Coffin Corner

The altitude at which an aircraft’s stall speed is very close to the maximum speed at which it can travel.

Speculation that errors in speed calculation might have caused the tragiccrash of Air France Flight 447 have brought to light the aviation slang “Coffin Corner.” A recent Reuters article explained the term:

The doomed Airbus A-330-200 was flying ever so close to its maximum altitude, in a zone pilots call the “Coffin Corner.” It refers to the edge of the so-called “flight envelope” of an aircraft.

At this altitude, the air is much thinner and that significantly narrows the swath of speed at which the airplane can safely operate. Because there are relatively few air molecules passing over the wings, they need to be moving faster to generate enough lift to keep the plane at altitude. They will stop flying (stall) at a much higher speed than they would on approach to an airport at sea level.

At the other end of the safe speed spectrum is the sound barrier. The wings on an airliner like the A-330 are not designed to break the speed of sound. Venture toward Chuck Yeager country and an airliner will begin buffeting. And as altitude increases, the sound barrier decreases — once again the dearth of air molecules is to blame.

So you see the squeeze play as a plane flies toward the Coffin Corner: the margin between the between the high and low speed limits gets thinner and thinner along with the air.