gary pearson wrote:

"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.

Unless it's changed in the last 15 years or so, Flight Level usage starts at or
above 18,000 ft. MSL, which is to say FL 180 or higher, depending on the
barometric pressure. The applicable FAR used to be 91.81, "Altimeter settings,"
with other sections such as 91.109 "VFR cruising altitude or flight level", and
91.119 "IFR cruising altitude or flight level", based on it.

Guy