Airspeed question

[09El_Boissevain]

Hello folks.
I was wondering because I had 2 different things reading differently.
When I cruised at 5,500ft, my airspeed read 251kias and M.41
When I cruised at 10,500ft, my airspeed read 245kias and M.44

Which one is faster? Do I go faster at 5,500ft or 10,500ft??

Thanx

Capt. El



Post Edited ( 04-14-06 13:14 )

[Drew]

10500

[AKLupine]

Keep in mind that the higher you are, the higher the mach number will be for a given idicated airspeed.

[Jetsim]

AKLupine wrote:
Keep in mind that the higher you are, the higher the mach number will be for a given idicated airspeed.

This is also due to the higher you are, the slower the speed of sound, therefore, this is why jets are more efficient and faster in the high
altitudes.

[09El_Boissevain]

Thanx I'll keep that in mnd.

Capt. El

[CowlFlapsOpen]

also IAS reads lower at the same true airspeed with increased altitude bc of the decreased air density

[Flagstaff Flyer]

That is understandable. But I am having trouble understanding that if IAS reflects decreased air density then why do I get "overspeed"
warnings when I am well below VMO? I guess I don't understand the relationship between IAS and true air speed. If IAS is showing how
fast you are passing through the air (at any altitude) how could your ac become damaged if you are below VMO? Is it that just because
there is less air at altitude you are still flying through it at a much higher speed?
Thanks, Mike

[09El_Boissevain]

I look at the MACH to reference true airspeed



Post Edited ( 04-16-06 09:46 )

[olseric]

Courtesy of IVAO: http://www.ivao.aero/training/tutorials/...Speeds.htm

[Flagstaff Flyer]

Thanks Eric!

[Pablo0007]

Hi

That is understandable. But I am having trouble understanding that if IAS reflects decreased air density then why do I
get "overspeed"
warnings when I am well below VMO?

I would like to say that when you go past your transitional height, your mach number takes over. When you have the auto throttle on IAS
instead of Mach, the autopilot will try to do what you tell it. It will keep your speed at whatever you ask it to do. At flightlevel 410, A380,
B777, B737-800, if you do not change the Auto throttle to Mach Number and want to keep flying at 290Kts IAS, you will over speed. In your
performance specks, it tells you what Mach Number your cruise should be. In the A380 At Flightlevel 430 or 43,000, Mach.83 is
239/240Kts IAS or there abouts.

I usaually change to the Mach number when I have reached 27000 ft as this is where the aircraft needs only around 5-700 ft Per Minute
climb. I change over to What ever the performance specks say. In the B737 PMDG (I have both programs NG & 800/900) the FMC in the
Climb Cruise & Desent pages show you the recommended speeds & Mach Number to have.

Once again, An autopilot will only do what is programed into it. You program wrong info, it will perform wrong thing. In the ITV Concorde
DVD, The Captian says if you program the FMC or in his case the INS correctly, you will have no problems. It is the same for normal FS9
Autopilot.

Hope that helps.
Cheers
Paul

[Flagstaff Flyer]

That is understandable - thanks Paul