Foreign object damage
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For the song by Green Day, see F.O.D..
F.O.D. stands for Foreign Object Debris or Foreign Object Damage. According to the National Aerospace Standard 412, maintained by the National Association of FOD Prevention, Inc, Foreign Object Debris is a substance, debris or article alien to the vehicle or system which would potentially cause damage. Foreign Object Damage is any damage attributed to a foreign object that can be expressed in physical or economic terms that may or may not degrade the product's required safety and/or performance characteristics. Typically, FOD is an aviation term used to describe debris on or around an aircraft or damage done to an aircraft.[1]
"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 costs the aerospace industry $4 billion USD per year and causes expensive, significant damage every year to aircraft and parts and may cause death and injury to workers, pilots and passengers.
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[edit] 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, or even directly injure those aboard the aircraft if the bird strikes the aircraft in such a way as to enter the cockpit and/or cabin.
- 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, and the winds created by a helicopter or prop-driven aircraft's rotors can send such objects airborne, creating hazards to nearby personnel.
- Hail: can break windshields and damage or stop engines.
- ice on the wings
- dust or ash clogging the air intakes (as in sandstorms in desert operating conditions or ash clouds in volcanic eruptions, respectively)
- tools, bolts, metal shavings, etc. mistakenly left inside aircraft during the manufacturing process.
All aircraft occasionally lose small metal 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 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".
[edit] 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".
[edit] Engine design that avoids FOD
Some Military aircraft have 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.
The Russian MiG-29 fighter has a special engine design to prevent injestion of FOD during take-off from rough airfields. The front air intakes could be closed and special inlets on the top of the plane temporarily opened. This would allow enough airflow to the engine for take-off but reduced the chances of the engine sucking up objects from the ground.
[edit] FOD damage examples
[edit] Concorde
The crash of a Concorde, Air France Flight 4590, at Charles de Gaulle International Airport near Paris was caused by FOD, in this case a piece of titanium debris on the runway which had been part of a thrust reverser and which fell from a Continental Airlines DC-10 that had taken off about four minutes earlier. All 100 passengers and nine crew on board the flight, as well as four people on the ground, were killed.
[edit] 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.
[edit] 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. Often airports employ a type of bird scarer that operates on propane to cause a loud enough noise to scare away any birds that might be in the vicinity. Airport managers use any means available (including trained falcons) to reduce bird populations. While this may seem cruel, the loss of human life from just one bird-related aircraft crash could be significant. As such, it is necessary to take all possible precautions to prevent such an accident from occurring.
