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Spoiler is open/activated The spoiler is back to normal/closed

A few days ago I flew on a Boeing 737-900ER. Luckily I was sitting just beside the left wing, seat 26B. I observed that the spoilers were activated (opened) just after the pilot spoke on the Passenger Address System announcing that the airplane was preparing for landing (if I was not mistaken) and began to descend. The spoilers then opened for around 3-5 minutes (picture 1) and then closed again (picture 2) until it touched down. In flight they were not as fully activated/opened as when the airplane landed.

My question:

  • What is their purpose? Are they brakes?

  • If they are brakes, does an airplane need brakes in the air?

  • If they are brakes, is not more efficient to reduce the engine's power rather than to keep it constant power but brake to slow down?

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    $\begingroup$ WRT your third point, sailplanes are commonly equipped with speed brakes and/or spoilers, and they don't even have engines. It's a question of whether aerodynamic drag will slow the plane quickly enough with no power applied. With the sailplanes I've flown, without using speed brakes you'd float all the way down the runway in ground effect. $\endgroup$ – jamesqf Nov 4 at 17:48
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    $\begingroup$ If you listen, you will notice that at that time, engine is already at idle. $\endgroup$ – user3528438 Nov 4 at 17:52
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    $\begingroup$ @user3528438: But then the question arises: "is it not more efficient to reduce the engine's power earlier?" $\endgroup$ – phresnel Nov 5 at 13:10
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    $\begingroup$ @phresnel That is explained already in the currently accepted answer. E.g. Air traffic control gives you a shortcut., which you didn't know at the time you would have had to idle the engines $\endgroup$ – PerlDuck Nov 5 at 13:14
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    $\begingroup$ the most fuel efficient way to drive your car is to never apply the brakes and cut the engine far enough that your car will slow to a stop as it enters your parking space. What's your point? $\endgroup$ – BooleanCheese Nov 5 at 15:44
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What you saw is called a speed brake, which is one of the functions of the spoilers. From the Boeing 737 NG FCOMv2 (9.20.5 Flight Controls - System Description):

Flight Spoilers

Four flight spoilers are located on the upper surface of each wing. Each hydraulic system, A and B, is dedicated to a different set of spoiler pairs to provide isolation and maintain symmetric operation in the event of hydraulic system failure. Hydraulic pressure shutoff valves are controlled by the two flight SPOILER switches.

Flight spoiler panels are used as speed brakes to increase drag and reduce lift, both in flight and on the ground. The flight spoilers also supplement roll control in response to control wheel commands. A spoiler mixer, connected to the aileron cable-drive, controls the hydraulic power control units on each spoiler panel to provide spoiler movement proportional to aileron movement.

The flight spoilers rise on the wing with up aileron and remain faired on the wing with down aileron. When the control wheel is displaced more than approximately 10°, spoiler deflection is initiated.

Speed Brakes

The speed brakes consist of flight spoilers and ground spoilers. Hydraulic system A powers all four ground spoilers, two on the upper surface of each wing. The SPEED BRAKE lever controls the spoilers. When the SPEED BRAKE lever is actuated all the spoilers extend when the airplane is on the ground and only the flight spoilers extend when the airplane is in the air.

The SPEEDBRAKES EXTENDED light provides an indication of spoiler operation in-flight and on the ground. In-flight, the light illuminates to warn the crew that the speed brakes are extended while in the landing configuration or below 800 feet AGL. On the ground, the light illuminates when hydraulic pressure is sensed in the ground spoiler shutoff valve with the speed brake lever in the DOWN position.

You noticed that the spoilers did not extend as much in the air as on the ground. This is intentional to reduce buffeting:

In-Flight Operation

Operating the SPEED BRAKE lever in flight causes all flight spoiler panels to rise symmetrically to act as speed brakes. Caution should be exercised when deploying flight spoilers during a turn, as they greatly increase roll rate. When the speed brakes are in an intermediate position roll rates increase significantly. Moving the SPEED BRAKE lever beyond the FLIGHT DETENT causes buffeting and is prohibited in flight.

The speed brake lever must not be moved above the FLIGHT DETENT while in the air. You can see it in the following image from the FCOM:

737 Speed Brake Lever


To answer your other questions:

If it is brake, do an airplane need brake in the air?

Yes, sometimes. Ideally a flight profile is calculated that does not require the use of brakes in the air. That means the Flight Management Computer (FMC) calculates a so called Top of Descent point at which you can start an idle descent. So the engines are retarded to idle thrust and the aircraft maintains the target speed by descending. This will get you on final approach at the desired altitude. Two things can happen now:

  • Air traffic control does not allow you to descend when you want to. This usually happens when there is conflicting traffic and you have to stay at a higher altitude than the idle descent profile for a short while. Afterwards you need to descend faster, which may require the use of speed brakes. You can also try to just increase the target descent speed (since higher speed means more drag and gets you down faster), but if that is not enough speed brakes are needed.
  • Air traffic control gives you a shortcut. This happens quite a lot. Some Standard Terminal Arrival Routes (STARs) involve some complex maneuvers and turns, but if traffic allows you can get a shortcut. Now your distance to the airport has just been reduced, requiring a steeper descent. Again, use of speed brakes helps here, especially when you are close to the airport already because below 10,000 ft you have to maintain 250 kt or lower, so speeding up is not an option.

If it is brake, is not more efficient to reduce the engine's power rather than to keep it constant power but brake to make it slower?

The engine will already be reduced to idle on a normal descent profile. Therefore using less power is not an option any more. Use of reverse in flight is usually prohibited for most aircraft, e.g. 737 FCOM:

Intentional selection of reverse thrust in flight is prohibited.

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    $\begingroup$ What does it mean, "prohibited"? Does that mean it's possible? $\endgroup$ – Harper - Reinstate Monica Nov 4 at 15:53
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    $\begingroup$ @Harper It should not be possible. From the FCOM: "The thrust reverser can be deployed when either radio altimeter senses less than 10 feet altitude, or when the air/ground safety sensor is in the ground mode." $\endgroup$ – Bianfable Nov 4 at 15:57
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    $\begingroup$ I know nothing of aviation, but I think in principle he is right that using idle earlier could save more power. In the same sense, a train could avoid breaking if it went on idle with so much advance as to reach speed 0 exactly at the station. The problem is, most of the time you want to get from A to B in the shortest possible time. In this case, the optimal controls have a phase of max acceleration, cruising, coasting, maximum breaking. This can be proven using Pontryagin's maximum principle. $\endgroup$ – Alberto Santini Nov 4 at 22:57
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    $\begingroup$ @AlbertoSantini Absolutely correct: whenever you use speed brakes, you basically waste fuel because you could have started an idle descent earlier (that's why the speed brake lever is sometimes called the lever of shame). The problem is that you didn't know that earlier in the cases I mentioned. $\endgroup$ – Bianfable Nov 4 at 23:02
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    $\begingroup$ @Haukinger It is about both (actually, most aviation is about money from tickets) and the company economics determines the cost index preferred, not an arbitrary choice of the pilot. Time in the air costs money, fuel costs money, it is a difficult process to find the optimum, but it is very important. Private flights may have different priorities but with airliners they are the minority. Normal airlines do not fly or cruise as fast as possible. $\endgroup$ – Vladimir F Nov 5 at 7:43
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Though your photos refer to a specific commercial airliner, your question is general, so I will describe a more specialized use case for speed brakes. A glider (also known as a sailplane) has no engine, and it is designed to have a very high glide ratio so that it can travel further between sources of lift. If a glider were to come in for a landing without using speed brakes, it would descend at a very shallow angle, starting far from the airport. A long, shallow approach is dangerous because the aircraft will be too close to the ground far away from the airport; also, lacking an engine, the glider has no ability to increase its speed and altitude simultaneously to avoid an obstacle. Further, when starting at a long distance from the airport and a low altitude, it would be virtually impossible for a pilot to judge the correct glide path to land at the desired point. Gliders deploy speed brakes or spoilers to enable a steeper glide path during the final approach. If an emergency occurs, such as a last-minute obstruction on the runway, the glider pilot may retract the brakes to gain speed or altitude, to gain additional time on approach.

Note that a pilot can't simply point the nose of the aircraft up or down to change the landing point, because that would change the airspeed of the aircraft. The aircraft's airspeed needs to be within a specific range in order to touch down safely.

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  • $\begingroup$ And with many GA planes, you get much the same effect with a slip. $\endgroup$ – jamesqf Nov 5 at 4:23
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As you can see on the photo, the flaps are slightly extended, allowing the aircraft to fly at a lower airspeed. This is common for aircraft to do during an approach, to keep a constant distant between incoming flights. An aircraft flying into an airport, can perform a continuous descent or a step down decent. Illustrated in the figure below:

A conceptual representation of step descent approach and continuous descent approach

From the figure it is seen that the for the step down approach, the aircraft needs to move relatively quickly from one flight level to the other. The pilot can do two things to achieve this, put the nose down, increasing airspeed and giving passengers a rollercoaster ride! Or, the other option would be to temporary reduce the lift of the wings without affecting the airspeed that much in order to keep a given separation between consecutive flights.

The spoilers on the wing you see deployed are also called lift-dumpers and are used to, while being extended, reduce the lift of the wing and thus have the aircraft "falling" out of the sky in a controlled fashion. They also reduce the airspeed of the aircraft due to increased drag, thus often as the spoilers are retracted again you notice the pilots increasing the throttle. In short, the lift dumper are used to move to a lower flight level in a controlled fashion without the need to push the nose down.

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  • $\begingroup$ In your diagram, the slopes of the descents in the step-down profile are less than or equal to that of the continuous descent. $\endgroup$ – sdenham Nov 6 at 18:37
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Yes, deploying the speedbrakes makes for a steeper descent - can be seen quite often.

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Yes, those are speed brakes, used to slow the aircraft down. In this case, they also double as spoilers to kill lift once the aircraft has touched down.

A propeller aircraft can be slowed down while in flight by throttling down and in some cases flattening the pitch on the prop blades... this created a lot of air resistance and slows the aircraft.

When turbine engines, both turbojet and turbofan, were developed for aircraft use, one drawback was that they offered no in flight braking ability like prop aircraft have. No big propeller to create air resistance when the prop is slowed down and/or the pitch is flattened.

That is why the aircraft you were on didn't reduce speed by cutting power... it would have almost no effect on the aircraft's immediate speed. Also, when approaching a field for landing, the aircraft is descending and tends to pick up speed, so the speed brakes are used to keep the aircraft's speed within what is desirable for a landing approach.

Imagine trying to turn off of a downhill highway into a parking lot with no brakes on your car, and you can see how this lack of immediate braking ability could be a problem, especially when approaching an airfield.

So, speed brakes, flaps that would open up to create air resistance, were developed for jet aircraft, so that they could slow down quickly when needed.

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The spoilers decrease the wing's lift coefficient. Yet in the context of flight, as opposed to the context of after touchdown, it's not really correct to imagine that they are forcing the wing to generate less lift. In a steady-state descent, lift is only slightly less than weight, unless we are talking about a really extreme dive angle. So what the "lift dumpers" are really doing is allowing the plane to fly at a higher angle-of-attack and thus make more drag for any given airspeed than would normally be the case. Also, the devices generate drag for other obvious reasons, independent of the increased angle-of-attack of the wing. The two effects in combination are what allow the plane to descend at a steeper glide angle than would otherwise be the case at the same airspeed and power setting. Or conversely, we could say the two effects in combination are what allow the airspeed to be lower than it otherwise would be at the same glide angle and power setting.

It's a bit counterintuitive that deploying flaps and deploying a "spoiler" or "lift dumper" or "airbrake" BOTH can have the effect of making the glide path steeper than it otherwise would be at the same airspeed and power setting, i.e. of making the airspeed lower than it otherwise would be at the same glide angle and power setting. Food for lots of thought on your next long airline flight.

To really start to come to an understanding of this apparent paradox, you'll need to look at how each device modifies the curve of L/D versus airspeed, which is also essentially a curve of sink rate versus airspeed. But that's probably more than you wanted to know!

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