One of the strangest flight characteristics to the uninitiated non-pilot types, or even fixed wing private pilots, is that jet aircraft have aerodynamic qualities in the upper flight level that are a design limitation: They can both overspeed and fall out of the sky, at pretty much the same speed.
Coffin corner is a great concept to explore since it is both the yin and yang of flight. Or put another way, it is a strange intersection of where too slow meets too fast. The most noteable accident that was a stall that started near coffin corner was Air France 447, which is a good example because after suffering the effects of it, the crew wasn’t able to diagnose that a stall had even happened.
But how can something that zooms through the air so gracefully encounter both a stall speed and an overspeed at the same speed? Well… the short answer is science and aerodynamics stuff, that we’ll translate into a simple distillation, without all the training and schooling to get a mental snapshot of what life is like up in coffin corner.
Private jets, or any jet aircraft that fly close to the speed of sound, measure their speed in Mach numbers at higher altitudes. A Mach number is a ratio of the plane’s speed in a medium (air of a certain temperature) vs. the speed of sound in that same medium. Every jet aircraft that flies in the upper atmosphere needs to pay attention to their max allowable Mach number or Mach CRIT – the critical speed. And here’s where the rubber meets the road with that speed and why it matters:
The way air flows at those speeds over a wing’s surface has characteristics that can actually change where the “up” vector is on that wing – also known as the center of pressure. While scientists and aerodynamics pros would shudder at this analogy, here is the easiest way to envsion it: Imagine you are watching a the wake of a boat from above – that is the same type of fluid dynamics that are happening around a wing. Except in the wings case the bow of the boat is the leading edge of the wing and you are now looking sideways at that wake. In an airplane, those wake lines create a shock wave once the wing is going through the air at higher speeds. This shock wave, creates more and more pressure as you approach the speed of sound. At some point, the amount of new drag (from this increased speed and jamming up of the shock waves, if you will) causes a big change in the aerodynamic characteristics of that wing – and what was once a happily balanced wing with a center of pressure that allowed the airplane to fly in level flight, now experiences a center of pressure that moves aft, or airplane / boat talk for “back on the wing.” As this center of pressure moves aft, the nose of the aircraft pitches down, since the weight ahead of this center of pressure is now greater than it was before the center of pressure moved – think see saw with a moving pivot point.
So to recap: Plane go fast, big waves make big drag and move the center of pressure aft and nose go down. Here’s the really bad news – we’ve already got a too fast situation and now that we’ve begun an uncontrollable nose down kind of thing, the airplane goes even faster.
The Mach tuck as it is known, or uncontrollable dive is bad news all around. As the aircraft continues to accelerate eventually it will break up or lose parts since it isn’t built or designed to go beyond a certain speed. The only way to prevent accelerating beyond the Mach CRIT number is to deploy speed brakes (things that shoot up out of the wing to disrupt airflow) or lower the landing gear which will certainly damage it, but also save everyone’s life hopefully.
The stall is a simpler affair: The aircraft stops flying. The problem with Coffin Corner is that the stall speed (at the higher altitudes where jet aircraft are more efficient and prefer to fly) creeps up to a higher and higher speed. In fact, the stall speed increases so much that it meets Mach CRIT specifically at a little junction known as … yep, you got it, Coffin Corner.
The reason why the stall speed rises is fairly simple to translate into non-aviation talk: The air is thinner. With less molecules to pound out of the way, the faster the plane needs to fly to keep that lift thing going on. (Lift is a longer subject of angle of attack vs. the shape of the airfoil, but for the purposes of this post, let’s just assume you need some speed to make the lift happen.)
A big misnomer also about “stalls” is that you’ll frequently here non-aviators confusing it with the engine. An aerodynamic stall has nothing to do with the engine stalling, rather a stall is that dip your paper airplane makes after you first huck it into the air and it pokes its nose up, before it drops and keeps flying a bit more before gliding to the ground.
In the case of Air France 447 all they needed to do (to get out of the stalled wing condition of being too slow while near Coffin Corner) was do that famous dip you’ve seen a paper airplane do so many times. Lower the nose, accelerate, get the wing flying again and you are back in business…. you are flying, no longer falling. Stalling and stall recovery is just that simple.
Now, you are an expert on stalls, Mach CRIT and Coffin Corner.
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