Aeroengine Safety

(Measures against bird strike damage are covered in Chapter 5.2.2.4)

• Identifying foreign objects is a prerequisite for specific preventive measures. Foreign objects can be identified by the geometry of the impact marks and a comparative microanalysis of the impact area (residue) and an undamaged surface.
  

• The periphery of running engines must be kept clean. Loose dense objects such as pebbles can even be sucked out of cracks and other apparently unreachable areas (or even out of running joints in the runway in the case of vortex formation!).
  

• Cleanliness and no loose part in the suction area of testing rigs. This includes cleaning rags and tools.
  

• Use of the prescribed tools and following the usual installation rules such as holding the ends of fuse wires when trimming them; confirming that all of the tools are present and accounted for immediately after finishing work.
  

• Not temporarily setting down auxiliary materials, tools, or replaced parts in the suction area, especially the inlet duct.
  

• Threaded fasteners and rivets must be inspected for completeness and they must sit tightly! If fastening devices such as pull rivets are used, the number of pull rods must be complete. When loosening fused bolts the removed fuse wires must be disposed of immediately.
  

• During assembly/installation work, especially on vertically standing engines, the compressor inlet must always be properly covered. Particles such as fuse wire trimmings cannot necessarily be removed by turning the engine and shaking it.
  

• The location of the engines on the aircraft should minimize the probability of foreign object ingestion. This refers to both sufficient height of the engine above the runway as well as the possibility of the landing gear tires throwing up foreign objects.
  

• Inlet/intake designs that allow air to be sucked in from less FOD-prone areas (such as on top of the wings of fighter aircraft) when the aircraft is near the ground.
  

• With long inlet ducts with complex configurations such as shutter systems, which are common in supersonic fighter aircraft, the designer must ensure that no parts of the adjustment mechanism can come loose.
  

• Mud guards for the hull landing gear of fighter aircraft; cargo aircraft with engines located on the rear fuselage and high sensitivity to FOD should be examined to see if a similar type of protection can be affixed to the landing gear.
  

• Proper use of thrust reversers in aircraft with engines in locations that are prone to FOD
  

• Following regulations that give the minimum distance which persons should be away from the inlet of the engine when it is running; securing objects which are usually carried loosely in clothing pockets such as cleaning rags, pens, and screwdrivers.
  

• There must be openings in the engine housing suitable for boroscopic inspections of the entire blading for foreign object strikes, as well as for doing limited reworking (fading out, hardening, etc.) without opening the housing.
  

• Using the most impact and notch resistant materials and coatings available; sufficient FOD behavior (such as a tolerable drop in dynamic strength) must be verified especially in brittle and hard blade coatings (armor, erosion protection).
  

• Engines located close to the runway may require measures against vortex formation (`vortex killer' Fig. "Vortex prevention").
  

• Solid design of the blading, which is especially sensitive to FOD. This includes relatively thick leading edges, a relatively straight profile (not curved) and a low dynamic basic load.
  

• Proper design of the engine inlet (also see Fig. "Bird strike-proof inlet design") decreases the FOD danger to the core engine decisively. This also made possible the use of a blisk in the core engine compressor of a large serial engine, which has been successful for a long time (the engine in Fig. "Optimal design of the inlet duct" is from General Electric Corporation).
  

• The fins of labyrinth seals and the seal coatings of bearing chamber seals should not contain any hard and abrasive elements (such as ceramic armor) which could cause damage if they entered the bearing (Example "Spalling aluminium oxide facing").
  The pressure ratios in and around the bearing chamber must be designed so that pressure is lower outside of the bearing chamber, ensuring that wear products are blown out.
  

• The number of compressor rotor blades should be even so that no imbalances occur when smoothing out a FOD nick on a blade; if the number is even, simply smoothing out the opposite blade will restore the balance. This procedure has proven itself in practice. However, designers often dislike even numbers of blades because these increase the probability of interference with the guide vanes and also the danger of the blading reaching resonant frequency.
  

• Sufficient inspection openings and good optical access to the blading.
  

• Easy replaceability of the blades without complex installation work or extensive disassembly of the part groups. This can be achieved through constructive measures such as axially split housings and suitable blade root fastening mechanisms.
  

• During testing rig operation, especially with field testing rigs, it must be ensured that no particles such as rust or flaking coatings come off of inlet ducts and tunnels. Any screens in front of the inlet ducts should be inspected for worn wires, since pieces of wire could break off due to a dynamic fatigue fracture.
  

• If possible, engines should not be run on the ground (for example, in field testing rigs or test runs) if there are any loose dusts or chemicals in the near vicinity (several hundred meters). Paint fumes, man-made fertilizer dust, and other materials have been shown to be damaging to compressors and hot parts.
  

• During manufacture and repair it must be ensured that any manufacturing residue such as metal shavings or auxiliary materials such as blast medium or cleaning fluids is completely removed from the engine parts. Oil and filters should be inspected for contaminants, especially in the first few hours of operation.
  

• Only the required amount of auxiliary materials such as sealants or lubricants should be used during maintenance and overhauls. Care must be taken not to press larger, excessive amounts into the inside of engine parts where they might block up the oil system or cause other problems.