Foreign Object Damage

In aviation and aerospace, foreign object debris (FOD), is any particle or substance, alien to an aircraft or system, which could potentially cause damage.

External FOD hazards include bird strikes, hail, ice, sandstorms, ash-clouds or objects left on the runway. Internal FOD hazards include items left in the cockpit that interfere with flight safety by getting tangled in control cables, jam moving parts or short-out electrical connections.

The term FOD is used to describe both the foreign objects themselves, and any damage attributed to them.

Jet engine design and FOD

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.

Engine and airframe designs which avoid FOD

Some military aircraft had 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.

A similar approach is used on many turboshaft-powered helicopters, such as the Mi-24, which use a "vortex-type" or "centrifugal" intake, in which the air is forced to flow through a spiral path before entering the engine; the heavier dust and other debris are forced outwards, where it is separated from the airflow before it enters the engine inlet.

The Russian Mikoyan MiG-29 and Sukhoi Su-27 fighters have a special intake design to prevent ingestion of FOD during take-off from rough airfields. The main air intakes could be closed with mesh doors and special inlets on the top of the intakes 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.

Another interesting design to minimize the risk of FOD is the Antonov An-74 which has a very high placement of the engines.

Boeing offered a gravel runway kit for early 737s that allows the plane to be used from unimproved and gravel runways, in spite of having very low-slung engines. This kit included gravel deflectors on the landing gear; foldaway lights on the bottom of the plane; and screens that prevented gravel, which would enter the open wheelwells when the gear was extended, from hitting critical components. The kit also included vortex dissipators, devices which would reduce the airflow into the engine from the bottom so as to reduce the likelihood of ingesting gravel.

Airbus engineers are investigating a novel approach to reducing FOD. By developing, in conjunction with Israel Aerospace Industries, the Taxibot, a tractor controlled by the pilot, aircraft will not need to use jet engines while taxiing, and therefore they will not be vulnerable to FOD on aprons or taxiways.

FOD damage examples

Vehicle tire track-in

Debris is often trapped in the treads of tires from vehicles coming onto an airfield. Types of debris trapped in a vehicle tire can include rocks, mud, stones, loose hardware (screws, washers, bolts, ect.) and many other forms of small materials. These can be crew & fuel trucks, maintenance vehicles and many others that inadvertently bring and deposit debris around a flight line. These types of FOD are very difficult to track and manage once they are introduced into the airfield. The debris can then easily be picked up by jet engine intake, engine blast and prop/rotor draft. This material, once loose around operational aircraft, can lead to serious safety concerns including personnel injury and equipment/property damage.

Runway debris

The crash of a Concorde, Air France Flight 4590, at Charles de Gaulle Airport near Paris on 25 July 2000 was caused by FOD; in this case a piece of titanium debris on the runway which had been part of a thrust reverser that had fallen from a Continental Airlines McDonnell Douglas DC-10 during takeoff about four minutes earlier. All 100 passengers and nine crew on board the flight, as well as four people on the ground, were killed.

A Gates Learjet 36A, registration number N527PA, was taking off from Newport News/Williamsburg International Airport in Virginia on March 26, 2007, when the crew heard a loud "pop". Aborting the takeoff, the crew tried to control the "fishtailing" and activate the drogue parachute. The parachute did not work and the Learjet ran off the runway, its tires blown. Airport personnel reported seeing rocks and pieces of metal on the runway after the accident. The National Transportation Safety Board said that the accident was caused by FOD on the runway. Failure of the drogue parachute contributed to the accident.

Volcanic ash

On 24 June 1982, British Airways Flight 9 en route to Perth, Australia, flew into a volcanic ash cloud over the Indian Ocean. The Boeing 747-200B suffered engine surges in all four engines until they all failed. The passengers and crew could see a phenomenon known as St. Elmo's fire around the plane. Flight 9 dived down until it exited the cloud allowing the airborne ash to clear the engines, which were then restarted. The cockpit windshield was badly pitted by the ash particles but the aircraft landed safely.

On 15 December 1989, KLM Flight 867, en route to Narita International Airport, Tokyo flew through a thick cloud of volcanic ash from Mount Redoubt, which had erupted the day before. The Boeing 747-400's four engines flamed out. After descending more than 14,000 feet, the crew restarted the engines and landed safely at Anchorage International Airport.

Item jettisoned from aircraft

An unusual case of FOD occurred on 28 September 1981 over Chesapeake Bay. During flight testing of an F/A-18 Hornet, the Naval Air Test Center of the United States Navy was using a Douglas TA-4J Skyhawk as a chase plane to film a jettison test of a bomb rack from the Hornet. The bomb rack struck the right wing of the Skyhawk, shearing off almost half the wing. The Skyhawk caught fire within seconds of being struck; the two persons on board ejected.

Bird strikes

On 20 November 1975 a Hawker Siddeley HS.125 taking off at Dunsfold Aerodrome flew through a flock of northern lapwings immediately after lifting off the runway and lost power in both engines. The crew landed the aircraft back on the runway but it overran the end and crossed a road. The aircraft struck a car on the road, killing its six occupants. Although the aircraft was destroyed in the ensuing fire, the nine occupants of the aircraft survived the crash.

On 17 November 1980 a Hawker Siddeley Nimrod of the