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When a U-2 spy plane flies very high, the air gets very thin. This causes the the amount of air going over the wing to fall, so the wing thinks it is flying slower and therefore will stall (lose all lift) and fall out of the sky at much higher real speeds. The U-2 has another limit, its critical Mach number. This is the highest speed before the airplane will begin to tuck under and lose control due to transonic effects on the wing and tail.
Normally, there is a huge gap between the stall speed of the airplane and its critical Mach number. But as you climb higher, these numbers begin to converge. At the U-2’s highest operational altitude, the difference between the stall speed and the critical Mach number can be as little as five knots. This is known to U-2 pilots as the “coffin corner.” In the coffin corner, if you fly any faster, you lose control. Fly any slower … and you lose control. It’s extremely dangerous, and normally the plane would only be flown on autopilot in this region.