Aeroengine Safety

Fig. "Dangerous situations by air duct leaks" (Lit. 23.5.2-6, Lit. 23.5.2-9, Lit. and Lit.23.5.2-16): Although `only' air was concerned, failures with leakages at bleed air ducts/lines can have dangerous consequences. The bleed air of the rear compressor stages is needed for the cabin supply, cooling of the turbine blading and the turbine casing. At modern aeroengines it possesses pressures above 40 bar (newest aeroengines in development up to 70 bar) and temperatures up to 600 °C (example 23.5.2-5). Under these conditions at a critical crack length, an explosion like failing of the line must be expected. Even smaller leaks can overheat neghbouring parts with air temperatures of several hundred °C.

Even if no fires develops, during exit of hot gas a fire warning should be triggered. This result of a hot gas exit at least an inflight shutdown, must be expected (example 23.5.2-4 and example 23.5.2-5).
Obviously certain lines are especially susceptible.

• Electronic and elekctrical lines (example 23.5.2-4).
• Near positioned tanks for fuel or oil and hoses can during overheating fail and leak. The hot air can easily ignite these flammable liquids (example 23.5.2-6).

Fig. "Resonance vibration caused by P-clamps" (Lit. 23.5.2-11): The investigation showed, that the fuel leak occurred at a broken dumping line. A similar fracture of the line already developed at the same aeroengine during a flight three days befor! The position was the same and had the same failure features. Then the airplane returned to the depart airport. There the line was exchanged. Also at an other operator, already two parallel cases existed. However, these concerned in each case different aeroengines.

The pipe line consists of stainless CrNi steel and is screwed with a ring line (detail sketch middle left). This distributes the fuel to the injection nozzles. In the heat affected zone at the transition of the weld to the end screwing (Fig. "Pipe line crack by tensioning" and Fig. "Fatigue cracks starting from inside"), a HCF fatigue crack (volume 3, Ill. 12.6.1-6) has developed. The picture of the fracture is typical for a one sided dynamic bending load.

Three P-clamps serve as support of the line (frame below). Its position was optimised in tests by the OEM, to minimize the vibration load of the line. As interlayer/bandage wrappings with teflon tape have been used. These act damping at vibrations and should prevent fretting at the line (Ill. 23.5.2-10). All three teflon bandages showed unusual heavy wear. This partially penetrated up to the pipe surface. The ends of the tape did not lay firm at the line. They have been frazzled and took off. Obviously the wrapping was very loose. There were besides the bandages at two spots markedly wear patterns. These indicated, that also other components came in contact with the line. This argued for deviations during the laying of the line.

A check of the form of the line showed slight deviations from the nominal geometry. The cause for this remained unclear. Also the position of the clamps was slightly offset, compared with the sheduled. A check of other aeroengines showed, that such deviations frequently exist without the development of a failure.

For the understanding of the failure causing vibration load the OEM carried out tests. Especially concerned was the resonance exitations of the line. These correlated the typical low frequency, however not usual combustion chamber fluctuations (rumble, volume 3, Ill. 11.2.4-11). Such vibrations can be transferred at the pipe line from the combustor casing, as well through the pipe ends as also the P-clamps. With these tests the positions of the P-clamps have been optimized for a low vibration load. Because these improvements obviously have been estimated as low, also the pipe line geometry was changed.

Conclusion: Obviously the dynamic load of the pipe line is aeroengine individual. It depends from scatters of the exitation frequency and the exitation intensity, as well as the natural frequency of the pipe line.

Comment: Most of the attention of the OEM at the pipe line support let suggest a weak point since long time. The unusual heavy combustor vibrations obviously could not be avoided.
In this connection it should be remembered at the influence of the fuel properties at the combustor vibrations in modern aeroengines (volume 3, Ill. 11.2.2.1-4.1). Even if these are inside the specifications, they can act failure causative. An unfafourable combination of component tolerances can promote the susceptibility of an aeroengine for such vibrations. Thereby e.g., the fuel injection or the flow through cross section of the turbine nozzel behind the combustion chamber may be concerned.

Fig. "Welds in components under thermal fatigue": A weld always means also a notch (volume 3, Ill. 13-18), even if it is specification conform. It acts as shape notch. The fatigue strength of the weld material (casting structure, weak points) and/or of the heat affected zone, is also lower as of the base material. With this the deteriorating effect of a vibration load intensifies (Fig. "Fatigue cracks at pipe line weldings"). For pipe lines additional the weldseams frequently are positioned at the pipe ends. They are the connection to the screwings. These zones are especially loaded from the usual bending vibrations of the pipe lines (Fig. "Causes of pipe line vibration").

In the displayed case, it came to the fracture of a scavenge pipe (detail). So hotgas could get into the oil system and ignite there an oil fire. This continued inside the pipe over a longer distance.

The pipe is guided through a strut of the turbine exit casing to the bearing chamber. Differnces in the thermal expansion between the strut and the scavenge line, as well assembly caused tensionings can, especially to the pipe ends, trigger a dangerously high prestressing. With the vibrations of the combustion chamber, the fatigue crack can be explained.

Fig. "Danger by a rubbing safety wire" (Lit. 23.5.2-2 11 and Lit. 23.5.2-2 12): During the flight at a military aircraft it came to the extinction of the aeroengine. Cause was the leak at the distribution line to the fuel injection nozzles (sketch right). With this the fuel supply failed.

The leak developed through a fatigue fracture (sketch below left), which started at a deep wear mark (fretting, sketch above left) at the fuel line. Obviously the wear mark was caused from a vibrating safety/securing wire. How dangerous wear during contact with safety wires can be, shows also the example 23.5.2-4. In this case, safety wires from nuts have been worn through. The nuts loosened and it came to a dangerous secondary failure.

Fig. "Assembly caused tension at pipe lines" (Lit. 23.5.2-13): During the aeroengine testing it came to an emergency landing. Cause was a long circumferential crack at an aluminium elbow of the fuel supply to the high pressure pump of the aeroengine. An investigation showed:
A production failure in the region of the seal at the connection to the fuel pump lead during the assembly to the pretension of the line. The so increased medium stress lowered the usable fatigue strength (Fig. "Pipe line crack by tensioning"). This pipe line underlied pressure shocks in the fuel during the actuation of the afterburner. Such pressure shocks act because of the deflection of the fuel at the elbow of the lines (Fig. "Fatigue cracks starting from inside"). Already after fifty of such shock loads the failure occurred. With this a LCF failure was concerned (volume 3, Ill. 12.6.1-6). As reaction to this incident, a design change was carried out:

• The aluminium elbow with the, for this material typocal low fatigue strength, was replaced with a pipe from CrNi steel.
• Changing of the contour from the line. Probably a `disarming' of the deflection.
• The pump was changed in a manner, so that dangerous pressure shocks are prevented.