Coffin corner is a high-altitude flight condition where an aircraft has very little safe speed margin. As altitude increases, the air becomes less dense, so the airplane must fly at a higher true airspeed to produce the same lift. At the same time, the aircraft gets closer to its Mach limit, where shock waves and high-speed buffet can begin.
This matters because a small speed change, gust, or maneuver can push the aircraft toward either a low-speed stall or a high-speed buffet.
Understanding Aviation: Coffin Corner
The important speeds near coffin corner are usually shown in different ways. A pilot sees indicated airspeed on the main airspeed display. This reading responds to air pressure flowing into the pitot system, so it is closely linked to how much lift the wing can make.
True airspeed is the aircraft's actual speed through the air mass. At high altitude, true airspeed can be much greater than indicated airspeed.
Mach number is shown separately because it tracks the aircraft's closeness to compressibility effects. A crew must watch both the low indicated speed limit and the high Mach limit.
A stall is controlled by angle of attack. This is the angle between the wing and the oncoming airflow. Pulling back on the controls raises angle of attack.
In a level turn, the wings must provide more lift because part of the lift is being used to turn the aircraft. The required angle of attack rises, which raises the stall speed. Even a modest turn can remove much of the available margin at high altitude.
Turbulence can briefly change angle of attack too. The aircraft may therefore buffet near stall even when its average speed looks acceptable.
The high speed boundary has its own cause. Air flowing over the upper wing surface can speed up beyond the local speed of sound before the aircraft as a whole reaches Mach one. Small shock waves can then form on parts of the wing.
These shocks disturb the smooth airflow and can cause Mach buffet, trim changes, or reduced control effectiveness. The exact boundary changes with aircraft weight, temperature, wing shape, and loading. Cold air lowers the speed of sound, so a given true airspeed produces a higher Mach number in colder conditions.
Pilots manage this region by flying a carefully planned cruise level and speed. A heavier aircraft needs more lift, so it has less low speed margin. As fuel burns, the aircraft becomes lighter and may be able to climb higher.
This is one reason for step climbs on long flights. Flight manuals and cockpit displays provide limits, but the limits are not a guarantee during rough air or turns.
Crews may slow down, descend, or avoid maneuvers when the margin becomes small. Autopilot use helps hold a steady attitude, yet pilots still need to monitor speed trends because an automatic system cannot remove the physical limits of the wing.
Students should separate speed through the air from speed over the ground. A strong tailwind can make an aircraft travel quickly over Earth without moving it closer to a Mach limit. A headwind can do the opposite for ground speed.
The useful link is that lift depends on the air around the wing, not on the landscape below. Coffin corner shows that flying higher is not simply easier because the air is thinner. It creates a narrow operating window where altitude, temperature, weight, turns, turbulence, and aircraft speed all matter at the same time.
Key Facts
- Lift equation: L = 1/2 rho v^2 S CL
- Stall occurs when the wing exceeds its critical angle of attack, not simply when the aircraft is slow.
- At higher altitude, lower air density rho means higher true airspeed is needed for the same lift.
- Mach number: M = v/a, where v is aircraft speed and a is the local speed of sound.
- Coffin corner occurs when stall speed and high-speed buffet speed become very close.
- Safe speed band = high-speed limit minus low-speed stall limit
Vocabulary
- Coffin corner
- A high-altitude condition where the safe speed range between stall and high-speed buffet becomes dangerously small.
- Stall speed
- The minimum speed at which an aircraft can maintain controlled flight for a given weight, altitude, and configuration.
- Mach number
- The ratio of an aircraft's speed to the local speed of sound.
- High-speed buffet
- Shaking or vibration caused by shock waves and airflow separation as an aircraft approaches its Mach limit.
- Flight envelope
- The range of speeds, altitudes, and load factors in which an aircraft can fly safely.
Common Mistakes to Avoid
- Thinking stall speed is only about indicated airspeed, which is incomplete because weight, bank angle, configuration, and altitude effects on true airspeed also matter.
- Confusing coffin corner with engine failure, which is wrong because coffin corner is an aerodynamic speed margin problem, not primarily a thrust problem.
- Assuming flying faster is always safer at high altitude, which is wrong because increasing speed can move the aircraft closer to Mach buffet and control problems.
- Ignoring bank angle during high-altitude turns, which is dangerous because a turn increases load factor and raises stall speed.
Practice Questions
- 1 At a certain altitude, an aircraft has a stall buffet boundary at 220 knots and a high-speed buffet boundary at 250 knots. What is the safe speed band in knots?
- 2 A jet is flying at Mach 0.78 where the local speed of sound is 295 m/s. What is the jet's speed in m/s?
- 3 Explain why coffin corner becomes more likely as altitude increases, even if the aircraft is flying at a steady cruise setting.