Foreign Object Damage

FOD is any item, article or substance that is not native to the aircraft or its system and one of the largest costs to civil aviation.

One of the largest costs to civil aviation is foreign object damage (FOD). It is estimated to cost the industry $13 billion dollars by direct damage to aircraft and their operating systems. Indirect costs from delays, AOG times and insurance claims are estimated by the industry to be at least 10 times that amount.

The definition of FOD is any item, article or substance that is not native to the aircraft or its system.

The most expensive damage occurs to the engines that are powering the aircraft either via foreign objects such as wildlife and parts of other machinery striking moving parts.

Although engines are stringently tested for ability to minimise impact to aircraft under severe damage (from bird strike or ice ingestion) the majority is from small atmospheric particles entering the engine whilst in transit. The recent eruption of the volcano Eyjafjallajokull in Iceland, that disrupted air travel in Europe in 2010, is a prime example of atmospheric FOD. The London VAAC advise that aircraft cannot fly in conditions over 2-4000 micrograms per cubic metre of ash.

Each PT6 engine has small holes in the engine casing to allow cooler air into the chamber to reduce the overall heat and therefore damage to the hot section parts. The damage that occurs over the period of the flight varies from location to location, however the most at risk areas are coastal areas, desert environments and flight paths in areas of volcanic activity. Over the course of operations these holes become clogged with the airborne particles and the engine cannot be cooled efficiently therefore losing the performance that is required for safe and economic flight.

Ways to minimise the impact and cost

Engine Prevention Systems.

Most modern engines, including the Pratt and Whitney Canada PT6 engine class, have built in FOD detection and prevention systems. For example, the Bell 412 helicopter uses the workhorse PT6T power plant, this engine has a deflection system installed to reduce the amount of FOD. As standard the PT6C and B models have a radial inlet screen. However these solutions do not prevent large FOD and often on inspections the damaged parts need to be cleaned, re-conditioned or at worse completely replaced.

Runways and Manoeuvring areas.

The FAA has strict guidelines on how to deal with foreign objects and modern airports and airfields employ sophisticated radar and scanning equipment to reduce the possibility of FOD. These methods are expensive and generally these solutions are not found in smaller runways where light aircraft that operate using the PT6 variant engines are based.

The International Civil Aviation Organisation (ICAO) have laid out guidelines for these operating areas that all manoeuvring areas for aircraft. However these are not fool proof for in some cases the only checks are the visual “walk” performed by ground crew during shifts.

If a runway becomes worn badly and areas of the tarmac start to disintegrate, then airline companies will investigate and if necessary halt flights.

Cleaning and Maintenance

To avoid particle FOD, OEM manuals for the PT6 engines advise that there needs to be a cool wash with demineralised water, then a subsequent dry run of the engine to ensure the water does not collect and cause further erosion. This is particularly important in areas of high salination and where missions are of a shorter length. We often see the majority of Hot Section damage as a direct result of these conditions.

Components that see the damage

When FOD occurs, there are a number of areas of the engine that can be affected. If the damage is from a large object then the main damage is to the turbine blades, the vane rings, CT shroud segments and the compressor inlet. Whilst the aircraft is in flight, atmospheric dust is sucked into the engine. Often with the smaller particles it is a matter of regular cleaning that assists the reduction to damage. From these particles come mechanical issues such as corrosion, erosion, blockages and in some cases engine flame out.

The erosion from volcanic or desert dust happens on the cold sections of the engine, the fan or the propellers. As the dust is irregular in size, it can create gaps in the material leading to loss of performance. If the dust lands on hotter areas of the engine such as the combustor walls and turbine blades then this causes corrosion as the particles for a glass layer and subsequently restrict the airflow through the engine. In the worst case scenario the glass layer that is created restricts the airflow that much that it causes a stall in the turbine blades then an engine flame out.

FOD damage can by inspected by ETM engine trend programs to see any loss of performance that the pilot may not have noticed during operation. The other way is Borescope Inspection, a service that mobile repair teams can perform when the aircraft is on the ground with minimal impact to operational use.

These steps are vital to ensuring the PT6 reaches its TBO and HSI times with minimum operator costs.

Conclusions

The costs of sweeping by manual exercise is the most cost effective method for smaller airports. Coupled with training of ground staff and vigilance will prevent this issue as much as possible.

In areas where aircraft are operating in desert regions, coastal areas and over volcanic areas, these forms of damage are most common and require more essential maintenance than other regions. In these cases, the washing of your engine is vital.  

At Euravia Engineering our MRT specialists are ready to assist and fix all forms of engine FOD that has caused further damage to engine hot sections. For more information you can contact us on 00441282844480 or enquiries@euravia.aero.