 = 6.77 percent. You will cross the obstacle at (.0677) (4.5) (6076) + 7680 = 9,531 feet, 281 feet above the obstacle. Once you thrown in the net takeoff flight path factor, you clear the obstacle by 281 + 219 = 500 feet.
You can easily compute your own reduced climb gradient by going through the charts and simply starting with a climb gradient reduced by the 24 percent ROC on a TERPS procedure or the 0.8 percent MOC on an ICAO procedure. You just need to be careful how you do that:
Figure: EFB-Pro Screen Grab, from Eddie's notes.
There are commercial products available that automate this process, such as CAVU Companies EFB-Pro. This program is available for many aircraft types and includes a database of airport runways. It does not, however, have a database of obstacle departure procedures. The program does what it advertises it will do but, in my opinion, there can be confusion behind what the numbers mean and the inherent risks.
When entering the obstacle departure procedure restrictions, you are asked to note if the procedure uses the ICAO or TERPS standard. If you select ICAO, the program subtracts 0.8 percent, the Minimum Obstacle Clearance, from what it calls the "Gross Gradient" to produce a "Net Gradient." Likewise, if you select TERPS, the program multiplies the value by (1 - 0.24), the TERPS Required Obstacle Clearance.
This can be confusing. TERPS and ICAO Doc 8168 do not use the terms "Gross Gradient" and "Net Gradient" to describe ROC and MOC. Using this program your climb gradient ends up being equal to the obstacle height plus the net takeoff flight path factor. In our example above, you cross abeam the controlling obstacle by 219 feet, provided you don't encounter a tailwind or temperature inversion, and provided you do everything by the book to that point.
That being said, using EFB-Pro you can load your aircraft to a higher weight since you are reducing the climb gradient by the MOC or ROC. In our original G450 performance problem, EFB-Pro allows us to load up to 55,720 lbs an increase of 7,493 lbs over what the aircraft's FMS computed. (That makes sense, since the FMS only subtracts the net takeoff flight path 0.8 percent factor, where as EFB-Pro subtracts the larger TERPS 24 percent ROC.)
There are many advantages to reducing your obstacle climb gradient by the TERPS Required Obstacle Clearance (ROC) of 24 percent or the ICAO Minimum Obstacle Clearance (MOC) of 0.8 percent:
The are also disadvantages:
I've used this method for many years after studying terrain charts and ensuring I fully understood where the threat was and even then, only if I had reasonable confidence the winds at altitude were not reversed. The best candidates were places in a large valley where I knew I would be able to navigate away from the terrain no matter the weather.
There was an accident many years ago, where an Air Force turboprop was unable to out-climb a mountain as result of a temperature inversion; so I added that to my list of things to worry about. Nevertheless, I continued to use the method until something better came along. And that leads us to . . .
We have already seen that reducing the vertical Obstacle Departure Procedure (ODP) climb gradient by the ICAO 0.8 percent Minimum Obstacle Clearance (MOC) or the TERPS 24 percent Required Obstacle Clearance (ROC) can pay dividends in allowing us to increase our payload while still providing adequate obstacle clearance in the event of an engine failure. (See above: A Low Tech Strategy: Reduce the Vertical Margins.) But this approach proves worrisome because it requires a number of steps on a chart or with a piece of software that can subject to user errors and it doesn't address low, close-in obstacles.
We have also seen that the lateral clearances afforded by all TERPS and ICAO obstacle departure procedures is very generous and that if we are able to navigate to tighter tolerances we can eliminate consideration of some obstacles that could be as much as 3 nautical miles left or right of course centerline. If your airplane can navigate to tighter tolerances, you should be able to eliminate the far off obstacles and reduce your required climb gradient even further. You are permitted to do this in accordance with U.S. AC 120-91 and ICAO Annex 6, Part I, ATT C. (See U.S. AC 120-91 (Lateral).)
Let's go back to our Aspen example. Recall that using the The Old School Strategy we were limited to a takeoff grossweight of 48,227 lbs using the G450 FMS which automatically reduces the entered ODP climb gradient by the net takeoff flight path factor of 0.8 percent for a two-engine aircraft. Recall also that using the A Low Tech Strategy: Reduce the Vertical Margins and the computer application EFB-Pro we were able to reduce the climb gradient by the larger TERPS 24 percent ROC which yielded a much higher takeoff grossweight of 55,720 lbs. Neither of these solutions addressed the low, close-in obstacle problem and both retained the very wide lateral obstacle consideration inherent in any TERPS obstacle departure procedure. What if we could narrow the obstacle consideration zone to no more than the 3,000 feet lateral zone required by U.S. AC 120-91? Recall finally that the on-course obstacles were only at a gradient of 2.33 percent, substantially less than the 5.75 percent gradient just one mile right of course. That reduction to 2.33 percent makes a big difference:
Figure: G450 Net gradient second segment climb, flaps 20, 2.33 percent, from Eddie's notes.
For the purpose of illustration, if we assume there are no obstacles higher than those found on course within 3,000 feet left or right of course, we can chase through the chart to see our takeoff grossweight goes way up to over 70,000 lbs. I've done this only to illustrate the potential. It would take a very large database of all obstacles and a very capable computer to compare each of these to our route of flight to do the job properly. Fortunately there are commercial vendors who have just this capability.
Aircraft Performance Group offers what they call "Runway Analysis" to evaluate declared distances, runway slope, weather, and departure obstacles. Their services are available on web-based or stand alone applications and through many flight planning service providers. The departure obstacle analysis program plans for performance that avoids every obstacle laterally or vertically, including low, close-in obstacles. In the event of an engine failure the scheduled performance keeps you away from obstacles by required ICAO and U.S. FAA minimums. This method is not perfect, but it is nearly so.
Back to our Aspen example. Recall that using the A Low Tech Strategy: Reduce the Vertical Margins and the computer application EFB-Pro we were able to reduce the obstacle departure procedure's climb gradient by the TERPS 24 percent ROC which yielded a much higher takeoff grossweight of 55,720 lbs. This gradient keeps us clear laterally of every obstacle in the very wide TERPS obstacle clearance area. The APG solution narrows our lateral obstacle distance to the 3,000 feet lateral zone required by U.S. AC 120-91 and produces OEI gross weights that are much higher:
Figure: APG KASE data, runway 33, G450, flaps 20, 30 Jan 2016.
The APG data shows we can depart under the same conditions at 69,279 lbs, slightly less than our attempt to manually derive on course obstacles. That makes sense because the APG data casts a wider net than our on course exercise. What about the "Requires use of attached special departure procedures" note attached to our selected 33DP column?
Figure: Example APG KASE special departure procedures 33DP, runway 33, G450, flaps 20, 30 Jan 2016.
Reading the verbiage provided with the APG data, we see these instructions precisely mimic the SARDD THREE procedure. (Shown at the top of this page.) In fact, it is more precise, offering bank angles, a turn based on position and not altitude, and a specific time to begin flap retraction and acceleration. We can, as a result, have confidence that we can load our G450 to 69,279 lbs. and:
Note, however, that there are more special procedures listed than just the DP1 that follows the ground track of the SARDD THREE . . .
Figure: Example APG KASE special departure procedures 33DP5, runway 33, G450, flaps 20, 30 Jan 2016.
It is unclear why you would select the special procedure 33DP5 other than, perhaps, being able to fly from waypoint to waypoint is appealing. This flight track is not duplicated by any of the airport's published procedures and it is doubtful you could file it as a matter of normal operations. I suppose that once you've lost the engine and declared an emergency you can fly any track you want and in some cases these APG hybrid procedures will result in further increased departure grossweight. (It does not in our Aspen example.) But if you plan using one of these unpublished procedures there is something you need to factor into the equation . . .
Figure: Comparing the KASE SARDD THREE to the APG KASE 33DP5, from Eddie's notes (using Google earth).
The ground track of this procedure is different than the published SARDD THREE and you will not be able to file it for your planned departure. That leaves you in the situation where your FMS, ATC, and your departure briefing are all based on one thing (the SARDD THREE) and your plan in the event of an engine failure is another thing entirely. Are you going to have the presence of mind to make these changes after an engine failure? Even if all that is involved is changing the active flight plan in the FMS and making a radio call, you may have your hands full and this is just an unneeded nuisance.
There are many advantages to using a computer application that melds a digital obstacle file and terrain maps to narrow the obstacle clearance area with aircraft performance data to produce a climb gradient reduced by the TERPS Required Obstacle Clearance (ROC) of 24 percent or the ICAO Minimum Obstacle Clearance (MOC) of 8 percent:
The are two disadvantages that I can think of:
I've been using APG data for over 10 years now and have had several issues, not the least of which are those unpublished procedures. But I've become comfortable using it and allow members of my flight department to use it provided they understand the trade-offs and follow the precautions which I am about the outline right now . . .
You can safely use all three of these methods, provided you take the right precautions and understand the trade-offs. My preference is as follows:
I've made these choices because APG is provided at no cost with our flight planning service provider (ARINCDirect) and the G450 has a very easy to use performance computer built into the normal performance and takeoff data pages of the FMS. If you have access to APG but your aircraft does not have a performance computer with this capability, you may opt to elevate the "low tech" strategy as your Option Two. I can't answer for you, but these are the choices I have made. No matter your choice, I recommend you understand what margins you are cutting (vertical and/or lateral), the need to navigate precisely and make the necessary GPS RAIM or other navigation accuracy checks, and keep an eye on winds and temperature at altitude. I've used these techniques for fifteen years now and they have often made the difference between being able to takeoff or having to wait for the weather to improve.
We often think of gray areas as something with no right or wrong answer, but that isn't right. I think a gray area is more than likely a problem you haven't thought through. I often get asked "How can I be sure I'm going to clear an obstacle if I lose an engine after I've retracted my flaps and have accelerated above V2 to V2+10 when the charts are based on losing an engine at V1, climbing out with the flaps set at those speeds? It gets worse. What if you do all that and lose an engine after your flaps are up?
Figure: Obstacle clearance, three scenarios, from Eddie's notes.
In the drawing there are four scenarios, two of which guarantee obstacle clearance and two which leave you with question marks:
This isn't so much a gray area as a pilot error in understanding what it takes to clear an obstacle. The only way to assure obstacle clearance is to fly the target speed in the correct configuration until the obstacles are cleared. Does that mean you need to subject your passengers to a rocket ship ride even if you don't lose an engine on the runway? Not necesarily. If you understand where the obstacles are you can adjust your clean up altitude appropriately.
In our Aspen Example, above, we know the required climb gradient is dictated by two obstacles at 4.5 and 7.2 nm north of the airport, the higher of which is 9,250' MSL (1,570' above the departure end of the runway). We should plan on maintaining V2 to V2+10 until we've climbed above this altitude, even if we don't lose an engine. Now we know we can beat the obstacle following an engine failure at V1, at our planned level off altitude, or anywhere in between.
When I started flying old turbojets with limited performance, beating the mountains with an engine failed was a constant worry. Our rules were simply to clear the obstacle with an engine out by any margin, what we called the "scrape paint" theory. One of the ways to do that was to simply spot the obstacle and avoid it. I hear this often as a civilian. The rationale goes like this: I can see the mountain. If I lose an engine I'll just turn the other way. Sometimes you don't see the mountain because there are so many of them that you don't know which one is the primary threat and you cannot avoid all of them. Sometimes the weather is technically okay but the visiblity isn't as good as they say or it is dark. But even if you can see the obstacle, with an engine failed you might not have as much directional control as you need. In short, it might be legal but it might not be smart. I get questions about this now and then, Larry put together a good answer.
A few of us have been having a friendly debate and wondering if you can answer the question. The scenario is as follows: Flying citations part 135 in the mountains in the summer. The example flight is out of EGE departing runway 25, weather is clear and a million, but a little warm. APG says we cannot make the custom alternate departure. I've heard the argument: "If we have an engine failure, we'll stay VFR and turn right to circle in the bowl to climb to a safe altitude." I've heard this debate before, whether or not "See and avoid" satisfies 135.379. I know what the safe conservative answer is, I'm curious what your thoughts are on whether or not this argument is legal.
An avid reader,
Dear Mr. Avid,
Thank you for your question. First, under 14 CFR Part 135 operations, large (aircraft of more than 12,500 lbs. maximum certificated takeoff weight), turbine-powered (turbojet and turboprop) airplanes must be operated under the performance rules in § 135.379 through § 135.387, as applicable (Subpart I of Part 135). Compliance with § 135.379 is mandatory on every takeoff; it does not matter if the airplane is operated under instrument flight rules (IFR) or visual flight rules (VFR) or if the weather conditions are instrument meteorological conditions (IMC) or visual meteorological conditions (VMC).
For at least the last 30 years and even today, and due partly to faulty training from Part 142 schools, misunderstandings within FAA inspector ranks, and lack of technically accurate training anywhere but in the 121 air carrier community, there persists a significant knowledge gap regarding takeoff planning and certification and operating rules with respect to one-engine inoperative (OEI) obstacle clearance requirements for operators of Part 25 aircraft.
A prevalent misunderstanding and misapplication of the rules is that an airplane with OEI must meet the required minimum climb gradient of a published standard instrument departure (SID) or obstacle departure procedure (ODP), when accepted as an element of an IFR ATC clearance. This is incorrect. IFR departure procedures assume AEO (all-engines operating) climb performance, and demonstrating that the airplane can meet these climb requirements after an engine failure is not required.
Yet there is another prevalent misunderstanding. Although an airplane with OEI is not legally required to meet the minimum climb gradient of a published SID or ODP, even if an airplane with OEI can meet the minimum climb gradient this still does not ensure terrain clearance (or §135.379 compliance) because U.S. terminal instrument procedures (TERPS) uses an uninterrupted surface defined by a gradient and Part 25 aircraft certification uses a segmented net takeoff flight path.
A special engine failure contingency procedure (or what you call a “custom alternate departure”) from a vendor such as Aircraft Performance Group (APG) may be an alternate acceptable means of compliance with the regulatory requirements of §135.379. These special engine failure contingency procedures may differ from the normal IFR departure procedure (e.g., SID or ODP) and are specifically designed as OEI “escape” procedures.
If conditions (takeoff weight, pressure altitude, outside air temperature, TODA) do not allow an airplane with OEI to meet the requirements of the special engine failure contingency procedure (“APG says we cannot make the custom alternate departure,” as in your example), an operator may use other methods to comply with the regulatory requirements of § 135.379 if those methods are shown to provide the necessary level of safety and are acceptable to the Federal Aviation Administration (FAA). (See: AC 120-91, ¶1, ¶21) Alternatively, an operator may reduce takeoff weight or depart when the outside air temperature is lower in order to meet the requirements of the special engine failure contingency procedure.
Principal Operations Inspectors (POIs) of Part 135 certificate holders are tasked with ensuring that each certificate holder's manual system (e.g., General Operations Manual, GOM) contains procedures that ensure compliance with applicable Part 135 Subpart I performance requirements. In other words, the use of the APG special engine failure contingency procedure must be specifically described and published in your GOM, which is an “FAA accepted document.” There should also be appropriate training (ideally in the simulator) on how to accomplish these alternate procedures. POIs and training center program managers (TCPMs) should ensure that appropriate takeoff emergency situational training is accomplished covering at least the following items:
1) Training to ensure that if an engine fails on takeoff in an airplane, a pilot shall avoid obstacles following the OEI contingency procedure developed by the operator.
2) Emphasis on pilots accomplishing immediate action items.
3) Declaring an emergency with ATC as soon as practicable.
4) Ensuring crews advise ATC of their intentions to fly the OEI routing developed during planning.
NOTE: Inspectors are to ensure that operators/pilots understand the IFR departure procedure is then no longer applicable, and ATC must assist as necessary for the emergency.
Without a doubt, the determination that safe takeoff obstacle clearance can be achieved following an engine failure on takeoff is not an easy task. Your Chief Pilot or Director of Operations can tell you if your GOM authorizes you to perform this assessment before each takeoff (e.g., "If we have an engine failure, we'll stay VFR and turn right to circle in the bowl to climb to a safe altitude.") Before you accept this responsibility, ask yourself if you possess the training, the tools, and the skills to analyze the engine out performance of your airplane. Can you determine where all the critical obstacles are along your takeoff path for every takeoff? (Including low, close-in obstacles not accounted for by TERPS criteria?) And do you have the tools necessary to calculate a takeoff weight that will clear these critical obstacles?
If the answer is maybe, no, or I don’t know to any of these questions, then you may want to consider using a special engine failure contingency procedure to comply with § 135.379. In any case, under FAR Part 135, you must be authorized to use any alternate method. (I would recommend getting this authorization in writing if not published in your GOM.) I doubt the phrase: "If we have an engine failure, we'll stay VFR and turn right to circle in the bowl to climb to a safe altitude" would be an acceptable means of compliance with § 135.379 and accepted by your POI as providing the necessary level of safety. Just as I doubt it would be an acceptable means of compliance for American Airlines, who flies Airbus 319s into and out of KEGE, or Skywest Airlines, who flies Embraer E-170s and E-175s into and out of KEGE. You can bet those airlines have published OEI guidance for their crews and those crews are trained in those procedures. Remember, Part 135 rules were designed to provide an equivalent level of safety as Part 121 rules for paying passengers.
I hope this information has been helpful and responsive to your question.
Transport Airplane Performance Working Group Video (Part 1 of 4): Planning for Takeoff Obstacle Clearance (youtube.com)
Transport Airplane Performance Planning Group/ACF/AFS410, Primary Part 25 Performance Subjects, Bruce McGray, FAA AFS-410, Sept 15, 2015 accessed here: https://www.faa.gov/air_traffic/flight_info/aeronav/acf/media/Presentations/15-02_TAPP_presentation_McGray.pdf
FAA Order 8900.1 Flight Standards Information Management System (current)
Advisory Circular 120-91, Airport Obstacle Analysis (current)
14 CFR Part 135 (current)
14 CFR Part 91 (current)
FAA-H-8083-15B, Instrument Flying Handbook (current)
NBAA “One Engine Inoperative Takeoff Planning and Climb Performance” Steve Leon, May 10, 2012, accessed here: https://nbaa.org/aircraft-operations/safety/in-flight-safety/aircraft-climb-performance/one-engine-inoperative-takeoff-planning-and-climb-performance/
It has taken me years to digest all of this and if you really want to understand the process, there is no better way than to start reading. Be careful, however. The rules and regulations from one source may draw you to a conclusion that is made unworkable by the next. You need to read all of it.
[14 CFR 25, §25.111 Takeoff path.
(a) The takeoff path extends from a standing start to a point in the takeoff at which the airplane is 1,500 feet above the takeoff surface, or at which the transition from the takeoff to the en route configuration is completed and VFTO is reached, whichever point is higher. In addition—
(2) The airplane must be accelerated on the ground to VEF, at which point the critical engine must be made inoperative and remain inoperative for the rest of the takeoff.
[14 CFR 25, §25.115] Takeoff flight path.
(a) The takeoff flight path shall be considered to begin 35 feet above the takeoff surface at the end of the takeoff distance determined in accordance with §25.113(a) or (b), as appropriate for the runway surface condition.
(b) The net takeoff flight path data must be determined so that they represent the actual takeoff flight paths (determined in accordance with §25.111 and with paragraph (a) of this section) reduced at each point by a gradient of climb equal to—
(1) 0.8 percent for two-engine airplanes;
(2) 0.9 percent for three-engine airplanes; and
(3) 1.0 percent for four-engine airplanes.
(c) The prescribed reduction in climb gradient may be applied as an equivalent reduction in acceleration along that part of the takeoff flight path at which the airplane is accelerated in level flight.
[14 CFR 91, §91.13 (a)] Aircraft operations for the purpose of air navigation. No person may operate an aircraft in a careless or reckless manner so as to endanger the life or property of another.
[14 CFR 91, §91.175 (f)] Civil airport takeoff minimums. This paragraph applies to persons operating an aircraft under part 121, 125, 129, or 135 of this chapter.
[14 CFR 91, §91.175 (f)(3)] Except as provided in paragraph (f)(4) of this section, no pilot may takeoff under IFR from a civil airport having published obstacle departure procedures (ODPs) under part 97 of this chapter for the takeoff runway to be used, unless the pilot uses such ODPs or an alternative procedure or route assigned by air traffic control.
[14 CFR 91, §91.175 (f)(4)] Notwithstanding the requirements of paragraph (f)(3) of this section, no pilot may takeoff from an airport under IFR unless:
[14 CFR 91, §91.175 (f)(4)(i)] For part 121 and part 135 operators, the pilot uses a takeoff obstacle clearance or avoidance procedure that ensures compliance with the applicable airplane performance operating limitations requirements under part 121, subpart I or part 135, subpart I for takeoff at that airport; or
[14 CFR 91, §91.175 (f)(4)(ii)] For part 129 operators, the pilot uses a takeoff obstacle clearance or avoidance procedure that ensures compliance with the airplane performance operating limitations prescribed by the State of the operator for takeoff at that airport.
[14 CFR 135, §135.379] Large transport category airplanes: Turbine engine powered: Takeoff limitations.
[14 CFR 135, §135.379 (d)] No person operating a turbine engine powered large transport category airplane may take off that airplane at a weight greater than that listed in the Airplane Flight Manual—
[14 CFR 135, §135.379 (d)(1)] For an airplane certificated after August 26, 1957, but before October 1, 1958 (SR422), that allows a takeoff path that clears all obstacles either by at least (35 + 0.01 D) feet vertically (D is the distance along the intended flight path from the end of the runway in feet), or by at least 200 feet horizontally within the airport boundaries and by at least 300 feet horizontally after passing the boundaries; or
[14 CFR 135, §135.379 (d)(2)] For an airplane certificated after September 30, 1958 (SR422A, 422B), that allows a net takeoff flight path that clears all obstacles either by a height of at least 35 feet vertically, or by at least 200 feet horizontally within the airport boundaries and by at least 300 feet horizontally after passing the boundaries.
[AC 120-91, ¶ 10.]
a. The Area Analysis Method defines an obstacle accountability area (OAA) within which all obstacles must be cleared vertically. The OAA is centered on the intended flight track and is acceptable for use without accounting for factors that may affect the actual flight track relative to the intended track, such as wind and available course guidance.
b. The Flight Track Analysis Method is an alternative means of defining an OAA based on the navigational capabilities of the aircraft. This methodology requires the operator to evaluate the effect of wind and available course guidance on the actual ground track. While this method is more complicated, it can result in an area smaller than the OAA produced by the Area Analysis Method.
Figure: Straight Out Departures, from AC 120-91, Appendix 1, Figure 1.
[AC 120-91, ¶ 11.] Area Analysis Method
a. During straight-out departures or when the intended track or airplane heading is within 15 degrees of the extended runway centerline heading, the following criteria apply:
(1) The width of the OAA is 0.0625D feet on each side of the intended track (where D is the distance along the intended flight path from the end of the runway in feet), except when limited by the following minimum and maximum widths.
(2) The minimum width of the OAA is 200 feet on each side of the intended track within the airport boundaries, and 300 feet on each side of the intended track outside the airport boundaries.
(3) The maximum width of the OAA is 2,000 feet on each side of the intended track. (See Appendix 1, Figure 1.)
On a straight out departure you have to be clear of all obstacles within an area that starts out 200 feet on either side and ends up no wider than 2,000 feet on each side.
Figure: Turning Departures, from AC 120-91, Appendix 1, Figure 2.
b. During departures involving turns of the intended track or when the airplane heading is more than 15 degrees from the extended runway centerline heading, the following criteria apply:
(1) The initial straight segment, if any, has the same width as a straight-out departure.
(2) The width of the OAA at the beginning of the turning segment is the greater of:
(a) 300 feet on each side of the intended track.
(b) The width of the OAA at the end of the initial straight segment, if there is one.
(c) The width of the end of the immediately preceding segment, if there is one, analyzed by the Flight Track Analysis Method.
(3) Thereafter in straight or turning segments, the width of the OAA increases by 0.125D feet on each side of the intended track (where D is the distance along the intended flight path from the beginning of the first turning segment in feet), except when limited by the following maximum width:
(4) The maximum width of the OAA is 3,000 feet on each side of the intended track. (See Appendix 1, Figure 2.)
On a departure with a turn of more than 15 degrees, you have to be clear of all obstacles within an area that starts out 200 feet on either side and ends up no wider than 3,000 feet on each side.
c. The following apply to all departures analyzed with the Area Analysis Method:
(1) A single intended track may be used for analysis if it is representative of operational procedures. For turning departures, this implies the bank angle is varied to keep a constant turning radius with varying speeds.
(2) Multiple intended tracks may be accommodated in one area analysis by increasing the OAA width accordingly. In a turn, the specified OAA half-widths (i.e., one-half of the OAA maximum width) should be applied to the inside of the minimum turn radius and the outside of the maximum turn radius. An average turn radius may be used to calculate distances along the track.
(3) The distance to an obstacle within the OAA should be measured along the intended track to a point abeam the obstacle.
(4) When an operator uses the Area Analysis Method, the operator does not need to separately account for crosswind, instrument error, or flight technical error within the OAA.
(5) Obstacles prior to the end of the runway need not be accounted for, unless a turn is made prior to the end of the runway.
(6) One or more turns of less than 15 degrees each, with an algebraic sum of not more than a 15 degree change in heading or track, may be analyzed as a straight-out departure.
(7) No accountability is needed for the radius of the turn or gradient loss in the turn for a turn with a 15 degree or less change in heading or track.
[AC 120-91, ¶ 12.] The Flight Track Analysis Method involves analyzing the ground track of the flight path This paragraph discusses factors that the operator must consider in performing a Flight Track Analysis.
a. Pilotage in Turns. The operator should consider the ability of a pilot to initiate and maintain a desired speed and bank angle in a turn. Assumptions used here should be consistent with pilot training and qualification programs.
(1) When using the Flight Track Analysis Method while course guidance is not available, operators should take into account winds that may cause the airplane to drift off the intended track.
(2) The operator should take into account the effect of wind on the takeoff flight path, in addition to making the headwind and tailwind component corrections to the takeoff grossweight used in a straight-out departure.
(3) When assessing the effect of wind on a turn, the wind may be held constant in velocity and direction throughout the analysis unless known local weather phenomena indicate otherwise.
(4) If wind gradient information is available near the airport and flight path (e.g., wind reports in mountainous areas adjacent to the flight path), the operator should take that information into account in the development of a procedure.
[ICAO Annex 6, Part I, ATT C, ¶1] The purpose of this Attachment is to provide guidance as to the level of performance intended by the provisions of Chapter 5 as applicable to turbine-powered subsonic transport type aeroplanes over 5,700 kg maximum certificated take-off mass having two or more engines.
[ICAO Annex 6, Part I, ATT C-2] The terms “accelerate-stop distance”, “take-off distance”, “V1”, “take-off run”, “net take-off flight path”, “one engine inoperative en-route net flight path”, and “two engines inoperative en-route net flight path”, as relating to the aeroplane, have their meanings defined in the airworthiness requirements under which the aeroplane was certificated. If any of these definitions are found inadequate, then a definition specified by the State of the Operator should be used.]
[ICAO Annex 6, Part I, ATT C, ¶2.8.1] The net take-off flight path is the one-engine-inoperative flight path which starts at a height of 10.7 m (35 ft) at the end of the take-off distance required and extends to a height of at least 450 m (1,500 ft) calculated in accordance with the conditions of 2.9, the expected gradient of climb being diminished at each point by a gradient equal to:
[ICAO Annex 6, Part I, ATT C, ¶5]
5.1 No aeroplane should commence a take-off at a mass in excess of that shown in the flight manual to correspond with a net take-off flight path which clears all obstacles either by at least a height of 10.7 m (35 ft) vertically or at least 90 m (300 ft) plus 0.125D laterally, where D is the horizontal distance the aeroplane has travelled from the end of take-off distance available, except as provided in 5.1.1 to 5.1.3 inclusive. For aeroplanes with a wingspan of less than 60m (200 ft) a horizontal obstacle clearance of half the aeroplane wingspan plus 60 m (200 ft), plus 0.125D may be used. In determining the allowable deviation of the net take-off flight path in order to avoid obstacles by at least the distances specified, it is assumed that the aeroplane is not banked before the clearance of the net take-off flight path above obstacles is at least one half of the wingspan but not less than 15.2 m (50 ft) height and that the bank thereafter does not exceed 15°, except as provided in 5.1.4. The net take-off flight path considered is for the altitude of the aerodrome and for the ambient temperature and not more than 50 per cent of the reported headwind component or not less than 150 per cent of the reported tailwind component existing at the time of take-off. The take-off obstacle accountability area defined above is considered to include the effect of crosswinds.
5.1.1 Where the intended track does not include any change of heading greater than 15°,
a) for operations conducted in VMC by day, or
b) for operations conducted with navigation aids such that the pilot can maintain the aeroplane on the intended track with the same precision as for operations specified in 5.1.1 a), obstacles at a distance greater than 300 m (1,000 ft) on either side of the intended track need not be cleared.
5.1.2 Where the intended track does not include any change of heading greater than 15° for operations conducted in IMC, or in VMC by night, except as provided in 5.1.1 b); and where the intended track includes changes of heading greater than 15° for operations conducted in VMC by day, obstacles at a distance greater than 600 m (2,000 ft) on either side of the intended track need not be cleared.
5.1.3 Where the intended track includes changes of heading greater than 15° for operations conducted in IMC, or in VMC by night, obstacles at a distance greater than 900 m (3,000 ft) on either side of the intended track need not be cleared.
5.1.4 An aeroplane may be operated with bank angles of more than 15° below 120 m (400 ft) above the elevation of the end of the take-off run available, provided special procedures are used that allow the pilot to fly the desired bank angles safely under all circumstances. Bank angles should be limited to not more than 20° between 30 m (100 ft) and 120 m (400 ft), and not more than 25° above 120 m (400 ft). Methods approved by the State of the Operator should be used to account for the effect of bank angle on operating speeds and flight path including the distance increments resulting from increased operating speeds. The net take-off flight path in which the aeroplane is banked by more than 15° should clear all obstacles by a vertical distance of at least 10.7 m (35 ft) relative to the lowest part of the banked aeroplane within the horizontal distance specified in 5.1. The use of bank angles greater than those mentioned above should be subject to the approval from the State of the Operator.
[ICAO Annex 8, Part IIIA, ¶2.2.3] Performance data shall be determined and scheduled in the flight manual so that their application by means of the operating rules to which the aeroplane is to be operated in accordance with 5.2 of Annex 6, Part I, will provide a safe relationship between the performance of the aeroplane and the aerodromes and routes on which it is capable of being operated. Performance data shall be determined and scheduled for the following stages for the ranges of mass, altitude or pressure- altitude, wind velocity, gradient of the take-off and landing surface for landplanes; water surface conditions, density of water and strength of current for seaplanes; and for any other operational variables for which the aeroplane is to be certificated. 184.108.40.206 Take-off. The take-off performance data shall include the accelerate-stop distance and the take-off path.
220.127.116.11.1 Accelerate-stop distance. The accelerate-stop distance shall be the distance required to accelerate and stop, or, for a seaplane to accelerate and come to a satisfactorily low speed, assuming the critical engine to fail suddenly at a point not nearer to the start of the take-off than that assumed when determining the take-off path (see 18.104.22.168.2).
22.214.171.124.2 Take-off path. The take-off path shall comprise the ground or water run, initial climb and climb-out, assuming the critical engine to fail suddenly during the take-off (see 126.96.36.199.1). The take-off path shall be scheduled up to a height that the aeroplane can maintain and at which it can carry out a circuit of the aerodrome. The climb-out shall be made at a speed not less than the take-off safety speed as determined in accordance with 188.8.131.52.
184.108.40.206 Take-off safety speed. The take-off safety speeds assumed when the performance of the aeroplane (after leaving the ground or water) during the take-off is determined shall provide an adequate margin above the stall and above the minimum speed at which the aeroplane remains controllable after sudden failure of the critical engine.
[ICAO Doc 8168 Vol II, ¶2.2.6] The standard procedure design gradient (PDG) is 3.3 per cent (Cat H, 5.0 per cent). The PDG begins at a point 5 m (16 ft) above the departure end of the runway (DER).
Cat H, by the way, applies to helicopters.
[ICAO Doc 8168 Vol II, ¶2.2.7] The standard PDG provides an additional clearance of 0.8 per cent of the distance flown from the DER, above an obstacle identification surface (OIS). The OIS has a gradient of 2.5 per cent (Cat H, 4.2 per cent).
[ICAO Doc 8168 Vol II, ¶2.2.8] Where an obstacle penetrates the OIS, a steeper PDG may be promulgated to provide obstacle clearance of 0.8 per cent of the distance flown from the DER.
[ICAO Doc 8168 Vol II, ¶2.5.1] The minimum obstacle clearance (MOC) in the primary area is 0.8 per cent of the distance flown from the DER. The MOC is zero at the DER.
[ICAO Doc 8168 Vol II, ¶2.5.2] The MOC is provided above an obstacle identification surface or, where an obstacle penetrates the OIS, above the elevation of the obstacle.
[ICAO Doc 8168 Vol II, ¶2.7.5] An increased gradient that is required to a height of 60 m (200 ft) or less, (normally due to low, close-in obstacles) shall not be promulgated (see Figure I-3-2-3). The position and elevation/height of close-in obstacles penetrating the OIS shall be promulgated (see Chapter 5, “Published information for departure procedures”).
1.7.1 The design of procedures in accordance with this section assumes normal operations and that all engines are operating.
1.7.2 It is the responsibility of the operator to conduct an examination of all relevant obstacles and to ensure that the performance requirements of Annex 6 are met by the provision of contingency procedures for abnormal and emergency operations. Where terrain and/or obstacle considerations permit, the contingency procedure routing should follow that of the departure procedure.
1.7.3 It is the responsibility of the State to make available the obstacle information described in Annexes 4 and 6, and any additional information used in the design of departures in accordance with this Section.
[ICAO Doc 8168 Vol II, ¶220.127.116.11] Departure with no track adjustment. The area begins at the DER and has an initial width of 300 m (Cat H, 90 m). It is centred on the runway centre line and splays at an angle of 15° on each side of the extended runway center line (see Figure I-3-3-1). The area terminates at the end of the departure procedure as specified in Chapter 2, 2.4, “End of the departure procedure.”
[ICAO Doc 8168 Vol II, ¶2.4] The departure procedure ends at the point where the PDG reaches the minimum altitude/height authorized for the next phase of flight (i.e. en-route, holding or approach).
[TERPS, Volume 1]
201. TERPS. Concept of Primary Required Obstacle Clearance (ROC). The title of this order, United States Standard for Terminal Instrument Procedures (TERPS), contains a key word in defining the order's content. The word is "STANDARD;" something set up and established by authority as a rule for the measure of quantity, weight, extent, value, or quality.
a. The TERPS document specifies the minimum measure of obstacle clearance that is considered by the FAA (the Federal authority) to supply a satisfactory level of vertical protection. The validity of the protection is dependent, in part, on assumed aircraft performance. In the case of TERPS, it is assumed that aircraft will perform within certification requirements.
b. The following is an excerpt from the foreword of this order: "These criteria are predicated on normal aircraft operations for considering obstacle clearance requirements." Normal aircraft operation means all aircraft systems are functioning normally, all required navigational aids (NAVAID's) are performing within flight inspection parameters, and the pilot is conducting instrument instrument operations utilizing procedures based on the TERPS standard to provide ROC. While the application of TERPS criteria indirectly addresses issues of flyability and efficient use of NAVAID's, the major safety contribution is the provision of obstacle clearance standards. This facet of TERPS allows aeronautical navigation in instrument meteorological conditions (IMC) without fear of collision with unseen obstacles. ROC is provided through application of level and sloping OCS.
[TERPS, Volume 1, ¶203.] Sloping Obstacle Clearance Surfaces (OCS). The method of applying ROC, in segments dedicated to descending on a glidepath or climbing in a departure or missed approach segment, requires a different obstacle clearance concept than the level OCS because the ROC value must vary throughout the segment. The value of ROC near the runway is relatively small, and the value at the opposite end of the segment is sufficient to satisfy one of the level surface standards above. It follows then, that a sloping OCS is a more appropriate method of ROC application.
[TERPS, Volume 1, ¶203.b.] The concept of providing obstacle clearance in the climb segment, in instrument procedures, is based on the aircraft maintaining a minimum climb gradient. The climb gradient must be sufficient to increase obstacle clearance along the flightpath so that the minimum ROC for the subsequent segment is achieved prior to leaving the climb segment (see figure 1-3). For TERPS purposes, the MINIMUM climb gradient that will provide adequate ROC in the climb segment is 200 ft/NM.
While this "MINIMUM climb gradient" of 200 feet per nautical mile is the same as the ICAO 3.3 percent Procedure Design Gradient (PDG) and the 152 feet per nautical mile Obstacle Clearance Surface is the same as the ICAO 2.5 percent Obstacle Identification Surface, the similarities end there.
[TERPS, Volume 1, ¶203.b.(1)] The obstacle evaluation method for a climb segment is the application of a rising OCS below the minimum climbing flightpath. Whether the climb is for departure or missed approach is immaterial. The vertical distance between the climbing flightpath and the OCS is ROC. ROC for a climbing segment is defined as ROC = 0.24 CG . This concept is often called the 24 percent rule. Altitude gained is dependent on climb gradient (CG) expressed in feet per NM. The minimum ROC supplied by the 200 ft/NM CG is 48 ft/NM (0.24 x 200 = 48). Since 48 of the 200 feet gained in 1 NM is ROC, the OCS height at that point must be 152 feet (200 - 48 = 152), or 76 percent of the CG (152 ÷ 200 = 0.76). The slope of a surface that rises 152 over 1 NM is 40 (6076.11548 ÷ 152 = 39.97 = 40).
[TERPS, Volume 1, ¶203.b.(2)] Where an obstruction penetrates the OCS, a nonstandard climb gradient (greater than 200 ft/NM) is required to provide adequate ROC. Since the climb gradient will be greater than 200 ft/NM, ROC will be greater than 48 ft/NM (0.24 x CG > 200 = ROC > 48). The nonstandard ROC expressed in ft/NM can be calculated using the formula: (0.24 h) ÷ (0.76d) where "h" is the height of the obstacle above the altitude from which the climb is initiated, and "d" is the distance in NM from the initiation of climb to the obstacle. Normally, instead of calculating the nonstandard ROC value, the required climb gradient is calculated directly using the formula: h ÷ (0.76d).
[TERPS, Volume 1, ¶203.c.] In the case of an instrument departure, the OCS is applied during the climb until at least the minimum en route value of ROC is attained. The OCS begins at the departure end of runway, at the elevation of the runway end. It is assumed aircraft will cross the departure end-of-runway at a height of at least 35 ft. However, for TERPS purposes, aircraft are assumed to lift off at the runway end (unless the procedures state otherwise). The ROC value is zero at the runway end, and increases along the departure route until the appropriate ROC value is attained to allow en route flight to commence.
Do not publish a CG to a height of 200 feet or less above the DER elevation. Annotate the location and height of any obstacles that cause such climb gradients.
Where obstacles 3 statute miles or less from the DER penetrate the OCS:
(1) Publish a note identifying the obstacle(s) type, location relative to DER, AGL height, and MSL elevation, and
(2) Publish standard takeoff minimums with a required CG to a specified altitude, and
(3) Publish a ceiling and visibility to see and avoid the obstacle(s), and/or
(4) Develop a specific textual or graphic route to avoid the obstacle(s).
NOTE: Where low, close-in obstacles result in a climb gradient to an altitude 200 feet or less above DER elevation, only paragraph 1.4.6a(1) applies.
14 CFR 25, Title 14: Aeronautics and Space, Airworthiness Standards: Transport Category Airplanes, Federal Aviation Administration, Department of Transportation
14 CFR 91, Title 14: Aeronautics and Space, General Operating and Flight Rules, Federal Aviation Administration, Department of Transportation
14 CFR 121, Title 14: Aeronautics and Space, Operating Requirements: Domestic, Flag, and Supplemental Operations, Federal Aviation Administration, Department of Transportation
14 CFR 135, Title 14: Aeronautics and Space, Operating Requirements: Commuter and On Demand Operations and Rules Governing Persons on Board Such Aircraft, Federal Aviation Administration, Department of Transportation
14 CFR 139, Title 14: Aeronautics and Space, Certification of Airports
Advisory Circular 120-91, Airport Obstacle Analysis, 5/5/06, U.S. Department of Transportation
Bombardier BD-700-1A10 Airplane Flight Manual, Rev 80, Jun 03/2014.
Gulfstream G450 Airplane Flight Manual, Revision 36, December 5, 2013
ICAO Annex 6 - Operation of Aircraft - Part 1 Commercial Aircraft, International Standards and Recommended Practices, Annex 6 to the Convention on International Civil Aviation, Part I, July 2010
ICAO Annex 8 - Airworthiness of Aircraft, International Standards and Recommended Practices, Annex 8 to the Convention on International Civil Aviation, July 2010
ICAO Doc 8168 - Aircraft Operations - Vol II - Construction of Visual and Instrument Flight Procedures, Procedures for Air Navigation Services, International Civil Aviation Organization, 2006
United States Standard for Terminal Instrument Procedures (TERPS), Federal Aviation Administration 8260.3B CHG 25, 03/09/2012
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