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newagebird
Topic Author
Posts: 63
Joined: Sat Sep 03, 2005 8:51 am

TAS Calculations

Sun Mar 11, 2007 8:10 pm

Hey
When posed with the question, 2 aircraft with different altitudes, which aircraft has the higher TAS?
is it the lower aircraft or the higher one.
The higher one because there is less density and so less drag? or
the lower one because the speed of sound is greater at a lower altitude, therefore travelling at a given Mach number produces a higher TAS at lower altitude.
Im hoping someone out there can help with the question.

Cheers newagebird
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: TAS Calculations

Sun Mar 11, 2007 8:52 pm

It depends on which aircraft is travelling at the highest airspeed...

Unless you want to specify the question a bit more, for example by stating that they are at the same IAS or the same mach number.

Typically you are travelling at a constant IAS up to a switchover point where you start flying a constant Mach number.

A given IAS will mean a higher TAS the higher you go, as IAS depends on dynamic pressure which will lower for a given TAS as density decreases.

A given Mach number will mean a lower TAS as temperature decreases, as the speed of sound depends on temperature. The temperature decreases up until you reach the tropopause, through which it remains largely constant.

Rgds,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
newagebird
Topic Author
Posts: 63
Joined: Sat Sep 03, 2005 8:51 am

RE: TAS Calculations

Sun Mar 11, 2007 9:19 pm

Thanks fredT
Yeh the question isnt specific on whether its IAS or Mach number so i was a little confused. This question was on a tech exam for an airline so i guess you would have to give the answer in terms of both speeds in order to clarify that you know your stuff.
Thanks, i think your right
cheers newagebird
 
FredT
Posts: 2166
Joined: Thu Feb 07, 2002 9:51 pm

RE: TAS Calculations

Mon Mar 12, 2007 5:42 am

What kind of job will they ask you that kind of interview questions for, if you don't mind me asking? Struck me as a question where you either don't need to know the answer or have papers telling them that you do know it.

Rule of thumb is TAS = IAS with 1.5% (or 2%, for easier arithmetics) added for every thousand feet ASL. Might make it easier to remember.

Rgds,
/Fred

[Edited 2007-03-11 22:42:48]
I thought I was doing good trying to avoid those airport hotels... and look at me now.
 
newagebird
Topic Author
Posts: 63
Joined: Sat Sep 03, 2005 8:51 am

RE: TAS Calculations

Mon Mar 12, 2007 9:11 am

Just preparing for a cadet pilot technical exam...they like to get very technical so yeh there you go, weird questions. Theres another one that says why does the 777 not have winglets? You can find a lot of technical exam questions on www.willflyforfood.cc.
Cheers newagebird

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