atpl question
 

Can anyone tell me the effect that a change in altitude will have on an aeroplane's TAS? I have a question which is as follows:

An aeroplane performs a straight and level horizontal flight at the same angle of attack at two different altitudes: (all other factors of importance being constant, assume ISA conditions and no compressibility effects)

A – the TAS at the higher altitude is higher
B – the TAS at both altitudes is the same
C – the TAS at the higher altitude cannot be determined
D – the TAS at the higher altitude is lower

Ref: AIR: atpl, cpl;

Ans: A
Is the answer above correct? If so please elaborate.

As you climb air density reduces therefore maintaining same IAS at a higher altitude TAS will be higher ( Considering ISA conditions ).

As speedover60 said, if the IAS remains constant the TAS would increase with height as the air density decreases.

Good old Bristol.

Sorry - absolutely nothing to do with ISA.

Perhaps someone should explain to the poster why we are assuming constant IAS in the question.

Happy New Year.

From the "ECTM" graph:

Constant EAS: CAS, TAS and MACH number increase with increasing altitude.
Constant CAS: EAS decreases with increasing altitude, TAS and MACH number increase with increasing altitude
Constant TAS: EAS and CAS decrease with increasing altitude, MACH number increases with incrasing altitude.
Constant MACH number: EAS, CAS and TAS decrease with increasing altitude.

Blimey Slav - you've remembered something I taught you.

Happy new year mate. :ok:

Its says "straight and level" and at the same AoA. So as the air is thinner you require a larger TAS at the same AoA in order to stay in straight and level flight.

Exactly, the answer comes from the lift equation:

L = CL * A * Rho/2 * V^2

Assuming weight is constant, the lift for straight and level flight must equal weight (L = W). The higher the altitude, the lower the density (Rho) of the air (mainly due to reduced air pressure). And if density decreases, you have to increase true airspeed (V) in order to create the same amount of lift and keep forces balanced.

...and now to stick my neck out...

Is this because the IAS has to be same and as the air is thinner, this will result in a higher TAS?

PM

Nothing to do with IAS. All the question refers to is the fact that at a higher altitude with a constant AoA you require an increase in TAS. Look at the Lift equation, as density is decreasing, something must increase in order to produce the same lift.

Hope its clear

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:ok:

Baggers said:

Really!!!!!!!!!!!!!!!!!!!!!!! If you really want answers - ask a :mad: professional.....................

Bye bye........