In article ,
Jim wrote:
Still thinking about VNE and whether it is usually stated as a TAS
rather than an IAS (one must read the POH to be sure, of course).

I've gotten the notion, probably from comments I've not understood
very well, that the "coffin corner" is the intersection of stall speed
as an IAS indication on the airspeed indicator, and VNE understood as
a TAS and thus occurs at a decreasing IAS with altitude.

I guess the consequence of this notion is that as aircraft altitude
goes up the stall speed TAS goes up to ultimately bump into
the VNE TAS. If VNE is published as an IAS, like stall speed,
then stall speed and VNE would never converge.

Or maybe it was the Manuevering TAS that
could bump into the VNE TAS.

IS this what is sometimes referred to as the "coffin corner"?

I think "coffin corner" is a totally different phrase relating to
high speed stall buffet. At some high mach number at high altitude,
a shockwave causes airflow separation from the wing and a
stall.

There is also for the wing a low speed "traditional" stall
which occurs with too much angle of attack.

There is some TAS where the curves of stall speed and
max mach number intersect. This happens at some altitude.
At that altitude and above, the aircraft WILL stall,
one way or the other, despite precise speed control.

I'm told the U-2 used to fly around at 80 kts IAS
at this coffin corner, and the FL was determined by
how much turbulence was expected (flying right at the
intersection altitude can only be done at perfect
airspeed in perfectly smooth air).

Another issue is designing an aircraft shape where
all the parts (underbelly, top, tail surfaces, wings) stall
at the same mach mach number (high and maybe low speed).
This prevents shaking and buffeting and allows a higher
mach speed, and better efficiency, and apparently
higher altitudes before reaching "coffin corner" or
equivalent "coffin corners" for turbulent conditions.

I watched a video at a 737 ground school about a
captain who flew a 737 into coffin corner, hit
some turbulence, low speed stalled, and spun the
737. After spinning through many thousands of feet,
he recovered. I heard the galley was out of
liquor a few minutes later...

I don't believe "coffin corner" is from high speed
separation over the wing over the wing.
But there may be some relation as the wavi air
goes over the wing and the rather narrow control
surface and waves over the aileron or elevator,
causing it to stall, engage, stall, engage, etc.
I.e. flutter.

So maybe flutter is related to narrowness of
glider control surfaces? And therefore each
control surface has its own little tiny
"coffin corner"? Who knows...