What is FOD?
FOD is an acronym referring to Foreign Object Damage. FOD is a significant hazard in the aviation industry, where even small foreign objects ingested into turbine engines can lead to major failures.
Foreign object debris may consist of loose material on a runway like garbage, rocks, and screws, or in-flight hazards like hailstones and birds. These objects are easily ingested into engines due to the intense suction generated by turbofan inflow. They can do catastrophic damage to engines in operation, or produce cracks in turbine blades that lead to unanticipated failure.
FOD prevention practices are standard at any airport or maintenance depot, and intensive FOD protocols are commonplace on aircraft carriers and Air Force bases where high-performance aircraft take off and land.
Tool Control Procedures – Aircraft maintenance facilities use strict tool control procedures any time work is done in or around engine systems. A stray wrench or fastener left inside an engine housing can cause major damage when the engine is activated. Most facilities use tool kits or tool boards where each object has a specified location, and all tools must be accounted for after any maintenance procedure. A single missing tool can lead to aircraft groundings or flight deck shutdown if there’s concern that a stray tool poses a FOD risk.
FOD Walkdown – Crew members aboard aircraft carriers routinely perform “FOD walkdowns”, where individuals line up and walk the length of the flight deck, scanning for any loose objects that could lead to engine damage.
FOD Detection Systems – Some airports install high-tech FOD detection systems that use radar or cameras to provide advanced detection of foreign objects on runways.
FOD Sweepers – Many airports employ FOD sweepers, which can be towed behind a vehicle and use brooms or vacuums to collect loose debris.
Consequences of FOD
A FOD incident can cause aircraft damage, engine failure, or even aviation disasters depending on the circumstances. The video below demonstrates the hazard FOD can pose to a jet engine:
The F-18 engine sparks and flames after ingesting an unidentified piece of foreign debris. Even something as small as a metal nut or washer can cause major damage to rapidly spinning turbine blades.US Airways Flight 1549
The most famous modern FOD incident occurred when US Airways Flight 1549 encountered a flock of Canadian geese minutes after taking off from New York City’s LaGuardia Airport in January 2009. The impact with the large birds resulted in the failure of both engines, and the pilots successfully ditched the powerless plane in the Hudson River. All 155 people on board the aircraft survived with mostly minor injuries, and a National Transportation Safety Board (NTSB) member called the incident “the most successful ditching in aviation history.”
Investigations later found bird remains including feathers from a Canadian goose in the right aircraft engine. The left engine contained dents and damage to the engine cowling, along with multiple fractured or missing guide vanes.
Given the serious FOD hazard they pose, aircraft engine manufacturers have actually tested bird impacts by firing dead birds into operating engines. The video below offers high-speed camera footage of a bird ingestion test, demonstrating the destructive impact birds can have on rapidly rotating turbine blades.
Laser Peening for FOD Resistance
Laser peening was originally developed as a method for mitigating FOD. In the early 1990’s, the U.S. Air Force was having FOD issues with General Electric F101 engines in the B-1 Lancer, leading to laborious manual inspections of all F101 first stage fan blades before each B-1 flight. The Air Force performed a study comparing the FOD resistance of laser peened turbine blades to that of standard blades receiving shot peen treatments, and the results revealed the superior enhancement effects provided by laser peening.
Sample blades were subjected to simulated FOD as notches were made using a chisel or an electrical discharge machine (EDM). The blades were then put through rigorous fatigue tests to compare the effect of different surface enhancement treatments on FOD failure.
The chart above demonstrates that laser peened blades with simulated FOD lasted more than twice as long as shot peened blades in most cases. What’s more, laser peened blades with simulated FOD even lasted longer than blades with no damage at all.
These landmark results led the Air Force to adopt laser peening as a standard production process for all F101 first stage fan blades.