Illustration: Visibility versus radii paradox.
If you are flying a circling approach in the United States or anywhere in the world where the approach is based on the older United States Standard for Terminal Instrument Procedures (TERPS) approach radii, be very careful. There are four factors you may not have thought about and each can kill you:
So this is a plea for you to set your personal visibility minimums higher than those on the charts if the approach is based on TERPS and doesn't have the "C" denoting an expanded circling approach area. How high? I can't answer that for you, but here is the answer I've come up with for myself: Eddie's Circling Approach Visibility Minimums.
More about all of this:
What follows are quotes from the relevant regulatory documents, listed below, as well as my comments in blue.
[14 CFR 97.3] Aircraft approach category means a grouping of aircraft based on a speed of VREF, if specified, or if VREF is not specified, 1.3VSO at the maximum certificated weight. VSO, and the maximum certificated landing weight are those values as established for the aircraft by the certification authority of the country of registry.
These categories are as follows:
These speeds are used to determine an aircraft's turn radius to ensure they fit inside the authorized circling approach radius, but they do not specify the pressure altitude used.
More about this: Instrument Procedures / Approach Categories.
[ATCM 51-3, pg. 178] The horizontal component of lift will equal the centrifugal force of steady, turning flight. This fact allows development of the following relationships of turning performance:
r = turn radius, ft
V = velocity, knots (TAS)
θ = bank angle, degrees
Doing the math for a 25° bank turn and converting V (in knots) to v (in nautical miles per minute) you get a good approximation:
Turn radius (nautical miles) = (nautical miles per minute)2 / 9
More about this: Basic Aerodynamics / Turn Performance.
Remember that for some circling approaches we will need to turn to a downwind which requires we have two radii displacement from the runway, as shown in the diagram. So our objective here is to have the authorized radius exceed the turn diameter.
From these formulas we can see that the authorized radii work perfectly if you were turning the airplane at sea level:
|Authorized Approach Radii||1.7 nm||2.3 nm|
|TAS / Turn Radius / Diameter / Margin||141 / 0.61 nm / 1.22 nm / 28 %||166 / 0.85 nm / 1.70 nm / 26%|
You can maneuver in this airspace with a 26 or 28% margin for error, not bad if the winds cooperate.
[Dole, pg. 25] True airspeed, coupled with ambient density ratio, produces the same dynamic pressure as EAS, coupled with standard sea level density ratio. That is
TAS2 σ = EAS2 σ0
But σ0 = 1, so:
TAS = EAS √ ( 1/ σ)
More about this: Basic Aerodynamics / Air Properties.
Running our table again with several pressure altitudes:
|Authorized Approach Radii||1.7 nm||2.3 nm|
|TAS / Turn Radius / Diameter / Margin||141 / 0.61 nm / 1.22 nm / 28 %||166 / 0.85 nm / 1.70 nm / 26 %|
|PA / TAS / Turn Radius / Diameter / Margin||5000 / 151 / 0.7 nm / 1.4 nm / 18 %||5000 / 178 / 0.98 nm / 1.96 nm / 15 %|
|PA / TAS / Turn Radius / Diameter / Okay?||9000 / 161 / 0.8 nm / 1.6 nm / 6 %||9000 / 191 / 1.12 nm / 2.24 nm / 3 %|
The higher your MDA, the slimmer your margin for error becomes. Of course with a wind pushing you away from the runway, even the healthier sea level margins could disappear without you knowing it.
It appears a committee at TERPS decided to base their original circling approach radii on aircraft turn radius at maximum category speeds and published that in nautical miles. Then another committee decided to come up with circling visibility minimums and as long as those numbers were less than the radii, that must be good. Of course they published their numbers using statute miles.
The biggest problem here is that airplanes can quickly use up all of that allocated turn radius at the higher end of the airspeed range given each category. They can quickly find themselves maneuvering outside the visibility minimums and lose sight of the runway. The problem is made worse when you realize that maneuvering is done in space measured in nautical miles. The radius number is 15% higher in statute miles.
Extracts from TERPS, Volume 1, ¶210.c, ¶260.a., and ¶3.3.3.
Example circling approach radii versus visibility, from Eddie's notes (Background photo from USGS).
Hanscom Field (KBED) Localizer Runway 11, circle Runway 5, Category D Aircraft.
Example: Runway 05 at Hanscom Field, Bedford, Massachusetts (KBED) does not have an instrument approach. If the winds or runway closures make Runway 05 your only option, you can expect to be cleared the ILS or LOC Rwy 11, circle to 05. The weather required for a category C aircraft is 800 and 2, and for a category D aircraft is 800 and 2-1/4. Note these visibilities are in statute miles. As of 18 October 2014 this approach into Bedford does not include the expanded circling area of increased minimums found in updated TERPS guidelines.
The airport can be open using this approach down to 2 or 2-1/4 statute miles visibility. We often brief, in a category D aircraft, "I'll stay within 2.3 miles of the airport. That 2.3 nautical miles comes to 2.65 statute miles. That won't work here where the visibility may be down to 2.25 statute miles.
If the winds are 020 / 30, which happens now and then, you may be flying this approach because the crosswinds on Runway 11 exceed your limits. You wouldn't have been able to keep within 2 statute miles on a no wind day, what are your chances with a 30-knot wind blowing you further south?
[Aeronautical Information Manual, ¶7-1-17.b.] Prevailing visibility is the greatest visibility equalled or exceeded throughout at least one half of the horizon circle, not necessarily contiguous.
Figure: Forward scatter visibility sensor, from ASOS User's Guide, figure 13
[ASOS User's Guide, ¶4.2]
Most airports will have their visibility based on the sensor next to the touchdown zone of the primary instrument runway. The sensor measures visibility at that particular spot and may not reflect the actual visibility within the circling radius of the airport or the approach end of other runways. For more about this, see Weather / Automated Weather / Visibility Sensors.
|Authorized Visibility Minimums (sm)||1-1/2||2|
|Authorized Radius (nm)||1.7 nm||2.3 nm|
|Authorized Radius (sm)||1.96 nm||2.65 nm|
If we need all the authorized airspace to maneuver we are already outside our visibility minimums and will lose sight of the runway, even if the units of measure matched. But they don't, and we are even further outside.
Figure: Guam International VOR-A, from Jeppesen Airways Manual, page PGUM 13-2, 9 Dec 2011.
[14 CFR 91, §91.175(e) Missed approach procedures. Each pilot operating an aircraft, except a military aircraft of the United States, shall immediately execute an appropriate missed approach procedure when either of the following conditions exist:
(2) Whenever an identifiable part of the airport is not distinctly visible to the pilot during a circling maneuver at or above MDA, unless the inability to see an identifiable part of the airport results only from a normal bank of the aircraft during the circling approach.
Is the visibility minimum high enough to allow you to keep sight of an identifiable part of the airport during a circling maneuver that ends in a stabilized approach at or above the MDA without excessive bank angle? The answer is a resounding no and the answer applies to more than just circling approaches in the United States without the expanded circling area, shown below, Fix of the Future. TERPS criteria are used throughout the world at countries that either adopted our standards rather than develop their own, or where there is or has been a strong U.S. military presence.
The VOR-A to Guam International, for example, is based on TERPS. You can tell that on a Jeppesen chart by the note in the bottom-left corner. Notice also that the circling visibility minimums are based on the old TERPS standard.
For years many of us have been saying the 1.7 Cat C / 2.3 Cat D minimums specified in TERPS were going to kill someone and in 2002 they did. While other factors were involved, had the circling visibility minimums been raised to match a more realistic radii, the accident could have been avoided. See: Mishaps / Air China 129.
The FAA corrected the problem but did not mandate all circling approaches be updated. Chances are the next circling approach in your future will have been based on standard circling approach maneuvering radius. The FAA appears to acknowledge the flaw by adopting expanded maneuvering airspace for future approaches:
Figure: Circling Approach Area Radii, from Aeronautical Information Manual ¶5-4-20, Figure 5-4-28.
If you take a look at the expanded circling approach maneuvering airspace radius in the table above, you will have a good idea of how much airspace your airplane needs to maneuver for landing for various altitudes and approach categories. I have decided that I am not going to accept a circling approach at visibilities less than what I can maneuver the airplane. The 5001 - 7000 foot line gives me a place to start:
3 or 4 up to 5 . . . I need 3 miles visibility (Cat C) or 4 miles visibility (Cat D) up to an MDA of 5,000 feet.
Portions of this page can be found in the book Flight Lessons 2: Advanced Flight, Chapter 17.
Aeronautical Information Manual
Air Training Command Manual 51-3, Aerodynamics for Pilots, 15 November 1963
Automated Surface Observing System (ASOS) User's Guide, National Oceanic and Atmospheric Administration, March 1998
Dole, Charles E., Flight Theory and Aerodynamics, 1981, John Wiley & Sons, Inc, New York, NY, 1981.
United States Standard for Terminal Instrument Procedures (TERPS), Federal Aviation Administration 8260.3B CHG 25, 03/09/2012