"TeleTech" wrote in message
. 193.32...

Hi.

I have heard some military comms on my scanner (not that I understand
everything I hear).

If I understand correctly, "Flight Level" or FL is used to describe
altitude. So, FL400 would be 40,000 feet?


"Flight Level" means a level of constant atmospheric pressure related to a
reference datum of 29.92 inches of mercury. Each is stated in three digits
that represent hundreds of feet. For example, flight level (FL) 250
represents a barometric altimeter indication of 25,000 feet.



My question is, if an airplane is way up there, can I hear the plane down
here? (I can hear them on the radio, but I mean, can I actually hear the
engine of the jet, as in the sound that a jetliner makes when it goes
overhead.)


I don't know if they're audible to the human ear at that height, but you
probably couldn't hear them over normal background noise anyway.



At the speeds used up that high, wouldn't I hear a sonic boom?


Only if they actually exceed the speed of sound. I remember hearing them
fairly often growing up in the sixties, haven't heard one in a very long
time.