View Full Version : Who can answer this ATPL type performance question?

27th Jul 2015, 14:39
Maintaining the same IAS descending from 35000 to 5000 feet, your pitch attitude will:

A. Increase then reduce
B. Be constant and unchanged
C. Gradually increasing
D. Gradually reducing

I am guessing here...C
Whats your answer with explaination?

Above The Clouds
27th Jul 2015, 15:26
Guessing A as the closest correct answer, but in reality I would say it would increase and then remain constant.

Due to temp change increase below the tropopause maintaining a constant Mach until cross over altitude say "27000ft" would require an increase in downward pitch to maintain a constant CAS/IAS as dynamic pressure increases therefore drag increases. Once below cross over altitude to keep a constant CAS/IAS there maybe a small reduction in the descent pitch attitude required to achieve a decrease in TAS, with throttles closed.

27th Jul 2015, 15:30
Tanks for your input.

With this question flying Mach until the changeover would not be flying a constant IAS.

I think the question wants for example to maintain 240kts indicated all the way down?


keith williams
27th Jul 2015, 16:34
The short answer to this question is that we need to maintain a constant pitch angle.

The long answer is

The ASI gives an indicated airspeed (IAS) by measuring the dynamic pressure.

So if our IAS is constant then our dynamic pressure must also be constant.

The speed down the glide path is determined by the relative magnitudes of the force acting up the glide path and the force acting down the glide path.

The force acting up the glide path is drag and the force acting down the glide path is the aircraft weight multiplied by the sine of the glide path angle.

Drag in any given configuration is determined by dynamic pressure and angle of attack.

If we maintain constant pitch angle we will maintain constant glide path angle and constant angle of attack.

So if we have constant dynamic pressure and we maintain constant pitch angle, we will have constant drag up the glide path and constant weight component acting down the flight path.

If the force up the path is equal to the force down the path the, then the TAS down the path will be constant. But this will not give us a constant IAS, because as we descend, the increasing air density causes the TAS : IAS ratio to decrease. We need to go slower to maintain the same speed indication.

So to maintain constant IAS we need to select a pitch angle which gives us a drag acting up the flight path which exceeds the weight component acting down the glide path by just enough to gives us the required deceleration rate to decrease the TAS just enough to maintain constant IAS. Having set this pitch angle we simply need to maintain it.

This explanation includes a number of simplifications, including the fact that the ASI actually measures impact pressure, which is not quite the same thing as dynamic pressure. This distinction is included in question in the Instruments syllabus but is often ignored in this type of question in the POF syllabus.

28th Jul 2015, 11:54
I would say it is a stupid question because it does not provide the information that is necessary for answering it.

Maintaining the same IAS descending from 35000 to 5000 feet the Mach number will reduce. Assuming that the change of Mach has no effect on the cL - cD - AoA relationship, the AoA and L/D will be constant. Pitch attitude is the sum of AoA and flight path angle, and the latter depends on L/D and the thrust setting.

So assuming zero (change of) thrust and zero Mach effects, the correct answer is "B".

P.S. The difference between EAS and CAS/IAS is one of the compressibility effects that are governed by Mach number.

28th Jul 2015, 13:40
Is this an actual ATPL question from exam or?

Certainly seems like we are focusing on producing pilots with relevant knowledge who will be ready to face the modern flight deck environment... :eek:


28th Jul 2015, 14:09
The answer would be B for the reasons given by others if the descent were at constant EAS, ignoring Mach effects. If instead it is IAS that is constant then EAS is increasing and will require a reduction in AoA.

As Keith remarks, the IAS/CAS vs. EAS distinction may be making a finer point than a questioner expects and so B might be acceptable anyway.

keith williams
28th Jul 2015, 14:36
It is (or for many years was) a real JAR ATPL question and the required answer was option B.

The logic employed by the author of the question was as I have explained above.

It is possible that the question has been removed or modified since the change over to EASA.

balsa model
28th Jul 2015, 23:45
A pedant correction to Keith's well reasoned (to me) answer about 1st order effects:
Even in idle thrust there would be some weight loss during descent. So perhaps gradually reducing pitch, after all?