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.
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.