There is absolutely a coffin corner on airliners.
For example, the FMC in a Boeing 757 calculates max altitude (at which the airliner can only climb 300 FPM when wings level, or at which it can only maintain 1.2 x stall speed). That altitude depends on gross weight, the speed at which the airplane is intended to be flown*, air temperature, and a few other things...
Indicated airspeed is a measure of dynamic pressure, so, with fewer air molecules hitting the airplane, it has to be going a lot faster through the air to get the same dynamic pressure. True airspeed is a measure of pure velocity through the air.
So, at sea level, if you see 230 KIAS, you're going about 230 KTAS (True Air Speed). 230 KIAS is often the clean maneuvering speed for a 757 with flaps up at high gross weight. The plane can be banked up to 30 degrees and flown well, but there isn't a ton of maneuverability at clean maneuvering.
Now, look at 230 KIAS when up at FL 370, and you'll see the TAS up around 500 KTS...mach number about .80...so, you're near the airplane's max speed, but without a lot of dynamic pressure (created by Indicated Air Speed) to create lift/forces over the wings and flying surfaces.
Airliners tend to be thrust limited. You simply can't get any higher...and the difference between stall buffet and mach buffet gets quite narrow at high altitude. For the U-2, for example, it's 4 Knots of Indicated airspeed. For a 757, I've seen about 30 KIAS between stall and max...not comfortable if you hit turbulence that could push the airspeed to either buffet regime.
You don't want either form of buffet.
"Nervous about buffet"...yeah, in the same way that we are "nervous about sliding sideways on black ice as we drive to the airport"...it's something you really, really try to avoid. Mach buffet will overstress some airplanes. Stall buffet leads to a control loss. For some folks, that's solved with a giant altitude loss...for some folks, it's worse...
Air France 447 experienced a high altitude stall...didn't work out well for them.
In a high altitude stall, you don't have the thrust to recover. Those HBPR fans make less power at high speed and less power at low air density...making high altitude a real challenge as the engines aren't making much net thrust. You have to accept a massive altitude loss to recover from a fully developed stall at high altitude, you're not going to power out of it.
For all airplanes flying up high, the behavior changes. The airplane is a body moving through space, acted on by the forces of wings, engines, control surfaces.
At high altitude, those forces are reduced because the air density is reduced. However, the airplane still follows Newton's laws and it is flying much faster. So, more inertia, but less force available to act on that body.
Turn radius then, is huge at high altitude, because the airplane is going much faster, and is acted upon by much smaller forces.
I know of pilots (who get extra training and this discussion in greater detail) who have tried to push an airplane higher than the FMC calculated max...and then hit stall buffet...which led to a huge altitude loss...and a potential loss of separation with other traffic. That loss of separation can lead to certificate action...like a traffic ticket for pilots...if traffic tickets came with a $10,000 fine and the permanent loss of your job.
The 757 is a good, honest jet. Handles well. Has good power. But at altitude it, like all other airliners, has to be flown gently. There isn't a performance margin to maneuver it aggressively. The autopilot imposes bank and pitch limits on the airplane up there...and the thoughtful pilot who is hand-flying does the same thing.
*Expressed as a mach number based on a parameter know as cost index. Higher CI = faster cruise speed, and sometimes, a lower max altitude.
For example, the FMC in a Boeing 757 calculates max altitude (at which the airliner can only climb 300 FPM when wings level, or at which it can only maintain 1.2 x stall speed). That altitude depends on gross weight, the speed at which the airplane is intended to be flown*, air temperature, and a few other things...
Indicated airspeed is a measure of dynamic pressure, so, with fewer air molecules hitting the airplane, it has to be going a lot faster through the air to get the same dynamic pressure. True airspeed is a measure of pure velocity through the air.
So, at sea level, if you see 230 KIAS, you're going about 230 KTAS (True Air Speed). 230 KIAS is often the clean maneuvering speed for a 757 with flaps up at high gross weight. The plane can be banked up to 30 degrees and flown well, but there isn't a ton of maneuverability at clean maneuvering.
Now, look at 230 KIAS when up at FL 370, and you'll see the TAS up around 500 KTS...mach number about .80...so, you're near the airplane's max speed, but without a lot of dynamic pressure (created by Indicated Air Speed) to create lift/forces over the wings and flying surfaces.
Airliners tend to be thrust limited. You simply can't get any higher...and the difference between stall buffet and mach buffet gets quite narrow at high altitude. For the U-2, for example, it's 4 Knots of Indicated airspeed. For a 757, I've seen about 30 KIAS between stall and max...not comfortable if you hit turbulence that could push the airspeed to either buffet regime.
You don't want either form of buffet.
"Nervous about buffet"...yeah, in the same way that we are "nervous about sliding sideways on black ice as we drive to the airport"...it's something you really, really try to avoid. Mach buffet will overstress some airplanes. Stall buffet leads to a control loss. For some folks, that's solved with a giant altitude loss...for some folks, it's worse...
Air France 447 experienced a high altitude stall...didn't work out well for them.
In a high altitude stall, you don't have the thrust to recover. Those HBPR fans make less power at high speed and less power at low air density...making high altitude a real challenge as the engines aren't making much net thrust. You have to accept a massive altitude loss to recover from a fully developed stall at high altitude, you're not going to power out of it.
For all airplanes flying up high, the behavior changes. The airplane is a body moving through space, acted on by the forces of wings, engines, control surfaces.
At high altitude, those forces are reduced because the air density is reduced. However, the airplane still follows Newton's laws and it is flying much faster. So, more inertia, but less force available to act on that body.
Turn radius then, is huge at high altitude, because the airplane is going much faster, and is acted upon by much smaller forces.
I know of pilots (who get extra training and this discussion in greater detail) who have tried to push an airplane higher than the FMC calculated max...and then hit stall buffet...which led to a huge altitude loss...and a potential loss of separation with other traffic. That loss of separation can lead to certificate action...like a traffic ticket for pilots...if traffic tickets came with a $10,000 fine and the permanent loss of your job.
The 757 is a good, honest jet. Handles well. Has good power. But at altitude it, like all other airliners, has to be flown gently. There isn't a performance margin to maneuver it aggressively. The autopilot imposes bank and pitch limits on the airplane up there...and the thoughtful pilot who is hand-flying does the same thing.
*Expressed as a mach number based on a parameter know as cost index. Higher CI = faster cruise speed, and sometimes, a lower max altitude.
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