‘Coffin corner’ is tricky

Q: What is the coffin corner? — Submitted by reader J.T. Ryan, Las Vegas

A: Coffin corner is a term used to describe a condition at high altitude when the maximum speed (limited by the spreading of supersonic shock waves) and the minimum (limited by amount of air passing over the wing) are nearly the same. This has caused cases where the airplane could not fly faster due to the high-speed limit or much slower due to the low-speed limit, making it difficult to control during turbulence or when descending.

One extreme example of the coffin corner is the U-2 — the difference between the famed spy plane’s high-speed limit vs. low-speed limits is quite narrow at the extreme altitudes of 60,000 feet or more. It is a very challenging airplane to fly because of the small acceptable airspeed window.

Most modern jetliners have good speed margins, making coffin-corner problems an issue of the past.

Q: When approaching stall speed in a commercial airliner, can a pilot without engine power maintain level flight, thereby avoiding a wing from dipping down and increasing the chance of a sudden crash? — Philip, St. Augustine, Fla.

A: Stalls are not related to engine power. A stall occurs when the air flowing over the wing is no longer attached to the wing and producing lift. When a stall occurs, many planes will experience a wing dropping because it stalls slightly ahead of the other one. Pilots can regain wing level by using the roll flight controls (ailerons and flight spoilers) and, if needed, the rudder. The critical action is to lower the nose to recover from the stall ( get air flowing over the wing), resulting in the symmetrical production of lift. A stall does not result in a sudden crash.

One example of the coffin corner is the U-2 — the difference between the spy plane’s high-speed limit vs. low-speed limits is quite narrow at extreme altitudes.