Spoilers and Speedbrakes are secondary flight control surfaces that can be deployed manually by the pilot or, under certain circumstances, that extend automatically. Speedbrakes are purely drag devices while spoilers simultaneously increase drag and reduce lift.
Aircraft are designed to be as aerodynamically "clean" as possible and drag is minimized as much as practical to improve performance and decrease fuel consumption. A side effect of this aerodynamic success is that, even at idle thrust, an aircraft does not tend to slow down quickly, especially when descending. High performance military aircraft have long used speedbrakes, interchangeably referred to as air brakes or dive brakes, to control speed during rapid descent or to quickly reduce speed during level flight. Early commercial aircraft types utilised extension of the undercarriage to provide additional drag when required. However, due to the altitude limitations of the landing gear and the the normally transient requirement for the addtional drag, this was not considered an optimum solution. In time, speedbrakes and spoilers were incorporated into commercial aircraft design.
Speedbrakes are high drag devices that are fitted to almost all high performance military aircraft as well as to some commercial aircraft types. In most cases, speedbrakes are fuselage mounted panels which, when selected by the pilot, extend into the airstream to produce drag. Dependant upon the aircraft type, the speed brake(s) may consist of a single panel or symetrically mounted pairs of panels. On the BA146, the speedbrakes are mounted on the tailcone. Speedbrakes may be used during the final approach to touchdown as well as after landing.
Spoilers are panels mounted on the upper surface of the wing that, when extended, both increase drag and decrease lift by disrupting the airflow over the wing. Dependent upon the aircraft type, spoilers can serve as many as three distinct primary functions:
- Ground spoilers
- Roll spoilers
- (Flight) spoilers or Speedbrakes
Some aircraft such as the AIRBUS A-320 and the EMBRAER ERJ 190-100 have all three spoiler functions whereas the BA146 only incorporates the ground spoiler function. Certain aircraft designs also utilize the spoiler panels for secondary functions such as turbulence damping.
Virtually all spoiler equiped aircraft have a ground spoiler function. During the landing ground roll or during a rejected takeoff, all spoiler panels are extended to their maximum angle. The primary purpose of the ground spoilers is to maximise wheel brake efficiency by "spoiling" or dumping the lift generated by the wing and thus forcing the full weight of the aircraft onto the landing gear. The spoiler panels also help slow the aircraft by producing aerodynamic drag. Depending upon aircraft type, the ground spoiler extension may be fully automatic when the system is armed provided that other deployment criteria such as weight on wheels, airspeed or throttle lever positon are met. Other aircraft may require the pilot to manually select the ground spoilers after landing or in the event of a rejected takeoff.
On many spoiler equiped aircraft, some of the spoiler panels have a flight spoiler function which is often referred to as "speedbrakes". In this application, the wing panels are symetrically extended by pilot selection. The maximum deflection of the panels while airborne is normally limited to an angle which is less than the deflection acheived in ground spoiler mode. Various aircraft have built in protections that will automatically command speedbrake retraction below a certain airspeed, with flaps selected beyond a given position or with thrust levers set above a specific angle. Wing spoilers should not be deployed during the final phase of the approach to landing as the induced loss of lift will result in a higher than normal stall speed and could result in a hard landing.
On many spoiler equiped aircraft, one or more of the spoiler panels will deflect in harmony with the aileron on the associated wing to enhance roll authority and response. Roll commands normally take priority over a speedbrake command and spoiler panels will extend or retract accordingly.