"Boomer" wrote in message ...
I 've noticed that many of the more modern fighters (F-16, SU-27) only reach
max alt while subsonic, whereas older fighters ( F-4, MiG-21 for instance)
reach max altitude around M 1.6 or more. Is there some general reason for
this?


Don't know where you picked up this info but it's bogus. Aircraft fly
(and stall) on indicated airspeed, which correlates with air density
(and thus developed lift). We are talking about level flight, right?
In zoom climbs you go on up until the aircraft runs out of energy and
starts falling back down. Usually it will be subsonic then but this is
in no way at a sustainable altitude. In level flight at max altitude
the faster you go the higher you can go. For instance M1.0 at 65000
gives an IAS of around 166 KIAS. This is way behind best L/D for
fighter aircraft. M2 gives double that, about 332 KIAS. A M2 16 or 27
should be able to cruise in level flight at rather better than 65000 -
barring any mechanical (engine) reason why it shouldn't be there.
Walt BJ