With higher altitude (= reduced air-density), you need a higher TAS to have the same IAS, thus the higher you fly, the higher will be your TAS to carry a certain weight (good for traveling).
A high speed stall is when the airflow over your wing reaches the speed of sound (M = 1 at this point of the wing) due to acceleration around the wings shape. A shockwave emerges and the airflow separates. [Note: you might have heard of the term Mcrit or critical machnumber. This is the aircrafts theoretical speed when its airflow over the wing reaches M = 1]
The higher you fly the lower is your TAS needed to reach M = 1 (due to lower temperature). The higher you fly, the easier you end up in a high speed stall.
So now we combine low and high speed stall: at a certain altitude, the minimum TAS you need to avoid a low speed stall will be the maximum TAS you can fly before entering a high speed stall. This point is called "coffins corner" and theoretically the highest altitude an aircraft can fly.
Thanks for that studi. Ive been on the look out for a consice 'coffin corner' explanation for years!
Not sure, but I think Concorde used to pump fuel fore an aft to help control it too.
Yes it did.