Q. about "Flight Level"

[TeleTech]

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?

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.)

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

[Mike]

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

You'll hear them at 40,000ft. That's only 7 miles away and there's nothing
to get in the way.

If they were travelling at mach speeds you'd hear the boom but they don't
tend to do that over land, or at least over populated areas.

[Ed Rasimus]

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

[Steve Mellenthin]

>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?

Nope.

http://en.wikipedia.org/wiki/Flight_level

>
>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.)

What you hear is the sound of the engines. It is louder than the "sound that a
jetliner makes"
>
>At the speeds used up that high, wouldn't I hear a sonic boom?
>

Not when the plane isn't going supersonic.

Wait til high school. Your physics teacher will explain.

[Ian MacLure]

TeleTech > wrote in
. 193.32:

[snip]

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

Not if they are subsonic.
Most classic jet aircraft cruise in the high 0.7-0.8 Machs.
Some cruise closer to 0.9 than 0.8 but thats not in the shock
producing region.
I used to lie on my back in the back yard with the radio tuned
to an air band. Every once in a while we'd hear a call sign
Speedbird ( British Airways ) reporting their altitude as FL5X0
to NY ARTCC. Only one airplane in the world that could be (Concorde)
By the time they would be below Mach 1 and no longer generating
a shock wave. So never heard a sonic boom.
Lived two years near Langly AFB and never heard a boom either

IBM

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[Steven P. McNicoll]

"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.

[gary pearson]

"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)
>

[D. Strang]

"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.

Hogwash. I hear them every day. As a matter of fact, I hear them about
20 miles after they pass overhead. You have to look where they might be
now, instead of straight up where you hear the sound.

[Guy Alcala]

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

[Ian MacLure]

"D. Strang" > wrote in
news:q4IIc.23480$r3.15887@okepread03:

> "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.
>
> Hogwash. I hear them every day. As a matter of fact, I hear them
> about 20 miles after they pass overhead. You have to look where they
> might be now, instead of straight up where you hear the sound.

Shock waves are funny things. They can reflect and travel long
distances. There were reports for years of mysterious
"sonic booms" on the east coast of Canada for many years which
I believe have a weak but not insignificant correlation with
Concorde schedule.

IBM

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[gary pearson]

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
>

[Andrew Bunting]

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

[M. J. Powell]

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