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roos112
Topic Author
Posts: 4
Joined: Tue Jun 16, 2020 1:51 pm

TAS question

Tue Jun 16, 2020 1:53 pm

How does TAS vary in a constant Mach climb in the troposphere ( under ISA conditions ) ?

Does TAS increase or decrease ?
 
User avatar
glen
Posts: 353
Joined: Thu Jun 30, 2005 4:43 pm

Re: question

Tue Jun 16, 2020 2:49 pm

TAS does decrease. The speed of sound is only a function of temperature. Speed of sound in m/s is c ≈ 331.5 + (0.6 x ϑ) , with ϑ=Temperature in °C.
Therefore speed of sound decreases with decreasing temperature. So with a constant Mach number, but a decreasing speed of sound due to lower temperature at altitude, TAS does decrease. You may check out google for articles about the crossover altitude, e.g.
https://www.quora.com/What-is-a-crossover-altitude
"The horizon of many people is a circle with zero radius which they call their point of view." - Albert Einstein
 
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Starlionblue
Posts: 19872
Joined: Fri Feb 27, 2004 9:54 pm

Re: TAS question

Wed Jun 17, 2020 12:15 am

The ECTM graph is a neat memory aid which most pilots memorise for the ATP/ATPL exams. You see people literally holding four fingers up in front of their face during the exam in order to visualise things. :)

As long as temperature decreases with altitude, this will always be the relative relationship.In the OP's case, M is constant, so that line will be vertical, meaning the top of the TAS line will be farther left. In other words decreasing with altitude.

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