Foreign object damage
From Free net encyclopedia
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.
"Internal FOD" is used to refer to damage or hazards caused by foreign objects inside the aircraft. For example, "Cockpit FOD" might be used to describe a situation where a clipboard, water bottle, or other 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 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.
FOD causes expensive, significant damages every year to many aircraft, and regularly causes death and injury.
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Examples
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:
- ice on the wings
- dust clogging the air intakes (as in sandstorms in desert operating conditions)
- anti-aircraft artillery (AAA) fire
All aircraft occasionally lose small metal parts, during takeoff and landing. These parts remain on the runway and cause damage to tires of other planes, hit the fuselage or windshield ("canopy"), or can be sucked up into an engine. Although airport ground crews regularly clean up runways, the Paris crash of a Concorde airliner demonstrated that accidents can still occur: in that case, the crash was caused by debris left by a flight that had departed only four minutes earlier.
On aircraft carriers, "FOD walkdowns" are conducted before flight operations begin. A line of crewmen walk shoulder to shoulder along the full length of the flight deck, searching for and removing any foreign objects. The objects removed are often also referred to as "FOD" although they haven't caused any damage. In this context a more appropriate translation of the acronym would be "Foreign Objects and Debris".
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 U.S. 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 engine's effective power, and thus the design was not repeated. However, many consider it an innovative solution to a challenging 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.
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 (including trained falcons) 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.de:Foreign Object Damage