by RICHARD FRIEND
After reviewing thousands of Royal Air Force FOD reports, I have determined that the majority of debris ingested into engines was either blown into the intakes by aircraft thrust reverse, taxiing too close, or by parts of aircraft structure falling off and into the intake.
Aircraft Generated FOD
Aircraft generated debris is especially noticeable on supersonic intakes that have auxiliary air intake doors; these doors supplement the intake airflow when the aircraft is stationary, or at a slow forward speed or vent air when traveling at high speeds when the intake is in ram effect. Older supersonic intakes have many moving parts — ramps and doors — that control the airflow though movable ramps that form shock waves to slow the airflow in supersonic flight or by bleeding air to present the engine with smooth controlled airflow at the right speed. The ramps and some doors are usually hydraulically operated and are supported by a series of mechanical components and switches that, in turn, have their own access doors, hatches and fairings. The access doors are secured with a range of fasteners and locking devices. On the Tornado, the Auxiliary Air Intake Doors are mounted on the side of the intake about two feet from the intake lip and are forward of the fan face.
Typically the engine fairing is removed using a speed brace and a Phillips cross-head screwdriver. Although this may seem a relatively simple task, the fasteners age with use and time and force is sometimes needed to extract them. Mechanics tend to lean onto the hand brace to prevent the Phillips bit slipping in the screw head and continue to apply this force until the fastener is removed. The fastener is held in place by a blind lock nut, which is attached by two small solid rivets. Time has shown that these rivets fail under loading and, while appearing secure, can come detached and be ingested in flight.
For the Tornado, this type of fastener failure has resulted in many impromptu engine removals and expensive overhauls. Moreover, due to the size of the fastener, once free in the intake, it has shown a habit of rebounding to cause extensive damage to the intake. This rebounding can occur many times until the fastener is chewed to a smaller size or presented at exactly the right angle to be ingested through the narrow gap between the blades.
Although I have focused on aircraft- generated FOD from intakes, any aircraft has access panels and these pose a source of debris, especially when they are fitted in front of or close to engine intakes. Special measures can be put in place, which call for an independent inspection by an unconnected third party trade person, after the work that refits the panels has been completed. The inspector would check for fit, form and function — as is usual with an independent inspection — and ensure that the panel and its associated fasteners do not become a debris hazard. Furthermore, it may be worth considering that any access bays be cleaned and checked prior to being closed; this could be as simple as using a vacuum and a flashlight. Further security and safety can be inexpensively applied. For years, when removing panels, the military has used fastener bags to secure loose fasteners and attach these bags to the panel once removed and placed on a parts stand. Remember, what may seem obvious to the trained eye has to be learned by someone new to a skill or work area, and we should share experience rather than make the uninitiated learn the lesson again, especially where safety is concerned.
Perhaps the prime contributor to FOD on fast jets is thrust reverse. Thrust reverse directs engine exhaust air forward to provide a braking mechanism. Thrust reverse disturbs the smooth airflow around the intake and ground, and can pick up debris and provide the lift to project particles into the strong airflow around the intake lip, down the intake and into the engine. Many airlines also use thrust reverse to help control speed on landing and prevent the aircraft brakes from overheating. The use of thrust reverse has to be strictly controlled and limited to forward airspeeds where particles cannot be thrown forward and ingested. Even at slow aircraft forward speed, the hot gas from the exhaust can be re-ingested and cause engine stall and surge.
For the Tornado, the design speed limit for the ingestion of particles using thrust reverse was validated using colored chalk chips arranged in color bands and strewn across a runway at a test airfield. As the approaching aircraft applied thrust reverse, the chalk chips were disturbed and some were ingested leaving their color marks in the intakes; these marks showed the cross over for the safe application of maximum-power thrust reverse against aircraft forward speed. For the Tornado, this design limit was re-appraised when the thrust buckets underwent structural change that altered the direction of the exhaust flow. This type of review should be considered when there are any significant changes that affect engine performance or if the aircraft structure is modified.
Many think that human-generated debris is a main contributor to FOD. Even daily routines can interact with operations and start a chain of events that could be hazardous. Several years ago, I was based at Royal Air Force Leuchars, Scotland, working on the Phantom F4. On one unusually bright and sunny day, the pilot and navigator walked to their aircraft. The pilot climbed up the front ladder and into the cockpit. The navigator used a rear ladder and was assisted into his ejection seat by one of the ground crew. The rear entry ladder fitted over the air intake, and was usually fitted on the port side, providing a platform for the navigator to step into the cockpit and a work base for the ground crew to help with the ejection seat straps. As the port side was busy, the navigator placed his map on the intake lip on the starboard side. Subsequently forgotten by the navigator, the map was ingested on engine start. It would have caused little damage except that attached to it was a ballpoint writing pen with metal fittings, which damaged the engine blades, and resulted in an engine change and its complete overhaul. Although the Phantom is no longer in service with the Royal Air Force, I wonder if this incident or something similar has ever been repeated in the many air forces where the Phantom still serves?
Unfortunately, intakes and intake lips do provide a convenient resting place for temporarily storing articles. High or low set, intakes remain a feature that must be checked and rechecked prior to engine start. On the VC10 transport aircraft, the intakes are simple bell-mouth shapes that form part of the pods that house the engines; they are fitted two on each side of the tail, about 12 feet off the ground. During maintenance to the intake, a small pot of rubberized sealing agent was applied to the outboard intake while the inner was used as a stand for the pot. Again, the work person was distracted and the pot was forgotten. During the subsequent pre-flight inspection, the intake was checked from the ground but because of the height of the intake, the intake lip made the pot invisible to both the aircrew and ground crew. On engine start up, the pot was drawn into the fan face and severely damaged the engine, which led to its replacement and overhaul.
Distractions like the ones above are common in aircraft operations. Even the most able people can be forgetful when occupied with multiple tasks and faced with a decreasing timeline. Accordingly, tool control and the control of what are classed C stores — screws, bolts, seals and washers — were brought into effect; tool control is mentioned later under its own heading.
While maintainers are trained and undergo refresher training to keep the dangers of FOD fresh in their memories, many visitors to aircraft operating bases will not have come across the acronym and may not understand the associated hazards. Strangely, many contractors with access to aircraft work areas may never get briefed. Military in the USA and UK generally recognize this loophole and have provided briefings and instructions to contractors working on their units. Moreover, each station has a FOD Prevention Officer who knows engine maintenance and has the ear of command and a nominated prevention officer for the whole service. This command organization helps provide visibility and takes action to help prevent recurrence of incidents.
However, mistakes are still made. Several years ago a picture on the front of a Royal Air Force magazine featured an engine mechanic entering an engine intake to carry out maintenance. Unfortunately, she was shown to be wearing a metal hair-grip that could pose a problem to an engine if it detached, fell into the intake and were left unnoticed. However, the picture generated a surge of letters that had a very positive effect in improving awareness of FOD. Moreover, each year the Royal Air Force runs FOD poster competitions that help reinforce the seriousness of the problem. Also, in recent years, this service has participated in the NAFPI (National Aerospace FOD Prevention Incorporation) conferences to help spread their experience and learn from others.
Lastly, on human-generated FOD, the Royal Air Force insists that aircrew and ground crew keep their pockets free from objects by providing working coveralls that have no buttons and personal lockers that are secured with a combination lock and used for clothing and small items like keys and money. In some areas, such as the flightline, additional small personal item lockers — similar to lockable mailboxes — are available to secure frequently used small items. In addition, smoking is not allowed in the work areas to maintain safety. For engine intake inspections, where the inspector has to enter the intake, many bases have special pocket-less coveralls.
Understanding the mechanisms of ingestion should help you visualize how debris gets ingested and provide the knowledge to generate effective working practices. Areas of work must be kept scrupulously clean. Any breakdown in the paved surfaces must be quickly repaired. Moreover, if at all possible, the damage should be isolated and cornered off or cleaned and checked on a regular basis to ensure that particles are not being generated. During the repair activities, debris can be a problem and work personnel should be thoroughly briefed, and the area checked, as the repair progresses. In recent years, work sites have been surrounded by plastic mesh that prevents debris migration. Moreover, as it is typically bright orange in color it provides a visual notice to avoid the area. On many bases, civilian contractors carry out repairs, and it is imperative that they be briefed about the dangers posed by uncontained debris. In the early 1990s, I recall an incident that left many engines damaged. Concrete runways are routinely overhauled and, after many landings, rubber tire marks have to be removed; on one Royal Air Force Station, a civilian contractor carried out this operation over a weekend. The contractor was probably not briefed or supervised and used metal balls — shot blasting — blown onto the marks at high speed. The runway was made up of twelve-foot square concrete blocks. The expansion joints between these blocks were sealed with asphalt sealant. The contractor cleaned the marks off the runway but left thousands of metal shot immersed in the sealant. When aircraft operations commenced, the shot was lifted, ingested and resulted in the removal of several engines from Tornado and Jaguar aircraft. It was a very costly lesson to learn, not just from a cost perspective but the loss of sorties and considerable disturbance to operations — both of which have no associated cost to the military, but obvious considerable value.
The need to keep operating and maintenance areas clean cannot be overstated. Mechanical sweeping and ad-hoc FOD Walks by all personnel are effective ways to minimize debris. Following the FOD Walks, the debris found can become an exhibit to be displayed on notice boards or custom FOD displays that can and should be moved to different areas of work. Good posters and photographs can have a great impact, but this impact is lost over time as the posters become like unnoticed old wallpaper. To work to maximum effect, posters should be routinely changed. Sharing posters and ideas with other services or other nations can bring new ideas and generate new interest.
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