Who can answer this ATPL type performance question?

[keith williams]

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.