Q. about "Flight Level"

On Sun, 11 Jul 2004 16:39:11 GMT, TeleTech
wrote:

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?

Altitude, measured traditionally by an altimeter, requires a
barometric pressure reading for a base. For most low altitude work,
the local barometric pressure is set in the altimeter and it then
reads field elevation in feet above mean sea level (MSL). As you
climb, the pressure is reduced and the instrument reads your altitude.

For high altitude operations, where aircraft will be transitioning
barometric pressure gradients, it is necessary to use a standard
altimeter setting so that all aircraft get the same altitude readings
and can maintain separation. This standard setting is 29.92 inches of
mercury. During climb out at the designated transition altitude,
altimeters are reset to standard then altitudes above that are
reported as flight level.

And, you are correct, FL400 is 40,000 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.)

If 6000 feet is a nautical mile, then the airplane is six and a half
miles away. Could you hear four jet engines at that distance?
Probably.

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

Only if the aircraft is supersonic. Typically airliners are operating
in the .85-.88 Mach airspeed region. Even tactical jets are probably
cruising sub-sonic. No booms for you.


Ed Rasimus
Fighter Pilot (USAF-Ret)
"When Thunder Rolled"
Smithsonian Institution Press
ISBN #1-58834-103-8