Coffin Corner

A bizarre term, but a possible explanation of what happened to that Air France airliner that disappeared on its way from Brazil to Paris.

What is it? Coffin corner describes the place in the performance envelope of a jetliner where it simply falls out of the sky.

At low speeds and at low altitudes we know this as a "stall," that is, there is not enough lift being supplied by the wings to support the aircraft. As the aircraft gets lighter, the stall speed decreases -- gliders are light enough that convection, heat rising is enough to support the craft. A brick, passing through the air in a suitable direction and angle of attack has lift too, but not enough to support that weight: it falls like a "brick."

At very high altitudes the amount of air passing over the wings (of say an Airbus A-330) at great speed only generates enough lift to barely support the aircraft. The higher you fly, the less air and hence, lift. The higher you fly the less resistance to the wind and more efficient the aircraft -- you go faster for less fuel.

So the tendency is to operate modern jet aircraft as high as practical, without unduly risking a simple stall for lack of lift. This determines the operating ceiling of an aircraft. A U-2 spyplane has very long wings and is relatively light: the amount of lift generated as compared to weight allows the aircraft to fly very high indeed, in very thin air. Concorde flew at 60,000 ... primarily because it was fast enough to generate the required lift to operate at that altitude ... sort of a balance: it was only able to be that fast because it could fly that high, and it had four massive afterburning Rolls-Royce Olympus engines sucking down special fuel.

But there is another element in this deal which creates the coffin bit of the stall corner ... critical mach number ("CMN"). CMN is where at a given speed airflow starts to separate from the wing surface -- at least in aircraft not designed to operate supersonically (impractical wing configuration for a heavy load passenger aircraft -- landing/take off speeds are too high). What this means is that while you are flying fast to generate lift to support a massive commercial aircraft in the sky (as fast as fuel consumption / power allows), you also can reach the point where all of a sudden the flow over the wing separates and suddenly you do not have lift anymore. This causes a sudden pitch nose forward.

Coffin corner can thus be described as the intersection between the line where you reach critical mach, and stall speed. Any slower, you stall and fall. Any faster, you exceed critical mach and you lose lift -- and fall. If, all of a sudden, apparent wind speed suddenly changes, you can easily fall out of coffin corner: in a thunderstorm, winds can veer 180 degree within a single cloud, with speeds over 100mph in that "wind shear." Instantaneous violation of the border of Coffin Corner. Your aircraft will fall out of the sky.

Of course, if your aircraft is well designed and stable, as soon as the apparent wind re-establishes and laminar flow over the wings reasserts itself, your airplane will fly again: provided that it didn't pitch straight nose-down, that your pilots are alert, and the aircraft is well built to sustain the stresses of falling 10,000 feet at near-supersonic speeds.

So far, it seems that Boeing 747s which are among the fastest (nay, it is THE fastest sub-sonic heavy-haul airliner) have been able to recover: there are many, many examples of 747s dropping 10-15,000 feet over the Pacific and recovering -- none that we know of have failed. Most likely that is good design, well trained pilots, a very tough airplane and a better weight/lift characteristic than "others." Also, it is a stable and old-fashioned plane: hydraulics, not electrics. It is emphatically not "fly-by-wire." In my book that is fine for military craft, but in the light of the foregoing, I am not sure I really want to fly in a thunderstorm in an electric aircraft near coffin corner.

So, picture this: you are in a fly-by-wire A-330 at 35,000 feet over the inter-tropical convergence zone over the Atlantic. You are comfortably settled in the corner of the aircraft's performance envelope: high and fast. You cannot go around the line of thunderstorms ahead. You cannot climb over them, as Concorde did. You must pass through them. The correct move would be to lose altitude, slow down (so you don't shake the plane apart), look for the weakest activity through your doppler radar ... and batten down the hatches, under manual control.

Alternatively, you can just set the autopilot and let it do its business: you are in a modern jet, with modern avionics and an unstable (but more fuel efficient) aircraft that is probably better handled by the computer. BUT you get hit by lightening. The computers try to reset as you hit wind shear and fly outside of coffin corner. Your plane pitches nose forward (if you are not strapped in, you will not be able to get back to your seat and you hit free fall in a nose down angle). You have no control, your computers are still trying to reset and provide control, you get hit by more lightening, the plane is hit by another wind shear ... it starts to spin.

Sayonara.

The passengers most probably would ride that aluminum tube right into the Atlantic. Maybe the wings would be ripped off by g-forces, maybe not. Maybe all the circuits simply go dead and the plane flutters to the ocean.

Happy thoughts....