Q. about "Flight Level"

No you are correct, I made a mistake. Sorry.

"Guy Alcala" wrote in message
. ..
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

gary pearson wrote:
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.

I live under the approaches to the MIMKU / GOMUP / NIBOG Atlantic entry
points and I can assure you that modern ( Stage III ) traffic is easily
audible from outside as it passes overhead, even up to FL370. There is
little commercial traffic above that level but biz jets are just about
audible at FL410.

On some nights, REACH Hercules flights crawling-out westbounds sound like
someone moaning in the garden outside...

The tone and volume of the audible sound varies considerably depending
upon cloud cover. However, it is simple and practical to sit on a
park bench on a heavily-overcast day and count the aircraft passing
overhead, high above the cloud deck, and to associate each aircraft
with traffic heard on the Scottish Upper frequency. On a clear
day, a pair of binoculars will confirm the livery, to a degree
( ``ah, quad with four red nacelles and a reddish tail; must be
VIRGIN 75...'' )

BTW, I'm in Belfast.

Thanks

Andrew

--
Andrew Bunting

In message , Andrew Bunting
writes
gary pearson wrote:
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.

I live under the approaches to the MIMKU / GOMUP / NIBOG Atlantic entry
points and I can assure you that modern ( Stage III ) traffic is easily
audible from outside as it passes overhead, even up to FL370. There is
little commercial traffic above that level but biz jets are just about
audible at FL410.

Has anyone noticed the phenomenon which I have named 'the acoustic
horizon'? I live about 5 miles from the end of the runways at
Manchester. I've noticed that I can see aircraft climbing towards me but
cannot hear them. Then, suddenly, the engine noise is audible and
rapidly increases. There is no slow fade in to audibility.

I've come to the conclusion that the change in atmospheric pressure with
height causes the sound to bend downwards in front of the aircraft so
that nothing is heard until this sound wave sweeps over my location.

Mike
--
M.J.Powell