Aviation Glossary

Spoilers

High-drag devices that can be raised into the air flowing over an airfoil, reducing lift and increasing drag. Spoilers are used for roll control on some aircraft. Deploying spoilers on both wings at the same time allows the aircraft to descend without gaining speed. Spoilers are also used to shorten the ground roll after landing.

source: FAA Airplane Flying Handbook (FAA-H-8083-3A)

Flight controls that are raised up from the upper surface of a wing to destroy, or spoil, lift. Flight spoilers are used in conjunction with the ailerons to decrease lift and increase drag on the descending wing. Ground spoilers are used to produce a great amount of drag to slow the airplane on its landing roll.

source: FAA Aviation Maintenance Technician Airframe Handbook (FAA-H-8083-31)

Devices on the tops of wings to disturb (spoil) part of the airflow over the wing. The resulting decrease in lift creates a higher sink rate and allows for a steeper approach.

source: FAA Glider Flying Handbook (FAA-H-8083-13A)

In aeronautics, a spoiler (sometimes called a lift dumper) is a device intended to reduce lift in an aircraft. Spoilers are plates on the top surface of a wing that can be extended upward into the airflow to spoil it. By so doing, the spoiler creates a controlled stall over the portion of the wing behind it, greatly reducing the lift of that wing section. Spoilers differ from air brakes in that air brakes are designed to increase drag without regard to affecting the lift, while spoilers reduce lift as well as increase drag.

Spoilers fall into two categories: those that are deployed at controlled angles during flight to increase descent rate or control roll, and those that are fully deployed immediately on landing to greatly reduce lift ("lift dumpers") and increase drag. In modern fly-by-wire aircraft, the same set of spoilers serve in both functions.

Spoilers are used by nearly every glider (sailplane) to control their rate of descent and thus achieve a controlled landing at a desired spot. An increased rate of descent can also be achieved by lowering the nose of an aircraft, but this would result in an excessive landing speed. Spoilers enable the approach to be made at a safe speed for landing.

Airliners are almost always fitted with spoilers. Spoilers are used to assist descent to lower altitudes without picking up speed. Their use is often limited, however, as the turbulent airflow that develops behind them causes noise and vibration, which may cause discomfort to passengers. Spoilers may also be differentially operated for roll control instead of ailerons; Martin Aircraft was the first company to develop such spoilers in 1948. On landing, however, the spoilers are nearly always used at full effect to help slow the aircraft. The increase in form drag created by the spoilers directly assists the braking effect. However, the real gain comes as the spoilers cause a dramatic loss of lift and hence the weight of the aircraft is transferred from the wings to the undercarriage, allowing the wheels to be mechanically braked with less tendency to skid. (Reverse thrust is also often used to help slow the aircraft on landing.)

In air-cooled piston engine aircraft, spoilers may be needed to avoid shock cooling the engines. In a descent without spoilers, air speed is increased and the engine will be at low power, producing less heat than normal. The engine may cool too rapidly, resulting in stuck valves, cracked cylinders or other problems. Spoilers alleviate the situation by allowing the aircraft to descend at a desired rate while letting the engine run at a power setting that keeps it from cooling too quickly. (This is particularly true for turbocharged piston engines, which generate higher temperatures than normally aspirated engines.)

source: Wikitionary / Wikipedia and Related Sources (Edited)