Airplane longitudinal control: pitch or power?

Question

I recently came across some slides from a flight school teaching, for the initial climb after take-off and for the approach, the strategy of controlling/adjusting

RoC/RoD with power

Speed with attitude

I tried to inform myself a bit on the subject, because I am more familiar with the idea of controlling

RoC/RoD with attitude

Speed with power

I came to understand that this is mostly a difference between VFR vs IFR flight. Is this true? Does the first approach actually have benefits in terms of ability for the pilot to control the plane in VFR flight? If yes, how and which?

Answer 1

I was wondering how long that it would take for this question to come up.

It is the old "pitch -vs- power" airspeed control technique debate that rages throughout the aviation world. There are strong proponents of both techniques (and they seem to view it almost as a religious debate in that the other side can never be right, no matter what), and both sides have good reasons for it!

Instead of saying that one is better than the other, let's just say that they are both techniques for managing the potential energy of the airplane (the airspeed that can be traded for altitude and vice-versa). In fact, this is how I like to think about it (and don't pick a side):

• If you change the pitch of the aircraft without changing power, the airspeed will stabilize out at a new value.
  • Pitch up and you lower the airspeed.
  • Pitch down and you raise the airspeed.
• If you change the power setting of the aircraft without changing the pitch, the airspeed will stabilize out at a new value.
  • Add power and you raise the airspeed.
  • Reduce power and you lower the airspeed.
• You can change both at the same time and get various results.
  
  • You can end up with the same airspeed that you had before.
  • You can end up with a lot more airspeed.
  • You can end up with a lot less airspeed.

Most often you need to change both at the same time in order to properly manage your airspeed.

So basically it comes down to what you want to accomplish. If you need to change the airspeed you have two ways to do it, but used individually each comes with a side-effect. In some situations this may be okay or even desirable, but you need to be aware of what your actions are going to accomplish.

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Let's look at a specific example:

Sometimes, especially on an instrument approach, you want to maintain your current airspeed while changing your vertical speed.

• Let's say that you are a little high on the glideslope.
  
  • If you simply pitch down to recapture the glideslope, you will gain airspeed but will recapture it quickly.
  • If you simply reduce the power to recapture the glideslope, you will lose airspeed but it will take longer than using pitch.
  • If you want to maintain your airspeed while recapturing the glideslope (typical on an instrument approach) then you would pitch down and reduce power at the same time, and when you recapture the glideslope you will pitch back up to maintain it and add power to maintain your current airspeed.

Answer 2

The practical argument for teaching the first approach to ab-initio students concerns developing safe responses to problems:

1. Falling short on approach (too high a rate of descent): If your first reaction is to raise the nose, you may well be creating the more dangerous 'too slow on approach' situation.

2. Power failure: If you think power controls speed, then you have lost the ability to maintain the correct speed. Furthermore, raising the nose will not keep you in the air (though it will slow you down).

From a more theoretical point of view, what does power allow a powered aircraft do that a glider cannot? It allows it to gain and maintain altitude relative to the airmass.

This point of view works well for flying small, low-powered, single-engined airplanes in VMC; beyond that, I cannot say.

Answer 3

Due to longitudinal stability, aircraft tends to maintain constant angle of attack. In straight flight the wing loading is constant, so speed remains the only significant factor and the aircraft tends to maintain constant speed (or rather oscillate around it in phugoid oscillation). If you increase the power and speed is maintained, law of conservation of energy dictates that the aircraft has to climb and when power is reduced it has to descend. So in theory you control speed with elevator (and trim so it is maintained without pressure on the controls) and vertical speed with power. See How It Flies, chapter 2 for a detailed discusson.

In practice changing power affects pitch trim too, so you need to always adjust both. And you have to arrest the phugoid oscillation.

It might be interesting to note that Airbus control laws change the rules so that attitude controls climb/descent and power controls speed by automatically adjusting the trim.

Answer 4

I think it also depends on kind of aircraft you are flying.

In jet aircraft speed is always controlled with power, lowering the nose to try and gain some speed on short final would destabilise the aircraft and probably lead to a more "firm" arrival.

Aircraft with underslung engines require forward pressure when applying power, which is quite noticeable on approach, due to pitch-power couple. The reverse is also true, take power off and you will need to trim back.