France A388 over Greenland on Sep 30th 2017, uncontained engine failure, fan and engine inlet separated
Last Update: September 25, 2020 / 15:02:05 GMT/Zulu time
The crew took off from Paris-Charles-de-Gaulle for a flight bound for Los Angeles, onboard an A380 equipped with EA GP7270 engines. The aeroplane suffered an uncontained failure of the engine installed in the right outer position (engine No 4) while in cruise climb to FL 380, overhead Greenland. No forewarning had preceded this failure. The fan separated from the engine bringing about the separation of numerous pieces of debris. The loss of these parts followed a random path but did not cause any substantial damage to the aeroplane. During the descent, the crew were surprised that they could not hold the driftdown level calculated by the FMS.
They adopted a strategy consisting in a step down descent to finally stabilize in speed and at a flight level around 7,000 feet below the expected level, known as EO MAX FL. The operational documents do not remind flight crews that this EO MAX FL is a ceiling that is achievable with a failed engine in that is windmilling, and that it may not be achievable in other situations. Without means to estimate the reduction in performance consecutive to the observed severe damage, the crew was not able to anticipate the final stabilization level. The diversion was continued to Goose Bay aerodrome (Canada) where the aeroplane was able to land without any other difficulty.
The engine debris fell into a deserted area in Greenland and the main elements were only found around 21 months after the accident. It was only possible to determine the failure process once a hub fragment had been found. In the meantime, inspection actions concerning the fleet in service had been carried out based on the information available. The scenario of damage occurring during a maintenance operation involving the removal of fan blade lock ring had been considered the most likely.
The examination of the fan hub fragment located in Greenland found that a cold dwell fatigue phenomenon caused the development and progression of a crack in the subsurface of a blade slot bottom. Neither the manufacturer nor the certification authorities had anticipated this phenomenon in this alloy during the design of the engine.
The following factors may have contributed to the failure of the fan hub on engine No 4:
- engine designer’s/manufacturer’s lack of knowledge of the cold dwell fatigue phenomenon in the titanium alloy, Ti-6-4;
- absence of instructions from the certification bodies about taking into account macro-zones and the cold dwell fatigue phenomenon in the critical parts of an engine, when demonstrating conformity;
- absence of non-destructive means to detect the presence of unusual macro-zones in titanium alloy parts;
- an increase in the risk of having large macro-zones with increased intensity in the Ti-6-4 due to bigger engines, and in particular, bigger fans.
The BEA reported:
The examination determined that the hub failure was caused by a low cycle fatigue cracking process which originated in the part’s subsurface. The origin of the crack was located practically in the centre of slot No 10 (Figure 17), around 14 cm (5.6 inches) behind the front face of the hub and 1.4 mm (0.055 inches) below the surface of the slot bottom. No material quality (composition, microstructure) or manufacturing related anomaly was found.
The BEA analysed:
In order to guarantee the airworthiness of the other engines in operation, in the absence of confirmation of the failure mode, the manufacturer and the certification authorities required inspections of the fan hubs to be carried out just after the accident. These observations found a number of fan hubs with surface damage, giving rise to a probable scenario linked to an inappropriate maintenance operation.
The perseverance in carrying out the search operations resulted in the finding and examination of a piece of fan hub debris twenty-one months after the accident. The results of the examinations invalidated the maintenance damage scenario considered the most likely up to this point and showed a failure mode which was originally ruled out as it was considered as highly unlikely.
This failure mode had already been seen on other titanium alloys, however, no titanium Ti-6-4 hub had failed in service under cold dwell fatigue before this on commercial airplanes.
The analysis below principally concerns the failure of engine No 4. It is structured so as to explain the failure mode of the fan hub. The factors which caused this failure along with the measures implemented during the design to prevent failure in a certified operating envelope are also set out in detail.
This accident revealed a failure phenomenon which had not been observed on the titanium alloy, Ti-6-4 (cold dwell fatigue) and was difficult to anticipate. The factors conducive to its appearance are being studied by manufacturers and need to be analysed in detail.
With respect to the actual failure of engine #4 the BEA analysed:
This uncontained failure was the result of the failure of the fan hub subject to a cold dwell fatigue mechanism (see paragraph 1.18.4), originating in a macro-zone present in the subsurface of a hub blade slot (blade root housing).
The failure occurred during a normal operational phase (cruise climb) without the appearance of any precursory event. The radial (upward and downward) expelling of the two fan hub high-energy fragments at the time of the separation was random. A lateral ejection (left and right) of the same debris might have had catastrophic consequences, for instance if the airframe had been pierced or if the wing spar, control surfaces or flight control systems had been damaged.
Due to fatigue, a crack was initiated and progressed in the subsurface of a hub blade slot, opposite the blade root. This crack progressed for around 1,650 cycles, the part failing at around 3,500 cycles since new, i.e. four times earlier than the minimum life shown by the designer for this titanium part (15,000 cycles).
The metallurgical examination of the failure confirmed that no maintenance induced damage was at the origin of the start and progression of this crack. Moreover, no scheduled maintenance action, at the time of the accident, would have detected it while it was still below the surface of the slot bottom.
This article is published under license from Avherald.com. © of text by Avherald.com.
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