Re: Flight Levels and Speed

From: (Ernie Fidgeon)
Date:         12 Jul 1999 19:14:27 -0400
Organization: MBnet Networking Inc.
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Others have replied to explain the airspeed physics.

Turbine engine thermodynamics says that efficiency of an ideal gas turbine cycle
is a function of the total difference in air temperature between the intake air
and the exhaust stream.  Therefore, high altitudes with low outside air
temperatures provide for efficiencies far higher than lower altitudes.

Second as noted, total drag is reduced at high altitudes allowing for lower fuel
burn coupled with higher cruise speeds, both from reduced parasite drag and
profile drag (wings are designed for cruise mach number/AOA idealization).

Third, for west to east travel the jet streams can be utilized for increased
ground speeds.  This would be really great if we traveled west to east only! :)

Fourth, travel at altitudes upwards of thirty thousand feet is more comfortable
as the local weather effects are below the aircraft for the most part,
especially thunderstorms and hail.

Fifth, travel at lower altitudes subjects the airframe to higher loads from
turbulence (higher density air, thus higher forces due to larger delta V) which
leads to less comfort and higher maintenance costs (fatigue).  Also, climbing to
higher altitudes requires a higher fuel load than your low altitude scenario
which actually reduces bending fatigue on the wings as the fuel is burned from
the inner wing out.   A low altitude scenario with less fuel would give
increased wing bending fatigue and additional flexure during turbulence,
hastening the fatigue.

Sixth, in the event of a loss of power and forced landing, the higher altitude
gives the pilots more opportunity for alternate airports to glide into.

Seventh, existing traffic demographics has most turboprop aircraft working at
you suggested altitude.  This would make routing/patterns a challenge to say the
least with the rabbit overtaking the tortoise!  Also, if we did this, our
utilization would go up as a result of shorter turn times, increasing workload
on the crews, and cycling the airframe more, both of which lead to higher
operations costs.  The crews can only fly so much and the airframes would
require checks more frequently, which would require more aircraft.  For example,
airlines that operate the 737 on short haul low alt. (28K max alt.) for example
have utilizations of up to 11 hrs a day in some cases, resulting in the need for
additional aircraft to account for the shorter C check/D check periods, unless
they take the lost revenue hit.

Eighth, you wouldn't be able to see as far and enjoy the glory of flight so
much! (GRIN)

Ninth, the flight attendants wouldn't have as much time to clean up during
descent!! (GRIN)

Sorry, couldn't resist the last two.

Hope this helps,

ME Incorporated wrote:

> I'm sorry to post a question about a computer aircraft simulator in these
> newsgroups, but it does have something to do with Real Life (tm), and I have
> to know!
> I was flying in Microsoft Flight Simulator 98, which is supposed to be
> pretty realistic when it comes to the flight models of the 737-300.
> I noticed that the higher you go, the lower the Maximum Mach needle was.
> This needle shows the fastest your aircraft is designed to go in terms of
> Mach (speed of sound), and moves according to air density (I guess.)  If
> your indicated airspeed is near or exceeds this marker needle, you may be in
> trouble.  In real life this is bad.  In a simulator, well, reset!
>  At 19000 feet, you can go almost 400kts (or faster?), but at 33000, you
> can't quite get to 300Kts without having the "overspeed" warning show up.
> Is this realistic?  It makes sense, given that the density of the air is
> less, and I think I remember that the speed of sound is slower due to the
> lower air density, so therefore, the maximum mach would be at a slower Kts
> reading.
> I am not a physics major, but wouldn't it be faster to fly at the lower
> altitude in real life?  I can't see any difference in fuel consumption in
> the simulator, but then again, it's a (to be honest) crude simulation.
> Wouldn't it make sense for the airliners to fly at the lower altitude, save
> the time, and turn around the aircraft faster?  This would allow the
> airlines to use less aircraft, less crews, and make more money with more
> paying passengers per aircraft per day.  (Provided, of course, fuel
> consumption stays constant at cruise.)
> Obviously, there might be a safety issue at the lower altitude, but that's
> for another discussion if my simple little theory is correct...
> Or is the simulator just screwed up?
> Just wondering... and thanks in advance!
> --
> "Flying is easy... buying the tickets, that's hard."
> -MikeyB