Continuous Descent Final Approach (CDFA)
Years ago, following a "dive and drive" mishap, a major airline had to remind its pilots that a non-precision approach must be flown with great precision.
In my distant past, I think the U.S. Air Force must have lost a few airplanes to "dive and drive" because our evaluation criteria required crews be busted for even an inch of deviation below the MDA. For most limits we allowed momentary deviations with signs of a positive correction, but the MDA was a brick floor, none were allowed to go below it until the runway was sighted and the airplane was in a position to land.
Levelling off at the MDA can be problematic if there are distractions or turbulence. Keeping the airplane at the MDA until the runway is sighted is another issue. But the worst problem may be resisting the urge to descend when you spot the runway too far out. Why not bypass all this?
Figure: CDFA versus conventional approach, from Eddie's notes.
- What is it?
- Why is it needed?
- Is it required?
- Do you need special equipment?
- What kind of approach is suitable for a CDFA?
- Procedures: Determining a Derived Decision Altitude (DDA)
- Procedures: Leaving the Final Approach Fix
- Procedures: When to go Missed Approach
What is it?
Figure: CDFA, from Eddie's notes.
A continuous descent final approach is what you do for every straight-in ILS and what you attempt to do for every visual straight-in approach: you hit the glide path and start down on an angle that ends up in the touchdown zone of the runway. You can also do this when in instrument conditions flying a non-precision approach, which we are probably better off calling an "approach without vertical guidance."
[AC 120-108 §4.c] CDFA is a technique for flying the final approach segment of an NPA as a continuous descent. The technique is consistent with stabilized approach procedures and has no level-off. A CDFA starts from an altitude/height at or above the FAF and proceeds to an altitude/height approximately 50 feet (15 meters) above the landing runway threshold or to a point where the flare maneuver should begin for the type of aircraft being flown. This definition harmonizes with the ICAO and the European Aviation Safety Agency (EASA).
[ICAO Doc 8168, Vol I, Part I, Amdt 3, Definitions] CDFA: a technique, consistent with stabilized approach procedures, for flying the final approach segment of a non-precision instrument approach procedure as a continuous descent, without level-off, from an altitude/height at or above the final approach fix altitude/height to a point approximately 15 m (50 ft) above the landing runway threshold or the point where the flare manoeuvre should begin for the type of aircraft flown.
Why is it needed?
Figure: Dive and drive problems, from Eddie's notes.
Some of us began our instrument jet experience without the benefit of an autopilot or flight director and flying an approach without vertical guidance was an invitation to making fatal mistakes. Even with the best technology, flying an approach without vertical guidance can be a problem:
- We often pull the throttles and head down from the final approach fix at 1,000 feet per minute or more, knowing we have to level off at the Minimum Descent Altitude. But we seldom fly without distractions and some pilots have missed this step right into the ground.
- Even if you succeed at leveling off at the MDA, you need to keep the airplane there until the runway is spotted and the airplane is in a position to land.
- In a low visibility approach we can't always perceive the angle to the runway and it can be tempting to push the nose over the minute we see the runway. But if you do this too early you don't have a good idea of the descent rate required or you could lose sight of the runway.
- If you are level at the MDA you should have an idea of where you will intercept a normal glide path to the touchdown zone. Once you've passed this point, you really ought to go missed approach. You could be tempted to try the landing, not really knowing how much pavement you have ahead of you.
Example: American Airlines 1572.
Example: Corporate Airlines 5966.
Example: Crossair 3597.
Example: Korean Air 2033.
[AC 120-108 §4] Controlled flight into terrain (CFIT) is a primary cause of worldwide commercial aviation fatal accidents. Unstabilized approaches are a key contributor to CFIT events. Present NPAs are designed with and without stepdown fixes in the final approach segment. Stepdowns flown without a constant descent will require multiple thrust, pitch, and altitude adjustments inside the final approach fix (FAF). These adjustments increase pilot workload and potential errors during a critical phase of flight. NPAs designed without stepdown fixes in the final segment allow pilots to immediately descend to the MDA after crossing the FAF. In both cases, the aircraft remains at the MDA until descending for the runway or reaching the missed approach point (MAP). This practice, commonly referred to as "dive and drive," can result in extended level flight as low as 250 feet above the ground in instrument meteorological conditions (IMC) and shallow or steep final approaches.
Is it Required?
In some countries: yes. In other countries: it is recommended but not mandatory.
ICAO Preferred Technique.
[ICAO Doc 8168, Vol I, Part I, Amdt 3, ¶ 1.7.1] Studies have shown that the risk of controlled flight into terrain (CFIT) is high on non-precision approaches. While the procedures themselves are not inherently unsafe, the use of the traditional step down descent technique for flying non-precision approaches is prone to error, and is therefore discouraged. Operators should reduce this risk by emphasizing training and standardization in vertical path control on non-precision approach procedures. Operators typically employ one of three techniques for vertical path control on non-precision approaches. Of these, the continuous descent final approach (CDFA) technique is preferred. Operators should use the CDFA technique whenever possible as it adds to the safety of the approach operation by reducing pilot workload and by lessening the possibility of error in flying the approach.
[EU Ops 1, OPS 1.430, Appendix 1, ¶(d)2.] All non-precision approaches shall be flown using the continuous descent final approaches (CDFA) technique unless otherwise approved by the Authority for a particular approach to a particular runway. When calculating the minima in accordance with Appendix 1 (New), the operator shall ensure that the applicable minimum RVR is increased by 200 metres (m) for Cat A/B aeroplanes and by 400 m for Cat C/D aeroplanes for approaches not flown using the CDFA technique, providing that the resulting RVR/CMV value does not exceed 5 000 m.
[AC 120-108 §5] The FAA recommends CDFA for all of the following NPAs published with a vertical descent angle (VDA) or glideslope (GS):
- Very high frequency (VHF) Omnidirectional Range (VOR),
- VHF omni-directional range station/distance measuring equipment (VOR/DME),
- Non-directional radio beacon (NDB),
- NDB/distance measuring equipment (DME),
- Localizer (LOC), Localizer Back-Course (LOC-BC),
- Localizer-type directional aid (LDA),
- Simplified Directional Facility (SDF),
- Area Navigation (RNAV), and
- Global Positioning System (GPS).
Required by Some Countries.
[ICAO Doc 8168, Vol I, Part I, Amdt 3, ¶ 18.104.22.168] Many Contracting States require the use of the CDFA technique and apply increased visibility or RVR requirements when the technique is not used.
Many countries do require CDFA techniques be used for flying approaches without vertical guidance, but the application of the technique is not consistent. Some countries list a CDFA approach with "CDFA" in the minimums section while others use "DA" or "DA/MDA" to signify such approaches. Even the countries that list "CDFA" are not consistent about the meaning. In India, for example, you are required to add the height loss additive to the CDFA altitude. But in France, you normally do not. The only way to ensure you are following the rules of the host nation is to look it up in the country's Aeronautical Information Publication or the Jeppesen Airways Manual ATC pages.
Do you need special equipment?
Photo: T-37 "Shot Gun" cockpit, from Eddie's sordid past.
If you have the equipment to fly an instrument approach, you have what you need to fly a continuous descent final approach. The more technology you have, however, the easier it will be to do that.
[AC 120-108 §6.a] CDFA requires no specific aircraft equipment other than that specified by the title of the NPA procedure. Pilots can safely fly suitable NPAs with CDFA using basic piloting techniques, aircraft flight management systems (FMS), and RNAV systems.
[AC 120-108 §6.b]
- Aircraft with FMS, barometric vertical navigation (baro-VNAV), wide area augmentation system (WAAS), or that are similarly equipped typically provide the published VDA or GS when the IAP is selected from the database.
- Aircraft equipped with Flight Path Angle (FPA) allow the pilot to enter an electronic descent angle based on the published GS or VDA. Pilots flying aircraft without either type of equipment must compute a required rate of descent.
What kind of approach is suitable for a CDFA?
Published Angle — Chart Legends
In the United States there is a stipulation in AC 120-108 requiring that you see a published angle on the chart. The presence of the angle varies with approach type and is also depicted differently between FAA and Jeppesen charts. There are four variations, described below.
Figure: Instrument approach procedure legend, from AC 120-108, appendix 1, figure 2.
[AC 120-108 §6.b]
- CDFA requires the use of a published VDA or barometric vertical guidance (GS) on the IAP. Figure 2, Instrument Approach Procedure Legend, shows the legend for an IAP and defines the GS and VDA.
- RNAV approaches with lateral navigation (LNAV)/vertical navigation (VNAV) minima are published with a GS.
- Non-RNAV NPAs or RNAV approaches with LNAV-only minima are published with a VDA.
[Jeppesen Airways Manual, Approach Chart Legend, 12 April 2013]
- Altitudes that correspond to the VDA
- Pull-up along the VDA at the DA/MDA is depicted relative to the missed approach point.
- A dotted gray line indicates the continuance of the VDA below the DA/MDA.
- Visual flight track is shown when the missed approach point is prior to the runway threshold.
[Jeppesen Airways Manual, Approach Chart Legend, 12 April 2013]
- Pull-up representing the DA/MDA or when reaching the descent limit along the GS/VDA.
- Pull-up arrow associated to a non-precision approach not using a CDFA technique.
Published Angle — Examples with Glide Slopes
Figure: KLAF RNAV(GPS) Rwy 28 profile view, from AL-200(FAA), 01 May 2014
On an FAA chart, an RNAV(GPS) will contain the glide slope angle in the profile view.
Figure: KLAF RNAV(GPS) Rwy 28 profile view, from Jeppesen Airways Manual, KLAF page 12-2, 16 Sep 11.
On a Jeppesen chart, an RNAV(GPS) will be depicted with glide feathers in the profile view.
Published Angle — Examples with Vertical Descent Angles
Figure: KASH NDB Rwy 14 profile view, from AL-5036(FAA), 01 May 2014
On an FAA chart, a non-RNAV or RNAV without vertical guidance will contain the vertical descent angle in the profile view.
Figure: KASH NDB Rwy 14 profile view, from Jeppesen Airways Manual, KASH page 16-1, 31 Jan 14
On a Jeppesen chart, a non-RNAV or RNAV without vertical guidance will depict the vertical descent angle with a dotted gray line below the MDA in the profile view.
Vertical Path Angle — Limitations
AC 120-108 does not impose a maximum vertical path angle, per se, but if you have OpSpec, MSpec, or LOA C073 you are already familiar with a VNAV DA(H) in lieu of MDA(H) provision that limits you to 3.77° (Category A, B, and C) or 3.5° (Category D and E). You really should adopt those limits for CDFA as well.
More about this: Vertical Navigation (VNAV).
[AC 120-108 §6.d] The VDA or GS is calculated from the FAF/precise final approach fix (PFAF) altitude to the threshold crossing height (TCH). The optimum NPA descent angle (VDA or GS) is 3.0 degrees. Descent angles are found in the following range when the optimum VDA is not possible: 2.75°–3.77° (IAPs w/≤ Category (CAT) C minimums), 2.75°–3.50° (IAPs w/CAT D/E minimums). On approaches with stepdown fixes, the goal is to publish a VDA that keeps the Vertical Path (VPATH) above the stepdown fix altitude. However, in some cases, the VDA is calculated from the stepdown fix altitude to the TCH.
Procedures: Determining a Derived Decision Altitude (DDA)
Figure: MDA vs. DDA, from Eddie's notes.
The MDA, under most cases, is still an altitude you cannot go below. The CDFA technique adds an altitude pad to account for the aircraft's tendency to go below the altitude at which the missed approach is initiated, usually due to autopilot reaction time. The new altitude, that resulting from the addition of this pad to the MDA, is known as the Derived Decision Altitude (DDA).
Increment Added to MDA.
[ICAO Doc 8168, Vol I, Part I, Amdt 3, ¶ 22.214.171.124] An increment for the MDA/H may be prescribed by the operator to determine the altitude/height at which the vertical portion of the missed approach shall be initiated in order to prevent descent below the MDA/H. In such cases, there is no need to increase the RVR or visibility requirements for the approach. The RR and/or visibility published for the original MDA/H should be used.
[AC 120-108 §6.F] Pilots must not descend below the MDA when executing a missed approach from a CDFA. Operators should instruct their pilots to initiate the go-around at an altitude above the MDA (sometimes referred to as a DDA) to ensure the aircraft does not descend below the published MDA. Operators conducting approaches authorized by operations specification (OpSpec) C073, IFR Approach Procedures Using Vertical Navigation (VNAV), may use MDA as a DA.
C073 is now available also as a Letter of Authorization. These approaches, those with the "Only Authorized Operators" ball note, take into consideration the aircraft's momentary dip below the MDA.
More about this: Vertical Navigation (VNAV).
Some countries add the altitude pad into the approach minimums, pilots need not "double add." Examples below.
G450 Autopilot Performance.
[G450 Airplane Flight Manual §1-22-20] Maximum demonstrated altitude loss for coupled go-around is 60 ft.
While some countries specify the method used to determine the DDA, most do not. In the case of a G450, pilots will add 60' to the MDA to determine the DDA.
Procedures: Leaving the Final Approach Fix
Figure: G450 VGP captured, from G450 Aircraft Operating Manual, §2B-08-00, page 92.
The objective of CDFA is to leave the final approach fix fully configured, on speed, and ready to land. You should not have to destabilize the aircraft by making airspeed or trim adjustments when spotting the runway. GV and G550 pilots often withhold the last increment of flaps as a courtesy to faster moving aircraft, but if flying a CDFA in instrument conditions, it may be wiser to configure earlier.
Methodology: High Tech
Some aircraft allow a CDFA be flown with ILS-like indications so that procedures can be identical. Others may require adjustments, such as lowering the altitude select to field elevation. The closer CDFA procedures can be to an ILS, the better. Pilots simply follow the needles down to minimums, being mindful of the decision altitude. (In either case, pilots should consult their aircraft manuals and practice these procedures in a simulator until comfortable.)
The G450 can adhere to CDFA techniques by either:
- Using LNAV/VNAV on most non-precision approaches, setting the altitude preselector to the minimum descent altitude plus 60 feet. If the runway is not sighted visually or using EVS, the autopilot would initiate a level off prior to the MDA/DA and the pilot would then accomplish a missed approach. If the runway is sighted, the pilot would press APPR which turns the VPATH into a VGP, causing the autopilot to ignore the altitude preselector. (At this point the altitude preselector would be set to the missed approach altitude.)
- Using APPR on most non-precision approaches, setting the altitude preselector to missed approach altitude and treating the MDA plus 60 feet as you would an ILS decision height, taking care to go around if you do not have what it takes to land at DA + 60 feet.
While the first method prevents you from going below minimums if you attain visual contact, it increases your work load if you do. I recommend the second method, making sure both pilots understand it will be flown like an ILS.
Methodology: Low Tech
[AC 120-108 §6.c]
- Find the published VDA.
- Find the descent gradient that equates to [the] VDA.
- Find the descent rate based on groundspeed.
If you don't have an FMS that does all this for you, or if you are flying an approach that prevents your FMS from doing all this for you, this table gives you an idea of what vertical descent rate you are going to need. If you don't have the table, you can approximate a 3° angle of descent by dividing your groundspeed by 2 and multiplying that by ten. A 120 knot ground speed, for example, yields 600 fpm.
You can check your progress during the approach by placing tick marks at each mile from the final approach fix to the missed approach point with the appropriate altitude. A 3° glide path should lose 318' every nautical mile. If, for example, the final approach fix altitude is 2,000 feet, you should be at 1,682' after one mile, 1,364' after two miles, 1,046' after three miles, and so on. You can figure these out during your approach briefing, marking each target on the approach plate.
Descent to MDA/DA.
[ICAO Doc 8168, Vol I, Part I, Amdt 3, ¶ 126.96.36.199] This technique requires a continuous descent, flown either with VNAV guidance calculated by on-board equipment or based on manual calculation of the required rate of descent, without level-offs. The rate of descent is selected and adjusted to achieve a continuous descent to a point approximately 15m (50 ft) above the landing runway threshold or the point where the flare manoeuvre should begin for the type aircraft flown. The descent shall be calculated and flown to pass at or above the minimum altitude at any step down fix.
Procedures: When to go Missed Approach
The CDFA places the aircraft right on glide path in a position to land in the touchdown zone of the runway. If the runway is sighted after this point, the aircraft will be too far down the runway to make that happen and a missed approach will be needed. This eliminates the judgement calls when sighting the runway early or late.
ICAO Doc 8168, Vol I, Part I, Amdt 3, ¶ 188.8.131.52] If the visual references required to land have not been acquired when the aircraft is approaching the MDA/H, the vertical (climbing) portion of the missed approach is initiated at an altitude above the MDA/H sufficient to prevent the aircraft from descending through the MDA/H. At not time is the aircraft flown in level flight at or near the MDA/H. Any turns on the missed approach shall not begin until the aircraft reaches the MAPt. Likewise, if the aircraft reaches the MAPt before descending to near the MDA/H, the missed approach shall be initiated at the MAPt.
ICAO Doc 8168, Vol I, Part I, Amdt 3, ¶ 184.108.40.206] It should be emphasized that upon reaching the MDA/H only two options exist for the crew: continue the descent below MDA/H to land with the required visual references in sight; or, execute a missed approach. There is no level flight segment after reaching the MDA/H.
CDFA Published — LFPB VOR Rwy 07
Figure: Le Bourget VOR Rwy 07, from Jeppesen LFPB, pg. 33-1.
The Jeppesen State Rules and Procedures pages for France: "The operational minima published on French non-precision approach charts have been determined based on the assumption that these approaches are flown using CDFA flight technique unless otherwise stated by the Authority for a particular approach to a particular runway.
This approach has already added the altitude pad so you can fly it down to the posted DA (590'). There is no provision in the French ATC pages stating you may fly this approach using RNAV.
By the book, you would have to fly this using the VOR as your navigation source, estimate your descent rate to fly the given altitudes on the chart, and 590 feet. Is that the best way to fly this approach? Probably not.
As technique, if you have the technology, the following will probably be safer:
- Fly the approach using APPR, the FMS as your navigation source.
- Have the raw data available, with CDI, some place in the cockpit. You could use the copilot's display or the standby flight director.
- Set the missed approach altitude in the altitude preselector.
- Fly the approach LNAV/VNAV, using the published CDFA as your decision point to continue or go around.
If you don't have the technology, you can determine your ground speed and compute an initial descent rate and a target altitude loss per nautical mile using the chart given above, Descent Table. Let's say you are moving along at 120 knots ground speed:
- At 120 knots ground speed your feet/nm is 318 and your rate of descent will be 637 feet/minute.
- Check first that you can clear the step down fix restriction using this descent rate. This fix is 4.5 nm from the final approach fix, which means a 3° glide path will lose (4.5) (318) = 1,431' and you will cross the step down at 3,000 - 1,431 = 1,569' which places you above the restriction.
- You can start your descent from the final approach fix using a 637 feet/minute descent rate.
- You can check your progress by subtracting 318 feet per nautical mile from the final approach fix altitude. Specifically:
- At 9.1 DME you should be at 3,000 - 318 = 2,682'
- At 8.1 DME you should be at 3,000 - 636 = 2,364'
- At 7.1 DME you should be at 3,000 - 954 = 2,046'
- At 6.1 DME you should be at 3,000 - 1,272 = 1,728'
- At 5.1 DME you should be at 3,000 - 1,590 = 1,410'
- At 4.1 DME you should be at 3,000 - 1,908 = 1,092'
- At 3.1 DME you should be at 3,000 - 2,226 = 774'
- At 2.1 DME you should be well on your way to landing or executing the missed approach.
DA Published — LSGG (GPS) VOR DME Rwy 05
Figure: Geneva VOR DME Rwy 05, from Jeppesen Airways Manual, LSGG, pg. 13-1.
The Swiss do not specifically mention CDFA on their approach plates or in the State Rules and Procedures pages, but a DA is listed on the non-precision approaches and the ICAO recommended technique should be employed. The Swiss do, however, specifically allow GPS overlays for specific approaches, including this one:
This approach can be flown almost like an RNAV (GPS) approach in the U.S. with the exception that 60' should be added to the published DA. Both PFDs can be set to LRN data with the SFD showing VOR or one PFD to VOR data. Unlike flying a U.S. RNAV (GPS) approach, the Swiss ATC pages specifically require the ground based navaids be operational and used as the primary means of navigation. I would have the ground based navaids in view on a display with a CDI, either the copilot's or the standby system. This way you can fly the approach using the APPR button, once again treating it like an ILS.
MDA Published with GS — KLAF RNAV(GPS) Rwy 28
Figure: Purdue University RNAV(GPS) Rwy 28, from Jeppesen Airways Manual, KLAF, pg. 12-2.
This approach gives you several options, depending on aircraft capability and crew training:
- If the aircraft and crew are LPV capable, the decision altitude of 856' would be used.
- If the aircraft and crew are LNAV/VNAV capable, the decision altitude of 1125' would be used.
- If the aircraft or crew were not VNAV capable, a DDA could be calculated (1060 + 60 = 1120' for an aircraft with a 60' pad), and the approach flown using CFDA techniques.
MDA Published with VDA — KASH NDB Rwy 14
Figure: Nashua NDB Rwy 14, from Jeppesen Airways Manual, KASH, page 16-1.
Without C073 authorization, crews would have an MDA of 880' with a local altimeter setting, 920' without. They could elect to calculate a DDA and fly the approach using CFDA techniques. (A G450 would use 940' with the local altimeter setting, 980' without.)
"Only Authorized Operators" Approaches
With C073 authorization (OpSpec, MSpec, or LOA), crews can fly this approach using CDFA techniques down to the published MDA's, using them as DA's. (An authorized G450 would use 880' as a DA with the local altimeter setting, 920' without.)
More about this: Vertical Navigation (VNAV).
CFDA Not Advisable due to Obstacles — PHTO RNAV(GPS) Rwy 21
Figure: Hilo RNAV(GPS) Rwy 21, from Jeppesen Airways Manual, PHTO page 12-1.
There are at least three clues on the approach plate telling you a CFDA is not advisable:
- There is no dotted gray line to the runway, which would indicate a vertical descent angle.
- There is no glide path feather, which would indicate the VNAV glide slope portion of a LNAV/VNAV approach.
- There is a warning in the profile view: "Descent angle (VNAV) not authorized.
Of course this begs the question: why?
Figure: Hilo Airfield Diagram, from Jeppesen Airways Manual, PHTO page 11-1, extract.
Looking at the airfield diagram we see three obstacles within a mile of the approach end of the runway that will required visual maneuvering at the last moment.
CFDA Not Advisable due to Excessive Descent Rate — KASE Loc DME-E
Figure: Aspen LOC DME-E, from Jeppesen Airways Manual, KASE page 11-1.
The approach contains the dotted gray line with a vertical descent angle given, 6.59°. The FAA version also has the requisite VDA depicted. Technically, you have what you need to fly this approach using a constant descent final approach, but it would be unwise. AC 120-108 suggests 3.77° is the maximum for a Category C aircraft.
There are many problems with such and approach, to name just a few:
- Few aircraft can descend at such a steep angle and not accelerate, even when fully configured.
- The normal, flight tested, go around maneuver may not account for such an extreme change in flight path vector.
- If you manage to make it to the runway on speed, you may not have the energy required to flare from this angle.
The CFDA is meant to give you an unchanging flight path, laterally and vertically, from the final approach fix to the runway. Approaches like this are meant for low altitude maneuvering, making them unsuitable for CFDA techniques.
Portions of this page can be found in the book International Flight Operations, Part VIII, Chapter 14.
Advisory Circular 120-108, Continuous Descent Final Approach, 1/20/11, U.S. Department of Transportation
European Union Regulation No 859/2008, Technical requirements and administrative procedures applicable to commercial transportation by aeroplane, 20 August 2008
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013
FAA-H-8083-15B, Instrument Flying Handbook, U.S. Department of Transportation, Flight Standards Service, 2012
FAA-H-8261-1A, Instrument Procedures Handbook, U.S. Department of Transportation, Flight Standards Branch, 2007
ICAO Doc 8168 - Aircraft Operations - Vol I - Flight Procedures, Amendment No. 3, Procedures for Air Navigation Services, International Civil Aviation Organization, 2006
Jeppesen Airways Manual