5.2 Foreign Object Damage (FOD)

- 5.2.1 Damage due to (non-biological) Foreign Objects
- 5.2.2 Bird Strikes

There are several possible answers to the question, “What is a foreign object that can cause engine damage?” For this reason, this chapter uses the term “foreign object” to include everything that enters the engine from outside (Foreign Object Damage = FOD) or originates in the engine itself (Own Object Damage = OOD or Domestic Object Damage =DOD) and is not a part of the normal inlet air or the specified auxiliary materials. Particles that contaminate the oil flow, as well as erosive (Chapter 5.3) and corrosive (Chapter 5.4) media in the air flow are also considered foreign objects. Bolts that come loose inside the engine are also foreign objects.

In the broadest sense, weather influences such as ice, hail, and rain which were dealt with separately in Chapter 5.1 can also be considered to be foreign objects.

The classification of foreign objects into “non-biological foreign objects” and “bird strikes” is arbitrary and was chosen because massive, hard foreign objects and foreign objects of technical origin can be differentiated from bird strikes and weather factors.

If corrosion and erosion are also included under FOD, then FOD is a deciding factor for engine cost, exceeding over 50% of the total purchasing cost in military machines, for example (Fig. "Overhaul statistics"). In most cases parts with FOD that exceeds specified limits (given in maintenance manuals and overhaul handbooks, etc.) must be reworked, and if this is no longer possible, replaced.

The increased use of electric and electronic engine parts such as regulators increases the danger of short circuits caused by extremely fine fragments of carbon fiber. These are created during burning or reworking (cutting) of fiber-reinforced parts such as fuselage sheeting (Ref. 5.2.1-1).
Ingested dust can cause erosive wear in the compressor, as well as block up cooling air ducts in the hot parts, leading to dangerous temperature increases.

However, some FOD is reversible and can be removed fairly simply. This includes fouling of the compressor blading through oil and dust, which sticks to the blades and can greatly decrease compressor performance. Washing and cleaning procedures that remove these coatings are a necessary part of routine maintenance.
Bird strike is one of the most feared types of FOD and is treated in a separate chapter (Chapter 5.2.2) due to its unique effects. However, even insects can have a similarly unallowable, if less spectacular, effect on engines. If insects carried by the airflow become stuck on the blading upon impact, it results in so-called “insect roughness” which can have a considerable effect on compressor performance. For example, in one case a small 100 kW gas turbine ingested a mosquito swarm which worsened the surge limit of the radial compressor so quickly that compressor surges made further operation impossible and required immediate cleaning.

Figure "Overhaul statistics": These diagrams depict an older (Ref. 5.2.1-2) statistical evaluation of military engine overhauls (percentages are rounded). The top diagram refers to a power-shaft engine type for helicopters. The bottom diagram covers a wide range of helicopters and winged aircraft used by the US Navy. If one goes by the broad definition of FOD used here, including erosion and corrosion damage, then this type of damage is the reason for more than 50% of overhauls. The notion that helicopters are highly susceptible to FOD is plausible, since they are operated in an environment with a large amount of foreign objects.

If one considers the total number of engines in the lower diagram then, in addition to the portion explicitly labeled as FOD, one could certainly include some of the cases from the “decreased performance” section (fouling, erosion, corrosion). This would mean that in all the studied military aircraft, roughly 30% of cases necessitated an overhaul due to FOD.

Figure "Foreign object damages": This diagram describes the damages characteristic of typical foreign objects (see Chapter 5.1.1). It shows the influence of FOD on operating behavior, i.e. the efficiency of the engine components and the engine as a whole. These are usually damage mechanisms with a long incubation period, such as erosion, buildup of deposits, or wear.
The creeping worsening of engine performance at the very least increases the probability of hot part damage (higher temperature levels required to achieve the necessary power) and, under poor operating conditions, the risk of compressor stalls/surges.

Larger birds:

  • Intake guide assembly/intake (front) bearing (V1): This type of construction is used in some military and older civilian aircraft types (Fig. "Influence of inlet guide vanes"). Parts of this assembly are stressed either by direct hits or indirectly through the inlet cone when it is struck. Experience has shown that this configuration has certain weaknesses when compared with configurations with rotating nose cones and no intake guide vanes.
  • Blading of the fan and low-pressure compressor (V2): Powerful forces on the blading cause damage including blade failure. The guide vanes are particularly susceptible to this (Fig. "Influence of flight speed") since they typically have thin profiles and are subject to high speed impact from large bird pieces that pass through the fan whole. However, the risk of damage to rotor blading is greater at slower flight speeds (low speed impact, see Chapter 5.2.3), because it must put more energy into accelerating the ingested bird mass, creating large bending force.Adjustable/variable guide vanes are at risk for damage to the adjustment mechanism and/or closing of the vanes.
  • High-pressure compressor blading (V3): An example of a dangerous situation is bird parts collecting on a housing strut ahead of the compressor and then striking the relatively filigreed compressor blading as a large mass (Fig. "Sensitivity caused by S-shaped ducts").
  • Spinner (rotating nose cone) (V4) : Imbalances caused by deformation of metallic spinners, fragments of fiber-reinforced synthetic spinners.
  • Main bearings (V5): Short-term overstressing of the main bearing through large axial forces (depending on the stiffness of the spinner, etc.) and/or dynamic overstress caused by imbalances (results: fractures, fatigue).

Flock of birds (small birds):

  • Guide vanes of the low-pressure compressor (VS1): Even small birds can cause dangerous damage to smaller engines, especially if these are of a very filigreed type and/or made from particularly sensitive materials such as fiber-reinforced synthetics or aluminum alloys.
  • High-pressure compressor blading (VS2): Damage can occur if large amounts of bird matter collect on struts or guide vanes and enter the compressor as a large mass (Fig. "Sensitivity caused by S-shaped ducts"). For this reason, the first high-pressure compressor rotor stage must be given special consideration.

Sand and dust:
Fan and low-pressure compressor (S1): Erosions wear, roughing, and small notches on the blading.

  • High-pressure compressor (S2): The centrifuging of the sand causes most of the sand to stay out of the high-pressure compressor in bypass engines. However, erosive wear, roughing, and small notches on the blading (especially near the tips) will occur. Erosion of the inner housing wall, especially of soft abradable coatings, will increase the clearance gap at the blade tips.
  • Labyrinths (S4): Erosion of labyrinths in the cooling air flow.
  • Cooled hot parts (S3): Blockages in the cooling air ducts causes overtemperatures with considerable reductions in the life span; damaging reactions (hot gas corrosion, sulfidation) with the inner and outer surfaces of the hot parts.

Small rocks and split:
Fan, low-pressure compressor (F1) and high-pressure compressor (F2); small notches that can usually be reworked.

Hail and ice (see Chapters 5.1.2 and 5.1.4):
Damage to the fan (for example, exit guide vanes made from Al alloys or fiber-reinforced plastics) and low-pressure compressor (H1); because the ice particles melt before impact, no damage is to be expected in the high-pressure compressor. However, a compressor stall (H2) due to the created water and/or unstable combustion or flame-out are possible results (H3). These effects are especially dangerous in hail, because hailstones are more massive than rain drops and less likely to be diverted into the bypass duct, causing a larger amount of water to enter the high-pressure area.

Rain and spray: Rain can cause rain erosion, i.e. rain drop impact in the fan (R1). While microscopic traces of rain erosion can be detected on titanium blades, there does not seem to be any acute risk of damage. Noticeable damage is possible on the surfaces of coated parts or those made from fiber-reinforced plastics. In the high-pressure compressor and combustion chamber, the same damage can occur as with hail ingestion (R2 and R3), usually in combination with hail, since rain is more likely to be directed through the bypass duct away from the high-pressure section.

Figure "Influence on operating behavior": Foreign objects and ingested contaminants can affect almost all components in the gas flow in many different ways, changing their operating characteristics and worsening the overall. The direct relationship is often not immediately discernible since there is a chain of effects that mutually influence each other. For example, the failure of a hot part may be causally related to erosion wear on seals in the cooling air ducts.
Mostly failure mechanisms which need longer periods of time, like erosion, the formation of deposits or wear processes are concerned.

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