by RICHARD FRIEND
Taking into consideration the exacting controls by military organizations over both personnel and work procedures, it is perhaps difficult to understand how their aircraft could suffer from damage caused by FOD. Work areas are kept scrupulously clean and well maintained, tools are kept in good condition and rigorously controlled, and personnel are specifically trained and regularly refreshed on the hazards of FOD and the need to be vigilant — even those not associated with aircraft operations. So why then have three nations this author has direct knowledge of lost military aircraft in the last ten years from FOD to fan or compressor blades on turbine engines? It is not sufficient to understand just what FOD is, but we need to understand how it occurs, understand the specific hazards unique to the many areas of work, and develop tailored prevention activities that aim to eliminate military FOD. To understand FOD is the first important step we must take before enforcing relatively simple and routine preventative measures. Indeed, if we understand the mechanisms of FOD, we establish the first and vital foundation block on which to build a prevention program to minimize the operational, economic and human costs of FOD.
FOD means different things to different people and can show itself in many ways on an aircraft and its components. When an aircraft taxis down a runway, the tires may pass over a stone or object that could cut the tread and lead to subsequent failure with an immediate puncture or, more likely, when the tire is rotated and loaded on take off. Few will forget the tragic events on 25 July 2001, when a French Concorde, Flight AF4590, while taking off from Paris, reportedly taxied over a piece of metal shed by another aircraft. A main undercarriage tire was cut, and the subsequent failure on take off shed a large piece of tread that hit the wing and caused a hydraulic shock wave in the fuel tank. A massive fuel leak ensued, to be ignited by an electrical arc from wiring in the undercarriage that had been damaged by the tire failure. Further damage was caused to hydraulic pipes and controls, stopping the undercarriage from being retracted and causing a loss of power on the number one engine and the second to cut out. At this stage it was too late to abort the take off. In the critical first moments of flight, fighting to overcome the high drag of the supersonic airframe, the aircraft was unable to gain height or speed. Fire engulfed the wing, engines and flying surfaces; the aircraft flew for less than three minutes before it crashed into a hotel in the small town of Gonesse killing 109 on board and four on the ground.
Whilst the tire loading on take off is high, especially at the Concorde’s very high take off speed — about 250 miles per hour compared with 170 miles per hour for most other subsonic aircraft — that loading is magnified many times over on landing. On landing, the wheel is shock loaded by the descending weight of the aircraft and the instantaneous acceleration to well over 100 miles per hour. A burst at this critical moment can lead to sudden loss of control and force the aircraft off the runway. In the early 1980s, I was on a detachment at RAF Waddington, near Lincoln, England, when an F4 Phantom blew a tire on landing. The aircraft immediately departed the runway, ran onto the grass in the center of the airfield, and sped toward cabins where the ground crew and I were operating. The pilot fought with the controls but as the aircraft sank into the muddy field, he was unable to adequately retard the speed or, more importantly, steer the nose wheels to control the direction. At less than five hundred yards from the buildings, the pilot was faced with a split-second decision, and he and his navigator ejected. The aircraft carried on its path, held firmly in its mud tracks, but as it continued to sink up to its axles and the landing gear doors, its speed rapidly declined and it came to rest a few hundred yards short of the buildings. Several seconds later the pilot and navigator, both now under parachute, landed safely on the airfield. Although no one was hurt, it is an event that I will never forget and one that has served as a reminder of the hazards of aircraft operations and of FOD.
Several years later, when I was Officer in Charge of the Tornado Engine Shop and the Station FOD Prevention Officer at RAF Leeming, in Yorkshire, England, I focused more on the dangers of debris being ingested into turbine engines. For the uninitiated, turbine engines can rotate at over ten thousand-rpm, and when a tiny piece of debris like a piece of safety locking wire or stone get ingested it behaves in a ballistic manner, striking the compressor blades as if fired from a gun. Depending on material grade, shape and size, the debris can cause damage to the first blade or any part of the intake they strike.
As the debris passes through the engine, its many contacts by fan blades can break it into smaller chunks or the debris can liberate pieces from the blades that multiply the debris field and increase the size and quantity of the damage. I have seen a small nick on the first stage of the fan and seen the rear stages of the compressor and most of the turbine rotors completely decimated, as if someone had taken barber shears and clipped the blades from the disk. Where the blades are highly loaded or are subject to vibration — sometimes referred to as HCF (High Cycle Fatigue) — the damage to the blades can grow rapidly to a crack and subsequently fail. Although engines are designed to contain failed blades, a blade failure can lead to the total destruction of the engine. Indeed, many posters exist that show how the ingestion of a small metal screw, which costs just a few pennies, can cause damage requiring the complete replacement of the engine, which costs many millions of pennies. However, it is not just hard debris that causes damage; a plastic bag can behave like a large sheet and blank the intake, stalling the airflow across the blades and causing the engine to surge. Engines can usually tolerate a surge — a reversal of the airflow — but, depending on severity, a surge can deform blades and necessitate the replacement of the engine.
It is easy to see why we must control debris to maintain safety and reduce costs. FOD is correctly seen as a flight safety risk, and everyone is responsible for keeping that risk as low as possible. Picking up debris that could migrate onto airfields is the first step that could avert a disaster. True, seldom does FOD result in a disaster, but it can be one link in a chain of events that, if left unbroken, could result in the failure of an engine, loss of aircraft and taking of life. Breaking that chain could be as simple as picking up litter. A key area to achieve maximum benefits is through training and education. More can be done if we target areas for improvement and coordinate increasingly limited resources to reduce problems and get the best return of investment.
Mechanisms of FOD
Understanding how engines ingest debris is important.
It is often believed that engines ingest debris through vortex ingestion. This phenomenon can sometimes be seen during cold and wet conditions when water is drawn up into the vortex and ingested.
A vortex is created by the disturbance of airflow around the intake mouth. In normal conditions, airflow is pulled from all around the intake. As the airflow is drawn from the rear of the intake, and as the flow interacts with the ground plane, a venturi is created by the ground and underbelly of the aircraft or intake. A vortex initiates due to the presence of a prevailing wind that blows at an angle to the intake face and forms a shear vector that causes the airflow-to-ground-plane interaction to swirl.
The speed and strength of a vortex is dependent on the speed and strength of the ingested airflow and wind vector. Preventing a vortex from forming can be as simple as pointing the aircraft into wind. However, because this mechanism was thought to be the predominant reason for ingesting debris, research was carried out in the UK in the early 1950s to measure and control the forces in vortices to prevent object ingestion. Various intakes were studied, including the common under-slung pod that is prevalent on many airliners and large cargo planes, and it was proven that pod intakes over three feet from the ground were unable to ingest objects heavier than sand grains. When a vortex forms and attaches to the ground, the periphery behaves like a rotating brush sweeping objects out of its path. However, if the objects were trapped, held in place between the gaps in the concrete and the eye of the vortex passed over them, then the objects may be picked up in the negative pressure at the eye and be ingested. Therefore, special attention needs to be placed on cleaning the gaps and ensuring that they are fully filled with sealant to avoid providing a temporary resting place for debris. Despite the research, it is still commonly believed that the vortex ingestion phenomenon is responsible for the majority of FOD.
Continue to Military FOD Prevention — Part 2.
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