"Steven P. McNicoll" wrote in message
nk.net...

"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.


Flight Level is an altitude of sorts but it is measered against a
theoretical datum. FL400 would be 40000 feet above theoretical Mean Sea
Level. When flying at an altitude you are actualy flying above a physical
reference point.
29.92 Inches or 1013(.2) Millibars is the altimiter setting used when you
are told to fly at a flight level. If you are told to use any other
altimeter setting then you are no longer flying at a flight level but an
altitude. In the US, all aircraft above 12500 feet (I think) should be using
flight levels. In Europe, it varies. In England FL as low as FL 35 is
possible.
As an aside, above flight level 290 you will only get odd numbers as 2000
feet separation is required, so FL400 does not exist UNLESS you are flying
in airspace which uses RVSM or reduced vertical separation Minima. This has
recently been introduced in Canada and across the Altlantic. I am not 100%
sure but I dont think it is used in the USA.




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.

Not a chance at FL 400 and very unlikely even at FL250.



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.
Supersonic flight is not permited over land (in peacetime operations)