How do planes descend for landing?

Question

I fly occasionally on a pretty short route on a turboprop (DHC-8). When the airplane descends for landing, I can look through the window and see the ground below getting closer over time. However I can't really tell what the pilots do to achieve that. I know that turns make you lose altitude, but it's very clear to me when the airplane makes a turn, so let's count that one out. So we're basically left with:

1. Slight nose-down attitude
2. Reduction of engine power
3. Combination of both

The reason I'm asking this is that during descent I do get this sensation of going down. However, during that time I cannot really see what the pilot did to achieve it. The nose-down attitude might be too minor for me to actually notice (I do not notice a slight nose-up cruising attitude to be honest). I hear a slight reduction in engine power a few times during a descent, but the feeling of going down and the audible noise reduction are separated in time. Is it that it takes time for an airplane to reduce speed (and as a result, descend) when you reduce engine power?

One additional question: I guess that with all the electronics in the cockpit pilots just set a target altitude or rate of descent, and then the airplane systems apply the correct measures to achieve this. How does the airplane know how much to reduce engine power and/or pitch to achieve this exact rate of descent?

Answer 1

If the pilot reduces the airspeed while he increases the rate of descent, you will not notice any change in pitch attitude when the aircraft descends. With jets you should be able to hear a change in engine pitch when thrust is reduced. Turboprops run their engines at constant speed, and a reduction in power here means a reduction in propeller pitch. This causes the propeller noise to change, but this change is harder to discern. With a little practice you should be able to hear the slight drop in propeller noise which signals the begin of the descent, though.

Gliders control their sink rate by flying in rising or sinking air, and additionally by flying slow (and near their minimum sink rate) or fast. To descend quickly, they can open speed brakes and/or sideslip. My favorite for a fast descent is to spin the aircraft, but airliners do not use this option.

In order to produce the necessary lift to keep the aircraft in the air, the pilot picks a combination of speed and angle of attack at which lift equals weight. Other combinations would result in an accelerating vertical speed and are only used in transitions from one flight state to the next.

To arrive at the aircraft's pitch attitude, you need to add the flight path angle to this angle of attack. If the plane descends with a flight path angle of -3° and the pilot reduces speed such that he needs to increase the angle of attack by the same 3° to produce the same lift, the resulting pitch angle of the aircraft would stay the same.

Normally, the correct answer to your question is 2. Reduction of engine power. But the other answers are also correct: If the speed is held constant, the attitude has to be reduced by the desired flight path angle change. Note that this is achieved by reducing thrust alone. If the pilot wishes to descend really fast, he might move to answer 3. Combination of both as in this answer.

Answer 2

The sequence that student pilots are taught is power, attitude, trim. Let's break down those three things:

1. Reduce engine power by closing the throttle. If you close the throttle partly, you get a powered descent; if you close it completely, you get a gliding descent. Airline operations typically prefer powered descents, mainly because turbine engines are very slow to add power, and you might need to stop descending at short notice (because the terrain is about to back into you). This lets your speed decrease to the desired descent speed (which is typically somewhat less than the cruise speed). While the speed is decreasing, you need to keep backward pressure on the stick to stop the nose dropping.

2. Once you've reached the correct speed, pitch the nose down to the descent attitude, to keep the speed constant. With the nose too low, the speed would increase, but the rate of descent would be too great, and would keep increasing. With the nose too high, the speed would decrease, which means the wings produce less lift, again causing a high rate of descent (and probably a stall). The combination of engine power and attitude together determine the airspeed and the rate of climb or descent.

3. Trim the aircraft. This means setting up the control surfaces so that the attitude is stable, and it's quite a big topic in itself, because different aircraft have different ways of trimming.

As for your supplementary point about autopilot and autothrust, the electronics don't actually need to know how much to change the power or attitude to achieve the desired effect. All they need to know is which direction to go (more or less power, nose up or nose down). At its core, the logic is very simple: airspeed too low -> nose down; airspeed too high -> nose up; rate of descent too small -> reduce power; rate of descent too great -> increase power. Because computers think in very small timescales, they can make that decision many times per second, so they never deviate far either side of the desired flight parameters.

Now in fact, it's a little more complicated than that, because the control loop needs to be tuned so it doesn't overreact to small changes, to cope with the delays caused by inertia and drag, and so it doesn't overshoot the target continually. There's a whole branch of engineering called control theory about this kind of system. But fundamentally, it knows how big a change to make by watching the effects of the changes it has made so far.

Answer 3

There are two different ways to climb or descend:

Constant rate climb or descent: using a constant power setting, the pilot applies pressure on the stick to set a desired attitude for the climb or descent. This is cross referenced against the vertical speed indicator (VSI) once the climb is established and adjustments in pitch attitude will be made to hold the aircraft at a desired rate of climb or descent. Trim will also be needed to alleviate elevator pressure as the speed of the aircraft will decrease in a climb and increase in a descent. Very rapid descent rates will also require a large reduction in power to avoid exceeding Vne or overspending the engine, if a fixed pitch propeller is used. To level off at a new desired altitude, elevator pressure must be applied to level off the nose and terminate the climb or descent. Again trim will have to be applied as the aircraft is returning to its cruise airspeed.

Due to the excessive control inputs and trimming, this type of climb is not preferred by pilots who hand fly the airplane. But it can be popular with aircrews using an autopilot when descending, as you can turn the potential energy you gained climbing to altitude into additional airspeed, allowing for a faster arrival to their destination; the autopilot can easily cope with the grunt work trend control and trimming.

Constant airspeed climb or descent: while in a cruise power setting, increase or reduce the throttle, to initiate a climb or descent from current altitude. Elevator input may be needed during the transition to help establish the correct attitude and avoid oscillations in airspeed. The aircraft will settle into the climb or descent at the cruise airspeed you have previously used. Throttle inputs can thence be used to control the attitude of and/or rate of the climb/descent. No trim will be needed to to alleviate control loads as the airspeed is constant throughout the climb or descent.

Answer 4

In one important sense, the pilot doesn't need to do anything to descend: gravity takes care of that all by itself.

A plane descends when its wings produce less lift than it weighs

In order to keep the plane aloft, the pilot needs to keep overcoming gravity - in physics terms, ensure that the aircraft produces more lift than the aircraft weighs.

A wing produces lift when it moves through the air

An airliner produces lift when air flows around its wings, which happens when the aircraft moves forward through the air. By changing the shape of the wings (for example, by extending flaps) or changing the attitude (angle) of the aircraft in the air, the pilot can adjust how much lift the wings produce.

More power = more speed = more lift

One very simple way to descend therefore is to reduce power. This will slow the aircraft down, so the wings produce less lift, and the plane slowly falls.

Why pointing the aircraft down (or up) isn't the way altitude is controlled

Another would be to point it firmly at the ground, but this would not be pleasant or safe, so airline pilots rarely do that.

Although it would seem intuitive that raising the nose will cause the aircraft to climb, and lowering it will cause it to descend, that doesn't necessarily follow.

Other things being equal (i.e. if the thrust produced by the engines does not change) pointing the nose down somewhat will cause the plane to descend, but it will also cause it to go faster.

This is for two reasons:

• the potential energy (height) of the aircraft has to go somewhere, so it becomes kinetic energy (speed)
• less of the engines' thrust is converted into lift, and more of it contributes to the plane's speed

This extra speed, other things being equal, will in turn cause the wings to generate more lift! Depending on how the controls are set, lowering the nose won't actually cause the plane to descend for very long.

The reverse applies when lifting the nose. Eventually of course if the nose is lifted far enough, although the aircraft will climb rapidly at first, soon it will lose so much speed that the wings fail to produce enough lift to keep it in the air, and it will plummet in what's known as a stall.

So the key overall is engine power, thrust. Smaller and more momentary adjustments will be achieved by moving the aircraft's control surfaces and controlling its angle of attack, but on the whole, a descent involves keeping the plane comfortably level and allowing it to fall very slowly through the sky - as far as possible, so that the passengers don't even feel it.