Foreign object damage

In aviation, foreign object debris is any substance, debris, or article alien to a vehicle or system which could potentially cause damage. The acronym FOD is now used to describe both the foreign objects themselves, and any foreign object damage attributed to them.

Internal FOD is damage or hazards caused by foreign objects inside the aircraft. For example, cockpit FOD is a situation where an item gets loose in the cockpit and jams or restricts the operation of the controls. Tool FOD is a serious hazard caused by tools left inside the aircraft after manufacturing or servicing. Tools or other items can get tangled in control cables, jam moving parts, short out electrical connections, or otherwise interfere with safe flight. Aircraft maintenance teams usually have strict tool control procedures including toolbox inventories to make sure all tools have been removed from an aircraft before it is released for flight. Tools used during manufacturing are tagged with a serial number so if they are found they can be traced.

Examples of FOD include:

All aircraft may occasionally lose small parts during takeoff and landing. These parts remain on the runway and can cause damage to tires of other aircraft, hit the fuselage or windshield/canopy, or get sucked up into an engine. Although airport ground crews regularly clean up runways, the crash of Air France Flight 4590 demonstrated that accidents can still occur: in that case, the crash was said to have been caused by debris left by a flight that had departed only four minutes earlier.

On aircraft carriers, as well as military and some civilian airfields, sweeps are conducted before flight operations begin. A line of crewmen walk shoulder to shoulder along the flight operations surfaces, searching for and removing any foreign objects.

Modern jet engines can suffer major damage from even small objects being sucked into the engine. The FAA (Federal Aviation Administration) requires that all engine types pass a test which includes firing a fresh chicken (dead, but not frozen) into a running jet engine from a small cannon. The engine does not have to remain functional after the test, but it must not cause significant damage to the rest of the aircraft. Thus, if the bird strike causes it to "throw a blade" (break apart in a way where parts fly off at high speed), doing so must not cause loss of the aircraft.

Some military aircrafthad a unique design to prevent FOD from damaging the engine. The design included an S-shaped bend in the airflow, so that air entered the inlet, was bent back towards the front of the plane, and bent back again towards the back before entering the engine. At the back of the first bend a strong spring held a door shut. Any foreign object flying in the intake flew in, hit the door, opened it, flew through, and then exited the aircraft. Thus, only small objects swept up by the air could enter the engine. This design did indeed prevent FOD problems, but the constriction and drag induced by the bending of the airflow reduced the engine's effective power, and thus the design was not repeated.

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