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Most of us have never had to worry about exceeding VNE - especially in level flight. And in a piston airplane, VNE is about as far away from stall speed as you can get.
But, the same isn't true in a jet. Especially a subsonic one. At a jet's operating ceiling, its Maximum Mach Number (MMO) is often extremely close to its stall speed. And that region of flight is called the "Coffin Corner"
The coffin corner's real name is the "Q Corner", because "Q" is the abbreviation for dynamic pressure. Coffin corner occurs from the interaction between stall speed and critical mach speed, which are both caused by pressure over your wing. So, "Q Corner" is the techie name, but coffin corner sounds more dramatic.
The region is deadly. Get too slow, and you'll stall the jet at high altitude (not something you want to do). Get too fast, and you'll exceed your critical mach number. The air over your wings will go supersonic, you'll pitch down, the aircraft will accelerate, and your wings will fall off. Also bad.
So why does this happen at a jet's maximum ceiling? As you increase altitude, true stall speed increases, and the true airspeed to reach MMO decreases. Coffin corner is the region just below their intersection.
As you climb, the air becomes less dense, and your wings need more airflow to generate the same amount of lift. So, as you climb, your true stall speed increases. This is true in a prop, turboprop, or jet.
As you climb, the true airspeed to reach MMO decreases. In sub-sonic jets, MMO prevents you from reaching your critical mach number. That's the speed where some air flowing over your wings begins traveling at the speed of sound.
As the air flows over your wing, it accelerates. At some point, the air in front of your wing may be subsonic, but it will accelerate past the speed of sound as it flows over the wing's upper surface. Once this happens, a shock wave forms. Turbulent flow develops behind the wing, causing a buffet called mach buffet.
You can also experience mach tuck after you pass the critical mach number. The supersonic air flowing past the leading edge of the wing generates far more lift than the subsonic (and often stalled) air aft of the shock wave. At this point, your center of lift is located at the leading edge of your wing. As you speed up, that shockwave moves aft. The high-lift region follows it aft, and the the center of lift moves aft, as well.
As your center of lift moves aft, your nose begins to pitch down. You accelerate more, and the center of lift moves farther aft. If your tail uses a conventional elevator, you may find that supersonic flow also limits its effectiveness, and you can't raise the nose. Now you're a missile.
Supersonic flow is the main limitation of a sub-sonic jet's MMO. And while MMO is a fixed number (e.g. 0.85 Mach), the true airspeed where you reach MMO decreases as the air gets colder.
As you climb in altitude, air temperature decreases. That's why jet aircraft have a moveable "barber pole" needle to show MMO, that automatically decreases with temperature. Glass panel aircraft use a similar digital marking.
As you approach the aircraft's maximum ceiling, you'll find that MMO and stall speed meet, or at least get close.
Most of today's jets have a fairly wide margin between stall and MMO, but a great example of a coffin corner aircraft is the U-2. At high altitudes, the U-2 can have as little as a 5 knots between stall and mach buffet. That leaves no room for error.
In the coffin corner, aircraft become extremely difficult to fly, and an autopilot is a necessity. Your margin for error is small, hence the name "coffin corner." But, despite its name, it still would be fun to try.
Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at firstname.lastname@example.org.