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5.2.1.3 Foreign Object Types

It is of fundamental importance to determine what kind of foreign object was the cause of engine damage. Identification of the foreign object enables conclusions as to its origin and is a prerequisite for designing appropriate preventive measures which, in turn make possible targeted measures for avoiding future damage.
Even though virtually everything found in and around engines has been found inside as a foreign object (Fig. "Human related foreign objects"), fastening devices such as bolts (Example "Ingested bolt during approach"), nuts, shims, and rivets or parts thereof are found most frequently.
The probability of foreign objects remain in the inlet area is certainly connected with the degree to which the inside of the engine is visible from the outside. In fighter aircraft, especially, the engines are integrated into the fuselage and require long, often curved inlet ducts with complicated internal structures (such as shutters and flow guide vanes; Fig. "Risk of complex inlets"), in which tools or fastening devices can easily “hide”. This type of problem is very unlikely to occur in the easily observable inlets of nacelle engines from modern commercial aircraft with large bypass ratios.
Engines in military use seem to be especially prone to FOD in comparison with civilian aircraft. This is probably due to the types of foreign object that are characteristic for military uses. These include parts of projectiles (shell housings, etc.) and rockets, as well as ricochets (see Example "Ricochets") that were ingested into the engine during shooting practice. However, even flight missions that occur primarily at low altitudes considerably increase the FOD risk, especially the probability of bird strike (Figs. "Increased risk at low altitudes" and "Influence of proximity to ground").

Figure "Human related foreign objects": Almost everything that is found around engines has been found inside them at some point. This diagram shows typical human-related foreign objects. There have been documented cases in which engine damage was traced back to these objects. Understandably, there is an especially high risk of the maintenance personnel`s equipment being sucked in when they pass by the engine inlet area. The vortex formation described in Chapter 5.2.1.2 is also important in relation to this type of foreign object ingestion. Vortexes can also suck in apparently securely fastened objects such as articles of clothing. There have been reported cases of FOD due to safety helmets “1”, ear protection “2”, hats “3”, a hood attached with a zipper “4” , and jackets “5”.

Personal equipment that has been sucked out of pockets includes pens “9” and cleaning rags “6”. Even a bucket “7” was torn from the hand of a maintenance worker and sucked into the engine.
With flashlights “8” and tools the greater risk is that they are accidentally forgotten in the engine during inspection of the inlet duct.

Example "FOD intake during take-off" (Ref. 5.2.1.3-1):

Excerpt: “…shortly after takeoff engine failure due to foreign object intake (unidentifiable bolt) followed by a crash…“

Example "Ingested bolt during approach" (Ref. 5.2.1.3-2):

Excerpt: ”……during approach (to the airport) engine failure due to damage from an ingested bolt…crash landing.”

Comments: This case concerns a single-jet fighter aircraft. Bolts are the typical foreign object. There are several similar cases listed in the reference literature. It would be important to determine whether the bolts had been sucked up off of the runway or if they had originated in the aircraft itself.

Example "Ricochets" (Ref. 5.2.1.3-3):

Excerpt: “…during shooting, engine failure due to ricochets into the fuselage and engine compressor; aircraft crashed.”

Example "Compressor stall" (Ref. 5.2.1.3-4):

Excerpt: “…during air-to-surface shooting, aircraft crashed following engine failure due to a compressor stall caused by foreign object ingestion.”

Comments: Typical “military FOD”; it is worth noting that missed shots from guns or rockets can evidently ricochet so far from the target that they endanger the aircraft, i.e. engine. There are frequent reports of this type of case.

Example "Maintenance flap" (Ref. 5.2.1.3-5):

Excerpt: “…crash due to engine damage following the loss of a maintenance flap on the left front fuselage.”

Comments: A typical case of FOD caused by an aircraft`s own parts.

Example "Forgotten maintenance tools" (Ref. 5.2.1.3-6):

Excerpt: “…immediately after takeoff engine failure due to a compressor stall; cause: a screw driver in the intake duct!”

Comments: Forgotten tools in the engine are not so rare as causes of catastrophic engine damage that leads to an aircraft crash, as it did in this single-jet type. For this reason, extreme care must be taken during maintenance to ensure that no objects are left in the inlet duct in front of the engine. Because of the darkness here are pocket lamps necessary, which in other cases already have been forgotten. With this in mind, after the end of maintenance work the tool set used should be carefully checked to ensure that all of the tools used are present and accounted for (Ill. 19.2.2-5).

Birds are not the only animals that cause FOD. There is also the following bizarre example:

Example "Frogs" (Ref. 5.2.1.3-8):

Excerpt: “…Then there was the case of a couple of frogs that jumped just at the wrong moment…the (small engine type) is really good about reducing the probability of ingestion but getting those fried frogs off the inlet screen was a real challenge.

Comments: Even relatively little foreign objects are dangerous for smaller engines. A frog in the engine can completely destroy the compressor. Here this was prevented by an intake grill. However, thereby the cross section is clogged and so a compressor surge can be triggered.

Figure "Small engines": Small engines are especially sensitive to apparently harmless foreign objects due to their filigreed, thin blading. For example, in the case depicted here a cleaning rag totally destroyed the compressor of a 400 kW class helicopter engine. It cannot be said with absolute certainty that a paper tissue would not cause unallowable deformation to the blading of a small engine.

Figure "Object origin": Often parts of the foreign object (pieces of shims, nuts, wires, etc.) remain in the blading. Analysis of the material and geometry of the foreign objects can indicate their origin.
If this is inconclusive, there are other important characteristics such as the geometric contours of the impact marks, the evaluation of which is made easier if there is FOD to multiple blades.
A nick with a crescent shape and smooth contour is typical for a smooth, cylindrical foreign object.
This assumption is supported if the blade edges of subsequent stages show smaller or larger nicks of increasingly irregular shape. These nicks were created by the already deformed foreign objects and/or its fragments.
It is also possible that the trailing edges of the blading will have impact marks. This is the case if, for example, the blades of the following stage threw the foreign object forward again (Fig. "Characteristic damage"). This is a relatively frequent occurrence. The top diagram shows a bolt imprint on the inside of a light metal compressor housing. Evidently the bolt was thrown free from the compressor or was flung radially outward by the rotor blades.
Many other foreign objects also leave characteristic imprints, even if they are made from a softer material than the blades. For example, the coin left a clearly identifiable imprint on a hard Cr-steel blade.
Unique patterns such as bolt thread imprints indicate not only the type of foreign object, but measurement of the imprint can show whether it was a metric or non-metric thread, which might indicate whether the foreign object came from within the engine itself or from a different source.
Analyzable foreign object fragments can even be found outside of the compressor in the gas duct. For example, the cooling air ducts in older combustion chambers are a good place to look for foreign object residue.

Figure "Identifying foreign objects" (Ref. 5.2.1.3-9): In order to specifically prevent further foreign object damage, the source of foreign objects must be determined with as much certainty as possible. A great deal of experience and specific knowledge of the engine and periphery (inlet duct with internals, etc.) is necessary for identifying foreign objects. It is extremely important to determine whether the foreign object came from the engine itself (through a loose part, etc.) or was itself consequential damage (delaminated coatings, etc.). The foreign object may also have entered during maintenance or mounting work (forgotten or overlooked nuts or the ends of fuse wires, etc.).
There are often remnants of the foreign object left in the blading which can be analyzed to determine their source.
Metallic foreign objects leave traces in the impact area through friction which can be EDX analyzed with an REM in order to determine their composition. Comparing these results with analyses of undamaged sections (base material composition) will indicate the material of the foreign object and therefore its origin.
If it small FOD is suspected of having caused serious compressor damage, then this can be confirmed by analysis of the dynamic fatigue fracture on the blade which initiated the damage (if it is still analyzable).

Figure "Increased risk following overhaul" : Damage statistics show that the likelihood of damage increases considerably immediately after an overhaul. This behavior can be seen in the typical damage history throughout the operating time (bathtub curve, bottom diagram). The increased damage rates in new parts and after overhauls are referred to as “infant mortality”, “wear-in”, or “burn-in”. This damage also includes foreign object damage caused by forgotten tools, fastening devices, auxiliary materials, etc.
Experience has shown that during normal work on a standing engine (top left diagram) there is a special danger of foreign objects such as fuse wire trimmings falling into the uncovered engine. This kind of foreign object can slide along the conical compressor rotor hub and fall between the disks. The disk band stops the foreign objects from falling out when the engine is turned over. Then during the test run the foreign object is flung into the blading where it can create dangerous nicks.
In engines with curved inlets, such as are not unusual in helicopters (Fig. "Curved inlet ducts"), objects that are set down or fall into the inlet can only be found by feeling around for them.
The increased damage rate of the wear-in phase levels off into a constant rate (random failures) throughout the estimated safe life span.
The increase in the damage rate near the end of the life span is referred to as “wear-out”. It is caused by normal aging processes such as fatigue, corrosion, and wear.
The bathtub curve is closely related to the “overhaul philosophy”, i.e. the determined overhaul intervals. If this is “conflict oriented”, then the first overhauls will be set early in the engine`s safe life span in order to ensure that the engines are in optimal condition in case of a conflict. On the other hand, a “peace oriented” overhaul philosophy recommends that overhauls be done based on the condition of the engine parts, i.e. at the beginning of the wear-out phase.

Figure "Curved inlet ducts": Helicopters often have curved air inlet ducts with relatively small diameters (top diagram). It is difficult to see into these ducts. In the case described, a foreign object was obviously left in the duct during mounting. It finally entered the engine after about an hour of flight. The flow combined with the normal vibrations during operations and maybe even special movement by the helicopter carried the foreign object into the engine inlet. The foreign object then oscillated between the first compressor rotor stage and the inlet housing (bottom diagram), causing extensive damage in this area (Fig. "Characteristic damage").

Figure "Characteristic damage": The foreign object that caused this FOD (see Fig. "Curved inlet ducts") was identified to a high degree of certainty by typical characteristics.
The damage to the rotor disk in the first stage and the inlet struts (right diagrams) indicated a foreign object that was too large to fit through the rotor blades. The burrs on the leading edges of the rotor blades (top detail) go against the direction of rotation, which was observed in parallel cases to be typical for a foreign object oscillating against the direction of flow (Ref. 5.2.1.3-11). The geometry of the impact marks (bottom right detail) indicated a six-edged foreign object, and the high silver content at the impact areas indicated that the foreign object was silver-plated.
The combination of these characteristics was typical for a silver-plated nut with a diameter of about 8 mm. This type of nut is used in several places on the helicopter fuselage.

Figure "FOD causes in air and oil systems": Many different particles from a wide variety of sources can contaminate the flow in the air and oil systems. They can occur at all stages of an engine`s life. In many cases they can potentially cause extensive consequential damages such as bearing track fatigue (Example "") or catastrophic labyrinth failure due to self-increasing rubbing. These foreign particles can primarily be prevented by a knowledgeable, careful, and conscientious crew and stable, safe processes during maintenance, manufacture, repair, and installation.

Example "Spalling aluminium oxide facing" (Ref. 5.2.1.3-10):

Excerpt: “Combination of aluminium oxide contamination and bearing loads has been identified as the cause of bearing wear found on some….engines in service…the electron

  • microscope examination of No.4 and No.5 roller bearings found evidence of aluminium oxide entering the bearing area and causing premature spalling of bearing surfaces.

Aluminium oxide, an abrasive material, had been used…to coat air seal teeth adjacent to the bearing area in the pressurized cavity in the oil system. The material facilitates mating of air seal surfaces during initial run-in of the engine.
…The engines ranged 40-4000 hr.use. A total of 18 engines have been delivered to the airlines with the aluminium oxide coating on the seals.
A four-week investigation found that flakes of aluminium oxide averaging 0.020th (0.5mm) of an inch size were coming off the knife edges of the seals, which keep the oil system pressurized and entering the bearing cavity area…
(the engine manufacturer) also sent samples of the flakes to both airlines for comparisons.
To rectify the problem, (the engine manufacturer) is stripping the aluminium oxide coating from the affected area in all …engines, and bearing and seal clearances are being increased to provide a higher tolerance to contaminants.”

Comments: The pressure distribution in the bearing chamber should normally be designed in a way that ensures that wear products and spalled bearing seal material are blown out of the bearing chamber and cannot enter into the bearing (Ill. 23.1.2-4).

Example "Vertical take off plane" (Ref. 5.2.1.3-11):

Excerpt: ”…(the aircraft developer) has suffered another in a series of minor bug nagging setbacks with the lift fan specially built for the short take off, vertical landing version of its Joint Strike fighter demonstrator aircraft….a part apparently vibrated loose from somewhere within the lift fan assembly during testing. The part then ricocheted around the spinning unit with enough force to knock metal filings off it and possibly from moving elements of the lift fan.“

Comments: This case seems to be similar to that shown in Fig. "Characteristic damage", in which a foreign oblect was repeatedly ricochetted/thrown aroundby the spinning rotor and caused extensive damage.

References

5.2.1.3-1 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 662.

5.2.1.3-2 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 595.

5.2.1.3-3 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 92.

5.2.1.3-4 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 307.

5.2.1.3-5 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 303.

5.2.1.3-6 G. Fischbach, “916 Deutsche F-104 Starfighter, ihre Bau- und Lebensgeschichten”, page 593.

5.2.1.3-7 NTSB Identification MKC89IA024, microfiche number 37058A.

5.2.1.3-8 D.A. Lombardo, “Controlling Foreign Object Damage”, periodical “AM” , April 1997, pages 27-31.

5.2.1.3-9 P. König, A. Roßmann, “Ratgeber für Gasturbinen-Betreiber” Vulkan Publishing Essen, page 57.

5.2.1.3-10 “Investigation Team Identifies Causes of CF6-80 Problem”, periodical “Aviation Week & Space Technology”, February 7, 1983, page 32.

5.2.1.3-11 D.A. Fulghum, “JSF Lift Fan Has Another Glitch”, periodical “Aviation Week & Space Technology”, September 11, 2000 page 37.

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