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FOD or Foreign Object Damage is an aviation term used to describe damage done to an aircraft due to collision with small foreign objects. Small, in this case, is anything smaller than another aircraft.
FOD causes expensive, significant damages every year to many aircraft, and regularly causes death and injury.
Examples of FOD include:
- Bird strikes: when a plane flies into a bird, the impact can cause severe damage to the fuselage or engine.
- Rock or other metal parts: Usually occurs when the aircraft is taking off or landing. The intake suction from a jet engine is often powerful enough to suck up loose material lying on the runway.
- Hail: can break windshields and damage or stop engines.
Things that are not considered FOD are:
It should be noted that large and small aircraft alike occasionally lose small metal parts during takeoff and landing. These parts remain on the runway and cause damage to tires of other planes using that runway, hit the fuselage or windshield ("canopy"), or can be sucked up into an engine. Thus, airport ground crews regularly clean up runways to remove such parts.
Jet engine design and FOD
Modern jet engines suffer major damage due to even small birds being sucked into the engine. The FAA (Federal Aviation Administration) requires that all engine types pass a test which includes throwing a fresh chicken (dead, but not frozen) into a running jet engine. 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. It is reputed that the chicken used in the tests is known in aviation circles to be a specific size and is thus known as an "aviation standard bird".
Engine design that avoids FOD
The US A6-E attack aircraft had a unique design to prevent FOD from damaging the engine. The design consisted of a 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 engines effective power, and thus the design was not repeated. However, many consider it an innovative solution to a tough engineering problem.
FOD example case: the B-1A
In the late 1970s, the B-1A bomber began production and the military began flight testing to determine its capabilities and limitations. Very shortly after the aircraft was accepted by the military, there was at least one very high profile crash, shortly after takeoff, from an airfield in North Dakota. The cause of the crash was determined to be FOD. Specifically, the aircraft flew into a flock of geese moments after it lifted from the ground. Many geese struck the leading edge of the wings. Unfortunately, the primary, secondary, and backup hydraulic system lines were all positioned within inches of each other in that section of the wing. The bird strikes dented the aircraft skin, which in turn dented the hydraulic lines and caused loss of pressure in all three systems. The aircraft became uncontrollable and crashed.
It was determined that the aircraft design was the major factor in this crash, not the birds. Any military jet should be expected to operate in unimproved conditions and probably incur FOD during normal operation. Thus, the design decisions were faulty in that (a) all three hydraulic systems were too close together, (b) they were very close to the outer skin of the aircraft, and (c) they had no extra shielding on the skin nearest this juncture point.
Thus, the B-1A was a short lived model. The B-1B incorporated many changes including rerouting, separating, and shielding the hydraulics.
Wildlife and wetlands near airports
Significant problems occur with airports where the grounds were or have become nesting areas for birds. While fences can prevent a moose or deer from wandering onto a runway, birds are more difficult to deal with. Airport managers use any means available to reduce bird populations, because while birds might be nice to look at and an important part of our ecosystem, the loss of life from one bird-related FOD-caused aircraft crash could be significant, is foreseeable, and thus should be prevented.
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