Aeroengine Safety

Fig. "Damage influences at multispline shafts" (Lit.23.2.2-1): Shaft couplings with multi splines are frequently used and are proven in gas turbines, especially in derivates. A big advantage is the easy assembly by pushing together. That benefits the assembly /disassembly of accessories like generators, control units and pumps. In spite of those advantages, multi splines are under unfavorable operation conditions also a potential weak point. They are primarily exposed to fretting (sliding wear). This leads to an abrasion of the loaded flank. In an extreme case, it comes to the fracture of the remaining tooth cross section and so to the failing of the coupling.

To prevent failures, an adequate lubrication is of great importance. It should be adjusted to the application.

• Lubrication oil is usual for components of the oil system.
• Leakage oil from neigboring seals (e.g., rotary shaft seals, chapter 4.2.3). This is a usual lubrication. Often this is not aware. For this reason exists the danger of disimprovement'. Endeavours, to cut also the last leakage, can lead to unexpected failures of multi splines. Do such failures occur not until longer operation times, the problem will be aggravated. Than the extent of the affected components/gas turbines is already big.
• Dry lubrication coatings (e.g., containing graphite or MoS₂ ) are used for couplings that can't be reached by oil. They ere adjusted for operation temperatures that are too high for an oil lubrication. This is frequently the case for couplings of main shafts.
• Fuel servs in shaft connections as lubrication to control units and pumps. The comparatively poor lubrication efficiency of the fuel requires an extra careful selection of the triboligical system.
  The lubrication additionally influences the wear with consistency and transport of the wear products. Mostly these are oxides, we speak about fretting remainders. Do those stay in the splining, their bigger volume, compared to steel, produces a `blasting effect'. This can favor fatigue cracks and fractures. Become the wear products transported out of the splining, the play widens and so the relative movements. Shock like loads occur (hammer wear). That boosts the wear further.

According to the lubrication, additional the materials combination of the contacting coupling teeth can be of essential significance for the wear behavior. This is to keep in mind, especially for repair coatings. If the coating in any way (e.g., pre-treatment product, deposit process) differs from specifications in the manuals, the OEM must be consulted. In case of doubt appropriate proofs are inevitable. Those require, as typical for wear processes, the relevant operation.
The existent type of wear is, not without cause, labeled as wear corrosion (fretting corrosion). The corrosion influence has great importance for the formation of the wear (abrasion) oxides. As electrolyte it comes into consideration primarily in marine atmosphere, respectively in the stand still, with sea salt contaminated condensate. The fine metallic abrasion is during formation chemically very active. It responses especially to oxidizing and corrosive influences.
Besides the lubrication conditions, the micro movements between the contacting tooth flanks play an important roll. They can have different causes, that can also act in combination:

• Alignment failure of the shaft ends.
• Vibrations of the shaft system.
• Start shocks.
• Different elasticity of the coupling components (dimensioning/design).

Causes for the crack formation in multi spline couplings are probably:

• Condition of manufacturing by grinding and/or blackening.
• Cracks by vibration fatigue can develop at wear grooves in the splining, also at the concave/bushing side of the coupling. Under dynamic torsion overload, axial cracks in the tooth root can be expected.
• High bending loads can trigger cracks in a predetermined breaking point of the neighbored shaft.

Fig. "Problems by changing a proven spline surface" (Lit. 23.2.2 -3): Concerned is a typical case of a disimprovement. It arises indeed only indirect in connection with the aeroengine, but is for multi spline couplings extremely instructive.
Few minutes before the accident witnesses observed, that the airplane flew with about 30-60 meters hight noticeable low. Suddenly a small smoke plume formed. The aircraft rolled to the left till the wings stood vertical. Then the nose of the airplane tilted downward. The aurplane hit the runway and was destroyed. The examination of the components from the left propeller showed, that the blades have been only 3° actuated, instead the dialed ca. 80°. This adjustment didn't correlate the position of the propeller adjusting spindle, poiting at an inner problem.

A following investigation of the causative concerned aeroengine showed:
The shafts of the propeller control unit (= PCU) and the propeller run concentric (sketch in the middle). PCU and propeller are connected with the oil bearing transmission tube, which runs through the propeller shaft. The torque is transferred from the outer spline toothing of the transmission tube to the multi spline coupling of the inner transmission tube from the ball spindle. So, by a spindle, the propeller is actuated.
The transmission tube has on the spline toothing to the ball spindle, instead of the previous nitriding layer, a new introduced titanium nitride coating. Its surfaces appeared dull. This can be traced back to an increased roughness. It was no alarming wear.
In contrast, the spline toothings in the PCU spindles of both aeroengines showed heavy wear. In the left aeroengine the spline toothing was almost removed (sketch below right). So the coupling parts could rotate free against each other. With this an actuating of the propeller was no more possible.

Change of the surface treatment from the outer spline toothing of the transmission:
The old version with nitriding layer had no problems. Anyway the surface treatment was changed, to facilitate the production. The nitriding layer had to be reworked after removing the epsilon layer and straightening (distortion during nitriding).
An „improvement“ with a titanium nitride coating was carried out in a product improvement program. The distortion during the old process and with this the straightening could be avoided, due to the lower coating temperature of the titanium nitride. The experiences of the producer let expect a 3-4 times lower wear in comparison to the old coating version. Comparable applications seemed to reassure this. Because of the low toothing load, the wear behaviour was evaluated as second-rank.

Test runs with the propeller actuation have been carried out with an other aeroengine type. After this the coupling teeth showed shiny golden, typical for not deteriorated titanium nitride. This finding should have shown, that the test of the wear behaviour of the coupling teeth with a different aeroengine type was insufficient.
The heavy wear of the teeth in the ball coupling was later traced back at the high hardness difference to the titanium nitride coating on the teeth of the transmission tube. The very thin and hard TiN layer could not adjust the unevenness of the tooth flanks, as it was acceptable for the former nitriding layer. Because the roughness requirements have not been adapted, the TiNlayer acted as a rasp.

Fig. "Material caused wear at a spline coupling" (Lit. 23.2.2 -4 up to -6): During rolling to the start, the right aeroengine accelerated autonomous. The associated warning light of a too high exhaust gas temperature flashed. In spite the withdrawal of the thrust, the warning light diaplayed further high thrust. All trials to decrease the thrust failed. The aircraft came damaged to rest on the runway. It was afterwards destroyed by a fire after a successful evacuation.

The following investigation of the dismounted fuel pump and the fuel control unit showed:
Control unit and pump are connect by a shaft with a multispline coupling at each side (sketch below left). Its rotational speed is proportional high pressure shaft/rotor (N₂ ). On the side of the control unit the spline toothing showed no wear at all. However the toothing at the side of the pump was nearly gone. So the parts could rotate free to each other. With this, the measuring of the rotation speed was no more possible. Even a partly loss of the N₂ signal is interpreted by the control unit as a demand to rise the speed again with an increased fuel supply. The only possibility against too high rotation speeds and exhaust gas temperatures is the cut off of the fuel. This is also possible with the „fire lever”.

Already 20 years before the accident, several toothing failures occurred at control unit shafts. These failures have been traced back to alignment failures. To clear the failure cause, at that time the producer of the pump carried out tests. These showed, that the torque at the shaft caused the misalignment (sketch below right) at the side of the pump. As remedy an axial offset was introduced with several service bulletins. How this measure avoided the wear can not be seen from the papers on hand.
In spite of this measure, in the last years according to the OEM about 40 similar cases of wear occurred. The aircraft OEM reports about further 26, also at other aircraft types (Fig. "Misalignment causing spline failure") with the same pump/control unit configuration. Even parts, which have been already changed during production, showed susceptible for wear.

There where drop outs at all flight situations like start, climb, cruise, descent and approach. A causative influence is attributed to misalignments.
There exists no fix inspection interval. Therefore several operators define these themselves. However usually a repair takes place only after a malfunction.
Some months after the accident; with a service bulletin a new shaft version is introduced. It consists of a through hardened steel (H11). With this as well the wear behavior, as also the fatigue strength should be improved. However, a proof by a test did not exist.

Fig. "Misalignment causing spline failure" (Lit. 23.2.2 -2): During run up of the aeroengine power before the start of the airplane, the left aeroengine accelerated unaccptable fast. So a fuel shut down got necessary. The nose gear got overloaded during the trial to compensate the pivoting of the airplane, caused by the asymmetric thrust - it broke. The following investigation showed, that a spline toothing of the drive to the fuel control unit was worn. So the connection got out of engagement. Now the control unit recorded an assumed too low rotation speed of the high pressure rotor (Fig. "Material caused wear at a spline coupling") and increased the fuel supply. The wear was traced back to misalignments, due to a rework caused dimensional deviation. Independent from this, the fracture of the nose gear was promoted from a fatigue crack. This crack started at a flaw , caused by too aggressive grinding during overhaul.

Fig. "Lubrication caused fretting wear": In favourable cases there are premature hints at failures on multispline couplings.
During the failure sequence in multispline couplings by wear, usually develop no larger chips, at least till the final failing. Wear products are fine iron oxides, mixed with remains of lubricant (volume 2, Ill. 6.1-3 and Ill. 6.1-4). They can be identified through an increased clearance and vibrations, caused by a failure betore the total drop out. According to the operating influences and the tribological system (material combinations, Fig. "Material caused wear at a spline coupling") it can come thereby, to different failure appearances (Fig. "Problems by changing a proven spline surface", volume 2, Ill. 6.1-3). A visual finding is naturally depending from the accessibility. This is in many cases given for according targeted maintenance work.
Does a multi spline coupling, lubricated with leakage oil, wear (Fig. "Damage influences at multispline shafts"), at the exit of the coupling red brown traces of a liquid ore past can be identified. These can show features of centrifuging (sketch left).
At dry lubrication, coating or lacking lubrication abrasion as powder develops. The red brown to black powder trickles out of the coupling toothing and disperses powder like in the neighbourhood (sketch right).

The fine wear abrasion let hardly expect in the oil system itself warning deposits at the magnetic plugs (Fig. "Monitoring particle formation in oil") or filters (Fig. "Ol filter helping for diagnistics" and Fig. "What deposits of magnet plugs can tell"). The chance for hints exists from the trend of the abrasion, caused oil contaminations (Fig. "Lubrication caused fretting wear" and Fig. "Wear particles point at problems"). Thereby new, continuous monitoring systems (Lit. 23.2.2-5) can be beneficial.
Resides the coupling in the fuel, for example in control units and pumps, an identification in time at the wear products is hardly possible.

Fig. "When leakage oil can not be missed": The lubrication of multi spline couplings from the drive shafts of accessory devices is in some cases only guaranteed by the leakage oil. Especially at older aeroengine types, where no more experiance is available, attention is demanded. Here „improvement measures” for the avoidance of leakage oil can lead to the surprisingly failing by wear. Then the failure potential is especially high. This is the case, if such a failure occurres not before a longer operation time. This means, that a multitude of aeroenbines can be concerned.

Fig. "Danger also without abrasion and chips": The wear mechanism at multi spline toothings produces very fine particles from iron oxide. In many cases, the abrasive does not get into the lubrication oil. Then a detection of the failure at the beginning is only promising with a targeted inspection during a maintenance process (Pand Fig. "Lubrication caused fretting wear" and example 23.2.2-1). Even if the wear paricles get into the lubrication oil, the chance that this attracts the attention in filters or at magnetic plugs is rather low. More promising in such a case is the trend of oil analysis (Fig. "Material specific particle content in oil" and Fig. "Wear particles point at problems").