Spoilers and Speed Brakes

A spoiler is a hinged panel on the upper surface of the wing, ahead of the flaps, that rises into the airflow to spoil lift and add drag. Jet transports carry a row of them on each wing and use them in three ways: asymmetrically, to help the ailerons roll the aeroplane; symmetrically in flight, as speed brakes; and all together after touchdown, to dump lift. The aerodynamics of roll spoilers is covered in roll control, adverse yaw and spoilers; this article covers the system that commands them.

Getting the spoilers right matters at both ends of a flight. In the descent they let the crew lose height or speed faster than idle thrust alone allows. On landing, ground spoilers that fail to deploy leave much of the aeroplane's weight on the wing instead of the wheels, and the landing distance grows, so crews check and call their deployment on every landing.

Flight and ground spoilers

Flight spoilers can be used in the air. They assist the ailerons in roll and, raised together, act as speed brakes. Ground spoilers, or lift dumpers, are panels, often the inboard ones, that deploy only on the ground. After touchdown all the panels rise together. Air/ground logic from sensors on the landing gear, traditionally a squat switch, keeps the ground spoilers from deploying in flight.

Spoilers are split between hydraulic systems so that a failure stays symmetrical. On the 737 each of systems A and B powers a different set of flight spoiler pairs, and system A powers all four ground spoilers. On the A320 each spoiler's servojack takes power from the green, yellow or blue system and is controlled by one of the three spoilers and elevator computers (SECs). If a spoiler fails, the symmetrical one on the other wing is inhibited.

View full schematic

Speed brake lever and use

A speed brake increases drag to steepen the descent or reduce speed. At a given thrust, more drag means a steeper descent gradient and a higher rate of descent, so the speed brakes help meet a crossing restriction, recover a high approach or slow to the turbulence penetration speed. In an emergency descent they go to the maximum in-flight position with the thrust at idle. The ideal speed brake adds drag with no loss of lift and no change of pitching moment. A fuselage-mounted air brake comes close; wing spoilers always reduce lift, a compromise accepted because the panels are already there for roll control.

The speed brake lever sits on the pedestal. On the Boeing 737 it has four positions:

• DOWN, in a detent: all flight and ground spoilers faired.
• ARMED: the automatic speed brake system is armed for landing.
• FLIGHT DETENT: all flight spoilers at their maximum in-flight deflection. The lever must not be moved beyond it in flight, because that causes buffeting and is prohibited.
• UP: all flight and ground spoilers fully extended, for use on the ground.

Boeing's operating limitations say not to deploy the speed brakes in flight below 1,000 ft radio altitude, and an amber SPEEDBRAKES EXTENDED light comes on in flight if the lever is beyond ARMED with the flaps beyond 10 or below 800 ft radio altitude.

On the A320 the lever has a hardpoint at the half position. With the autopilot engaged the maximum speed brake deflection is reached at half lever, and moving it further gives nothing more. Speed brake extension is inhibited with the flaps at FULL, with angle of attack protection or alpha floor active, with a thrust lever above MCT, with an elevator fault, or if SEC 1 and SEC 3 both have faults. If an inhibition occurs with the speed brakes out, they retract and stay retracted until the condition clears and the crew resets the lever. Extending them raises VLS, so the crew must have a speed margin first, especially before a turn, to avoid triggering alpha floor. On the E190 E1 the speed brakes retract automatically with flaps 2 or more, or with the thrust levers above 70° TLA in a go-around.

Roll spoilers and spoiler mixing

A spoiler mixer turns the roll input into commands for the spoiler actuators. It is driven from the aileron system and raises the spoilers on the wing whose aileron goes up, the descending wing, while those on the other wing stay down. The deflection is proportional to the roll demand, and a follow-up from each spoiler actuator closes its control valve when the panel reaches the commanded position.

On the 737 NG the first officer's control wheel is cabled to the spoiler PCUs through the spoiler mixer. The flight spoilers begin to rise once the wheel is turned more than about 10°. On the 737 MAX the spoilers are signalled electrically, which made room for manoeuvre load alleviation, a landing attitude modifier and emergency descent speedbrakes. On the E190 E1 a small wheel input moves only the ailerons, and the multifunction spoilers join in as the wheel angle grows. On the A320 the computers combine ailerons, spoilers 2 to 5 and rudder in normal law.

The two functions add. With the speed brakes out, a roll input raises the spoilers further on one wing and lowers them on the other from the selected setting. When the sum would exceed the travel available, roll wins: on the A320 the corresponding surface on the other wing is retracted until the difference between the two equals the roll order.

Outboard aileron lockout

On a long, flexible swept wing, an outboard aileron at high speed twists the wing so much that its effect falls and can reverse. Many large jets therefore carry inboard ailerons as well and use outboard aileron lockout: with the flaps retracted, the outboard ailerons are inhibited and roll control comes from the inboard ailerons and the spoilers; when the flaps are extended for low-speed flight, the outboard ailerons are brought back in. Spoilers suit this role because they spread their lift loss across the chord instead of concentrating it at the trailing edge, so they twist the wing far less, and they stay effective at transonic speeds. The 737 and the A320, with one aileron on each wing, rely on the spoilers to add roll authority instead (see flutter and aeroelasticity).

Automatic ground spoilers

After touchdown the wing is still producing much of the aeroplane's lift. Ground spoilers destroy it, transferring the weight to the main wheels so that the brakes, whose force depends on the load on the tyres, become fully effective, and they add aerodynamic drag. Without them the landing roll is longer, and on the A320 the autobrake does not activate unless they extend.

The automatic ground speed brake function is armed by the crew before landing and deploys when its conditions are met, typically main wheel spin-up, air/ground sensing and thrust levers at idle, or reverse selection. If the aeroplane goes around or bounces back into the air, the spoilers retract when the wheels spin down or the thrust levers are advanced.

On the 737, compression of any landing gear strut enables the flight spoilers, while compression of the right main strut opens the ground spoiler interlock valve mechanically. An amber SPEED BRAKE DO NOT ARM light shows an abnormal condition in the automatic system. At touchdown the pilot monitoring checks the lever and calls "SPEED BRAKES UP", or "SPEED BRAKES NOT UP" if it has not moved. The A320 adds phased lift dumping, partial deployment once one main gear is on the ground, and if no spoilers extend the crew sets idle and full reverse and brakes manually. On the E190 E1 the ground spoilers deploy with weight on wheels, thrust levers below 26° TLA and wheel speed above 45 kt or airspeed above 60 kt (see anti-skid and autobrake).

Spoiler blowback

Spoiler blowback, also called blowdown, is the retraction of a raised spoiler by the airflow. At high speed the air load on a fully deflected panel can exceed the force its actuator can produce, so the panel is pushed back towards the wing and gives less than the selected deflection. Wing speed brakes are generally cleared up to VMO/MMO, with blowback limiting their effect at the top of the range.

Loss of power has a similar effect. On the A320 a spoiler that loses hydraulic pressure keeps the deflection it had, or a smaller one if the air load pushes it down, whereas a spoiler that loses its electrical control or develops a fault is retracted, and a SEC failure retracts the spoilers it controls. A raised spoiler cannot flutter, one of the reasons spoilers are trusted with roll control at high speed.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.