Turbine Pilot

Spoilers and speed brakes, the spoilsport of lift and speed.

Considering the amount of effort required to design an efficient airplane, the notion of adding spoilers to destroy lift would seem be an anathema. But it is the efficiency of some aircraft that deems spoilers necessary.

Jet-powered airplanes are clean (aerodynamically speaking) and do not have the drag of windmilling propellers to help slow them down. They have glide performance that is double that of piston-powered airplanes. Jet pilots often cannot comply with a request from a controller to go down and slow down at the same time.

Spoilers, however, change the rules of the game. Deploying them allows the aircraft to descend at a great rate without necessarily increasing airspeed. (Spoilers on piston-powered airplanes allow a pilot to descend rapidly without having to reduce power so much as to risk shock cooling the engine.)

Although there are various types of spoilers, the most common consists of one or more rectangular plates that lie flush with the upper surface of each wing. They are installed approximately parallel to the lateral axis of the airplane (depending on wing stucture and geometry) and are hinged along their leading edges. When deployed, spoilers deflect up and against the relative wind, which interferes with the flow of air about the wing. This both spoils lift and adds drag. Spoilers usually are installed forward of the flaps and not ahead of the ailerons because this would interfere with roll control.

It's easy to confuse spoilers with speed brakes. Perhaps this is because both devices look the same when installed on the upper surfaces of the wings.

The primary purpose of spoilers is to spoil lift even though some drag is created in the process. The purpose of speed brakes, however, is to produce drag even though some lift might be sacrificed in the process. The difference between them is their location along the chord of the wing. If the devices are relatively far forward on the wing, they are most effective at dumping lift and are called spoilers. When relatively far aft, they are most effective at creating drag and are called speed brakes. It all depends on the designer's intent.

A simple test a pilot can perform to determine if an aircraft is equipped with spoilers or speed brakes is to note the effect of deploying the devices while holding a constant and approximately level attitude. If a significant sink rate develops and airspeed decay is minimal, they probably are spoilers. But if the results are initially the other way around, they most likely are speed brakes.

Despite what they are called, however, spoilers and speed brakes are used in the same manner and for the same purpose. When either are deployed, the descent profile can be steepened without the usual increase in airspeed. Or they can be used to reduce airspeed by keeping sink rate in check.

Raising spoilers against the force of the relative wind obviously requires muscle, which is why hydraulic power most often is used. Sailplane pilots obviously must deploy their spoilers manually. To assist these pilots, their aircraft often are equipped with balanced spoilers. These consist of conventional spoilers on the upper wing panels. They operate in conjunction with speed brakes under the wings. These "air brakes," however, are hinged at their trailing edges and deploy backwards as shown in the figure.

When the sailplane pilot pulls the spoiler handle to begin a descent, the relative wind strikes against the rising spoilers (on top of the wing) as if attempting to push them back into their stowed position. At the same time, however, the wind pushes against the inner surface of the opening speed brakes (on the bottom of the wing), which assists in opening them farther. Because the speed brakes are linked to the spoilers, the power of the relative wind is used to help raise the spoilers, which lightens operating forces.

Pure speed brakes are those installed on the fuselage of an airplane such as the dive brakes on military jets. Included, of course, is the landing gear, which often makes a splendid speed brake.

Many jet pilots do not like to use spoilers during descent. One reason for this is that spoilers often create a rumbling buffet that can be disconcerting to passengers. This is most noticeable in the rear of the cabin. Another reason for not wanting to "pop the boards" is that this might be interpreted to mean that a pilot did not plan his descent properly and that spoilers must be used to correct for not getting down soon enough.

There are, of course, several reasons for using spoilers, not the least of which is unexpected descent clearances from ATC. Another is the need to descend rapidly through a band of reportedly harsh turbulence without exceeding the turbulence-penetration speed.

Because spoilers destroy some lift in their immediate vicinity, wing loading elsewhere on the wing necessarily increases. This has the effect of reducing gust-induced G loads, which softens the ride somewhat.

Spoilers also can be modulated to prevent gaining altitude in the powerful updrafts of strong mountain waves instead of shoving the nose down and risking excessive airspeed.

One of the most important roles for spoilers is to enhance a rapid descent from high altitude necessitated by a loss of cabin pressure.

Training for such a maneuver often involves teaching pilots to react rapidly to the simulated emergency. Almost without thinking, we are expected to quickly don an oxygen mask, raise the spoilers, lower the nose, and maintain an airspeed somewhat shy of the barber pole (redline) all the way down.

But there has been a change of philosophy. Pilots now are taught to first evaluate the emergency instead of simply reacting to the decompression with a steep dive. If the pressure loss is caused by structural failure, descending at high airspeed could worsen the damage.

Because such a rapid descent can result in pegging the vertical speed indicator and descending at many thousands of feet per minute, it is not difficult to overshoot the target altitude. To prevent this, plan to raise the nose and reduce the sink rate when no less than 1,000 to 2,000 feet above the target. Spoilers should be fully retracted by the time there is only 500 feet to go.

Although spoiler deployment is allowed in some aircraft with the flaps extended, this procedure ordinarily should be avoided because of the hazards associated with high sink rates near the ground while in a high- drag (and probably low-power) configuration.

On other aircraft, using spoilers and flaps together is prohibited. One reason for this is that the combination can result in significant buffeting, which eventually can lead to structural fatigue. This is particularly true of the tail, which usually is most susceptible to buffeting.

On the Boeing 727, simultaneous use of flaps and spoilers is banned because this would create such large openings in the wing that continued and controlled flight would be difficult at best.

Another problem with using spoilers during an approach is that — on some aircraft — there is so little buffeting that a pilot could forget that the boards had been deployed. This is something that I once observed from a cockpit jump seat and almost did not survive.

Deployed spoilers have curiously little effect on stall speed and seldom affect stall quality. They do, of course, make it more difficult to recover from a stall with a minimum loss of altitude.

Spoilers also are invaluable when deployed immediately after touchdown. They obviously add substantial drag to assist in slowing the airplane. They also kill a great deal of lift (as much as 80 percent in some cases). This immediately adds more of the airplane weight on the wheels, which substantially improves braking performance.

While recently being "retreaded" at a recurrent-training class, my classmates and I were treated to a movie that showed the effect of spoiler deployment on a Boeing 747. The camera was mounted beneath a wing so as to frame one of the landing-gear struts. As the aircraft touched down, the oleo strut obviously compressed. But when the spoilers were deployed, strut compression doubled. It was very impressive to see and made it easier to understand why spoilers should be deployed as soon after touchdown as possible. This also is why some aircraft are equipped with automatic spoilers that deploy as soon as the wheels are on the ground. (Although spoilers are important because they contribute to the reduction of landing distance, they are most valuable during an aborted takeoff when there often is less available distance within which to stop.

When landing some airplanes with an aft center of gravity, deploying spoilers (and possibly applying reverse thrust) can cause the nose to pitch up enough to cause a tail strike. This can be avoided by lowering the nose slightly and immediately after touchdown. Do not hold the nose high while raising the spoilers.

Although spoilers are used primarily to vary the descent profile, they also are used on some aircraft to control roll because some ailerons lose effectiveness during high-speed flight. A design advantage of using spoilers to supplement roll control is that this allows the use of smaller ailerons, which makes room for larger flaps. In the case of systems that use only spoilers for roll control — such as on the Mitsubishi MU-2 — the trailing edge of the wing can be devoted entirely to full-span flaps. Roll-control spoilers operate differentially (one wing at a time). Entering a right turn requires use of the spoiler(s) on the right wing, and vice versa.

Another advantage of using spoilers for roll control is that they typically create less adverse yaw than ailerons do.

There is another use of spoilers that cannot be found in operating manuals and is seldom discussed by those who use this "special" procedure. (Please regard the following as confidential.)

It is common practice for captains to share landings with their copilots. This can lead to an element of competition between the two pilots, especially if the copilot is at least as skillful as the captain, which occasionally is the case. The idea, of course, is to see who can make the smoothest landings.

Sadly, some captains have egos the size of watermelons and cannot stand the thought of losing a landing contest to a "mere" copilot. And so, if it seems to the captain during the landing flare that his first officer is about to grease one on, he sneakily reaches for the spoiler handle and moves it slightly, just enough to force the airplane onto the runway with a thump. The captain wins again.