Aeroengine Safety

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.

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.