Figure: G450 "Coffin Corner" chart, parts hidden, from G450 Airplane Flight Manual, §05-08-00, figure 1.
Every now and then I hear somebody refuse to fly a Gulfstream above FL350 because they fear the . . . cue dramatic music . . . coffin corner. I've spent hours at FL510 so I must be fearless. Right?
I collect aeronautical engineer books, yes I am a pretty dull person. None of them mention "coffin corner." An FAA advisory circular, AC 61-107B, mentions it while describing the aircraft maneuvering envelope. So what is this corner where you are sure to meet your demise? It is simply where your aircraft's operating envelope narrows in terms of altitude versus speed. Too slow: you stall. Too fast: you exceed design speeds.
Oh yes, I clipped part of the G450 operating envelope diagram above and took the unusual step of using three explanation points. It is as deceptive as I could make it. The truth lies below.
Most of this comes from my notes from freshman aero at Purdue but I did lean heavily on the references listed at the bottom of the page. My comments appear in blue.
[Advisory Circular 61-107B, ¶1-4.e.] Critical Mach Number (Mcr). The free stream Mach number (M) at which local sonic flow such as buffet, airflow separation, and shock waves becomes evident. These phenomena occur above Mcr, and are as follows:
[Advisory Circular 61-107B, ¶1-4.q.] Q-Corner or Coffin Corner. A term used to describe operations at high altitudes where low IAS yield high TAS (as indicated by Mach number) at high angles of attack (AOA). The high AOA results in flow separation, which causes buffet. Turning maneuvers at these altitudes increase the AOA and result in stability deterioration with a decrease in control effectiveness. The relationship of stall speed to the Mcr narrows to a point where sudden increases in AOA, roll rates, and/or disturbances (e.g., clear air turbulence) cause the limits of the airspeed envelope to be exceeded. Coffin corner exists in the upper portion of the maneuvering envelope for a given gross weight and G-force.
High altitudes increase an aircraft's stall speed as do G-forces, narrowing the aircraft's operating envelope.
Figure: Variation of significant speeds with altitude, from D.P. Davies, figure 6.8.
[D.P. Davies, page 165.]
The so-called coffin corner can be seen in this example at 50,000 ft. This aircraft can only operate around 210 knots, 0.87 Mach, plus a very small margin. It would be prudent to fly at a lower altitude.
Figure: Mach buffet boundary, from FAA Handbook FAA-H-8083-3A, figure 15-11.
[Advisory Circular 61-107B, ¶3-2.b.(4)] An airplane's indicated airspeed (IAS) decreases in relation to TAS as altitude increases. As the IAS decreases with altitude, it approaches the airspeed for the low-speed buffet boundary where pre-stall buffet occurs for the airplane at a load factor of 1.0 G. The point where high-speed Mach, IAS, and low-speed buffet boundary IAS merge is the airplane's absolute or aerodynamic ceiling. Once an aircraft has reached its aerodynamic ceiling, which is higher than the altitude limit stipulated in the AFM, the aircraft can neither be made to go faster without activating the design stick puller at Mach limit nor can it be made to go slower without activating the stick shaker or pusher. This critical area of the aircraft's flight envelope is known as coffin corner.
[FAA Handbook FAA-H-8083-3A, page 224]
In the example diagram from the FAA Handbook, a 15,000 lb aircraft can fly to 45,000 feet and have a maneuvering envelope from 0.6 to 0.75 Mach and still be able to pull 1.4 G's. At 40,000 feet this margin expands: 0.46 Mach all the way to VMO.
Figure: G450 "Coffin Corner" chart, from G450 Airplane Flight Manual, §05-08-00, figure 1.
Here is the G450 buffet boundary chart with the altitude axis shown. On an ISA day flying at 0.80 Mach, you can make 45,000 feet at 58,000 lbs. The red line represents an aircraft at its maximum weight for the altitude and temperature in a 30° banked turn or in turbulence of similar G-loading. The chart says the aircraft will experience stall buffet at 0.65 M and reach VMO at 0.874M.
So does the G450 have a coffin corner? It would seem only possible if your somehow found yourself at 55,000 feet. So don't climb that high, okay?
14 CFR 25, Title 14: Aeronautics and Space, Federal Aviation Administration, Department of Transportation
Advisory Circular 61-107B, Aircraft Operations at Altitudes Above 25,000 Feet Mean Sea Level or Mach Numbers Greater than .75, Change 1, 9/9/15, U.S. Department of Transportation
Air Training Command Manual 51-3, Aerodynamics for Pilots, 15 November 1963
Connolly, Thomas F., Dommasch, Daniel 0., and Sheryby, Sydney S., Airplane Aerodynamics, Pitman Publishing Corporation, New York, NY, 1951.
Davies, D. P., Handling the Big Jets, Civil Aviation Authority, Kingsway, London, 1985.
Dole, Charles E., Flight Theory and Aerodynamics, 1981, John Wiley & Sons, Inc, New York, NY, 1981.
FAA-H-8083-15, Instrument Flying Handbook, U.S. Department of Transportation, Flight Standards Service, 2001.
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013.
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.