Re: Flight Levels and Speed

From: (Mike in Miami)
Date:         12 Jul 1999 09:44:22 -0400
Organization: InfiNet
References:   1
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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

We are talking indicated airspeed here and therein lies the confusion. 
As you go higher, air density drops and the speed displayed on the
airspeed indicator (a pitot-static instrument) drops. One of the many
functions of both manual and installed flight computers is to correct
the imdicated airspeed for temperature and altitude to arrive at "true"
airspeed. This is useful for flight planning and performance analysis.

The airspeed indicator itself will display the mach (or knots)
limitations referenced to the indicated airspeed so the pilot can simply
maintain his speed within limits without any mental gymnastics to do the
conversion in real time.

If you are at FL290, indicating 250 knots you are traveling quite a good
deal faster in terms of true airspeed (if I had my trusty E-6B here with
me, I'd do the conversion for you but alas...). But the indicator may
show 270 knots as VNE....

Hope that helps...

Mike in Miami