Foreign object damage

FOD damage to the compressor blades of a Honeywell LTS101 turboshaft engine on a Bell 222, caused by a small bolt that passed through the protective inlet screen.

FOD deflection system on a PT6T installed on a Bell 412. Air enters from upper right, and pure air follows the curved ramp down to the turbine inlet (also covered by a screen). Any debris being sucked in will have enough momentum that it won't make such a sharp bend, and will hit the screen on the upper left, and will be carried out to the left, getting blown overboard.

Foreign object damage or foreign object debris (FOD) 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 both the damage done to aircraft by foreign objects, and the foreign objects themselves (i.e. any object that has, or is likely to, cause damage.)

"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 an 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 manufacturing or 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. Tools used during manufacturing are tagged with a serial number so if they're found they can be traced.

FOD costs the aerospace industry $1.1-$2 billion USD per year in direct costs, and as much as ten times that amount in indirect costs from delays, aircraft changes, incurred fuel costs, unscheduled maintenance, and the like for a total of $12 billion USD per year and causes expensive, significant damage to aircraft and parts and death and injury to workers, pilots and passengers.

It is estimated that FOD costs major airlines in the United States $26 per flight in aircraft repairs, plus $312 in such additional indirect costs as flight delays, plane changes and fuel inefficiencies.

In the United States, the most prominent gathering of FOD experts occurs at the annual National Aerospace FOD Prevention Conference. It is hosted in a different city each year by National Aerospace FOD Prevention, Inc. (NAFPI), a nonprofit association that focuses on FOD education, awareness and prevention. Conference information, including presentations from past conferences, is available at the NAFPI Web site.

Examples

Examples of FOD include:

• Rocks, broken pavement, vehicle or aircraft 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 or by a jet blast can send such objects airborne, creating hazards to nearby personnel. One way to counter this is to install a gravelkit, which exist to certain planes like the early B737.
• Parts from ground vehicles
• Garbage, maintenance tools, disposed champagne bottles etc. mistakenly or purposely deposited on tarmac and/or runway surfaces.
• Hail: can break windshields and damage or stop engines.
• Ice on the wings, propellors, or engine intakes
• Dust or ash clogging the air intakes (as in sandstorms in desert operating conditions or ash clouds in volcanic eruptions). For helicopters, this is also a major problem during a Brownout.
• Tools, bolts, metal shavings, lockwire, etc. mistakenly left behind inside aircraft during the manufacturing process or maintenance.

Generally speaking, Bird strikes (when an aeroplane flies into a bird, the impact can cause severe damage from a bird striking the fuselage, engine, etc) are not considered to be FOD strikes. Bird strikes are treated separately.

All aircraft occasionally lose small metal or carbon parts during takeoff and landing. These parts remain on the runway and can cause damage to tyres of other aircraft, hit the fuselage or windshield/canopy, or get sucked up into an engine. Although airport ground crews regularly clean up runways, the crash of Air France Flight 4590 demonstrated that accidents can still occur: in that case, the crash was said to have been caused by debris left by a flight that had departed only four minutes earlier.

On aircraft carriers, as well as military and some civilian airfields, FOD walkdowns are conducted before flight operations begin. A line of crewmen walk shoulder to shoulder along the flight operations surfaces, 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".

One spectacular accident involving FOD happened during the USSR war in Afghanistan when an Il-76 got too close on the taxiway to a MiG and sucked in the smaller plane's drogue chute into one of its engines.

Early reports suggest that the Biman Bangaldeshi Airlines crash at Dubai International Airport in March 2006 was caused by FOD. An image of the downed aircraft can be found on a variety of news sites including [1], and a video of the aircraft moving down the runway in“shower of sparks” can be seen at [2].

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 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. A chicken gun is used to perform experiments on bird strikes.

Engine and airframe designs which avoid FOD

Some military aircraft have a unique design to prevent FOD from damaging the engine. The design consisted of 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. However, many consider it an innovative solution to a challenging engineering problem.

The Russian MiG-29 fighter has a special engine design to prevent ingestion 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.

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. This kit included gravel deflectors on the landing gear, foldaway lights on the bottom of the plane and screens that prevented gravel entering the open wheelwells when the gear is extended from hitting critical components. It 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.

FOD damage examples

Air France Flight 4590

The crash of a Concorde, Air France Flight 4590, at Charles de Gaulle International 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 which fell from a Continental Airlines McDonnell Douglas 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.

Bombardier Learjet 36A

A Bombardier, Learjet 36A, was taking off from Newport News/Williamsburg International Airport Va., on March 26, 2007, when the crew heard a loud “pop. Aborting the takeoff, the crew tried to control the “fishtailing” and activate the drag chute. The chute didn’t 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 NTSB said that the Learjet accident was caused by foreign object debris (FOD) on the runway. Failure of the drag chute contributed to the accident.The chute had been inspected three months before the accident but it was not deployed and repacked by mechanics in accordance with the maintenance manual instructions.

British Airways Flight 9

On 24 June 1982, British Airways Flight 9 on 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 at Jakarta, Indonesia where passengers were disembarked without injury.

NATC Douglas TA-4J

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

B-1A crash

In the late 1970s, the B-1 Lancer 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, a B-1A 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.

Nimrod crash

On 17 November 1980 a Hawker Siddeley Nimrod of the Royal Air Force crashed shortly after taking off from RAF Kinloss. It flew through a flock of Canada geese, causing three of its four engines to fail. The pilot and copilot were killed; the pilot was subsequently posthumously awarded an Air Force Cross for his actions in maintaining control of the aircraft and saving the lives of the 18 crew. The remains of 77 birds were found on or near the runway.

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 control. 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.

On 29 April, 2007, a Thomsonfly Boeing 757 sustained engine damage from a bird strike during takeoff. The damaged engine exhibited repeated compressor stall until being shut down; the plane returned to the airport safely. The entire incident was caught on video (viewable here).

Technologies capable of detecting FOD

• Thermal imaging cameras

example: Magna

• Lidar (Light Detection and Ranging)
• Radar

example: QinetiQ and Trex Industries

note that although the first QinetiQ systems were radar only, the newest installations combine the radar with high power telescopic cameras with night-time illumination systems.

• Camera systems [visible band imagery (standard CCTV) and low light cameras]

examples: Farran and Stratech

• Camera / Radar:

example: Xsight

Technologies capable of removing FOD

• Walk-behind Sweepers

examples: Billy Goat , F.O.D. Control , and Tennant

• Tow-behind Sweepers

examples: Aerosweep and AZ Industries

• Sweeper Trucks

examples: Elgin , Tymco , and Tennant

• Magnetic Bars

examples: F.O.D. Control , Monroe , and Thompson

Studies on FOD

There have only been two detailed studies of the economic cost of FOD for civil airline operations. The first was by Brad Bachtel of Boeing, who published a value of $4 billion USD per year. This top-down value was for several years the standard industry figure for the cost of FOD. The second work, more recent (2007) and more detailed is by Iain McCreary from the consultancy Insight SRI Ltd. This report breaks down the cost of FOD into direct and indirect costs for the top 300 global airports, with detailed footnotes on the suporting data. The Insight SRI research has become the new standard cost value, being quoted by regulators, airports, and technolgy providers alike.

Eurocontrol and the FAA are both studying FOD. Eurocontrol released a preliminatry assessment of FOD Detection technologies in 2006, while the FAA is conducting trials of the four leading systems from Qinetiq (PVD Providence T F Green Airport), Stratech (ORD CHicago O'Hare Airport), X-Sight Systems (BOS Boston Logan Airport), and Trex Industries during 2007 and 2008. Results of this study should be published in 2009.

External links

• FOD News
• Insight SRI
• FOD Detection at Heathrow AirportJune 13 2008
• F.A.A. Tests Ways to Detect Runway Junk NYTimes May 23 2008
• Best Practices for the Mitigation and Control of Foreign Object Damage-Induced High Cycle Fatigue in Gas Turbine Engine Compression System Airfoils. RTO Technical Report TR-AVT-094
• List of Various FOD Mitigation Studies, Including Links to Corresponding Technical Papers