Coffin Corner

Critical Mach Number (M_cr). 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:

1. Subsonic. M below .75.
2. Transonic. M from .75 to 1.20.
3. Supersonic. M from 1.20 to 5.0.
4. Hypersonic. M above 5.0.

Source: Advisory Circular 61-107B, ¶1-4.e.

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 M_cr 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.

Source: Advisory Circular 61-107B, ¶1-4.q.

• The bottom end of the achievable speed range is the stall; the top end is V_DF or M_DF. [DF - Maximum demonstrated diving speed.] These are absolute minimum and maximum values and are not available or permitted for normal operation. Within each limit there is a more prudent limit; the lower one can be taken as pre-stall buffet or a defined margin above the stall and the upper one as V_MO or M_MO. Stall speed increases with altitude as previously explained and increases with weight. So, therefore, will any function of stall speed. The lines of V_MO and M_MO and V_DF and M_DF do not vary with weight.
• [The example] is drawn for a mid-weight, say 250,000 lb.. If 1.15 V_s is acceptable as a minimum speed and V_MO/M_MO must be accepted as the maximum speed then the size of the permitted speed range at any altitude can be read off. Notice how it gets smaller with altitude above 30,000 ft. due to the limiting Mach number bringing down the airspeed.

Source: D.P. Davies, page 165

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.

Source: Advisory Circular 61-107B, ¶3-2.b.(4)

• An airplane's indicated airspeed decreases in relation to true airspeed as altitude increases. As the indicated airspeed decreases with altitude, it progressively merges with the low speed buffet boundary where pre-stall buffet occurs for the airplane at a load factor of 1.0 G. The point where the high speed Mach indicated airspeed and low speed buffet boundary indicated airspeed merge is the airplane's absolute or aerodynamic ceiling. Once an airplane has reached its aerodynamic ceiling, which is higher than the altitude stipulated in the FAA-approved Airplane Flight Manual, the airplane 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 stick pusher. This critical area of the airplane's flight envelope is known as "coffin corner."
• Mach buffet occurs as a result of supersonic airflow on the wing. Stall buffet occurs at angles of attack that produce airflow disturbances (burbling) over the upper surface of the wing which decreases lift. As density altitude increases, the angle of attack that is required to produce an airflow disturbance over the top of the wing is reduced until the density altitude is reached where Mach buffet and stall buffet converge (coffin corner).
• When this phenomenon is encountered, serious consequences may result causing loss of airplane control. Increasing either gross weight or load factor (G factor) will increase the low speed buffet and decrease Mach buffet speeds. A typical jet airplane flying at 51,000 feet altitude at 1.0 G may encounter Mach buffet slightly above the airplane's M_MO (.82 Mach) and low speed buffet at .60 Mach. However, only 1.4 G (an increase of only 0.4 G) may bring on buffet at the optimum speed of .73 Mach and any change in airspeed, bank angle, or gust loading may reduce this straight- and-level flight 1.4 G protection to no protection at all. Consequently, a maximum cruising flight altitude must be selected which will allow sufficient buffet margin for necessary maneuvering and for gust conditions likely to be encountered. Therefore, it is important for pilots to be familiar with the use of charts showing cruise maneuver and buffet limits.

Source: FAA Handbook FAA-H-8083-3A, page 224