The term FOD can stand for either Foreign Object Debris, or Foreign Object Damage, depending on the context. In either cases, it references an object in a location which it does not belong, such as a maintenance tool left in the landing gear of an aircraft, or a loose object which flies around inside the cabin of a plane during flight. Many people use the term “FOD” to refer specifically to aircraft, although it can also be used in the context of various other machines and vehicles.
Foreign objects can cause a great deal of damage to aircraft, even when they are small. For example, a tool left in the undercarriage of an aircraft could prevent the landing gear from descending, which would be catastrophic when the plane came down to Earth. FOD can also cause a failure of cabin pressure, or the failure of a jet engine. Such objects could also potentially jam controls and various mechanical features on an aircraft, impeding its normal operations.
Because FOD can be very costly and potentially deadly, many airlines and manufacturers work to prevent or reduce FOD on their aircraft. For example, jet engines have intakes which are specially designed to deflect foreign objects, preventing them from being sucked into the engine, and maintenance crews follow specific protocols when they service planes, which include using a checklist to collect all tools at the end of a servicing, ensuring that no tools are left in or on the aircraft. Many airports also have rumble strips on their runways, which are designed to shake foreign objects loose before takeoff.
Inside a plane, the cabin crew reduces FOD by stowing all items very carefully. As people who have flown on aircraft know, passengers are typically instructed to stow all belongings during takeoff and landing to prevent FOD, and to control objects in their vicinity during flight; if an object inside a plane got enough momentum, it could damage the structure of the cabin, potentially causing a loss of pressure. Loose objects can also injure crew and passengers, in addition to causing FOD.
When a foreign object causes damage to the outside of something, it is known as external FOD. Examples of external FOD include bird strikes on aircraft and tool damage. Internal damage is known, imaginatively, as “internal FOD.” Internal FOD often happens in the cockpit of an aircraft, where a dislodged item could hit any number of vital controls, potentially causing navigational problems.