Alex,
I think the Pegasus problem you refered to is one of high altitude HP compressor surge. In the case of the Pegasus it is (or was some years ago) referred to as NF Root Theta surge. As altitude increases at constant NF, the air mass flow decreases while the HP spools RPM increases. This increases HP compressor ratio to the point where it surges. The old Mk 103s and 104s used a hydro-mechanical pressure ratio limiter in the fuel system, to limit maximum HP pressure ratio a above 10000 ft. The test was done at 0.8M at 40000 ft.
JT,
I must confess I was thinking only of the types where FADEC moves the throttle vavle without moving the power levers in the cokpit. You are of course correct that in some systems the autotthrotle (or pilot) moves the levers. But whichever method is used, it seems to me the overall effect is the same. There are two limits, thrust and turbine temperature, which must not be exceeded. Whatever control method is used, the engine is throttled up until it hits the lower of these two limits. By having a surplus of thrust capacity, it is the thrust limit that is reached within the range of ambient conditions for which the engine has been flat rated.
[ 20 February 2002: Message edited by: Keith Williams. ]</p>