TUI B738 at Manchester on Dec 24th 2019, wheel overtakes aircraft on landing
Last Update: October 1, 2020 / 14:12:10 GMT/Zulu time
On Oct 1st 2020 the British AAIB released their final bulletin concluding the probable cause of the serious incident was:
Bearing failure investigations such as this are often inconclusive due the severity of the material damage within the bearing destroying evidence of the initiation. Therefore, it was not possible to determine the cause of the bearing failure, or to discount the possibility that there was a pre-existing fault, or the bearing had become damaged as a result of the ingress of debris or moisture.
It is possible that preload torque applied was slightly below the minimum required; however, it was still considered enough to ensure that the bearing assembly was correctly seated and makes it unlikely to have affected the bearing running condition. However, a combination of the possible causes set out in this report cannot be ruled out.
The AAIB will review the findings of the operator’s investigation into the bearing failure on G-FDZB and will provide an update to this report if it provides further clarification on the cause of the bearing failure on G-TAWG.
The AAIB described the debris trail:
Marks and debris on the runway indicated that the wheel detached approximately 1,800 m from the threshold and continued for a further 700 m before coming to rest on the grass to the right of Runway 23R, between Taxiway AF and AG. A runway inspection carried out immediately after the landing found metallic debris from the wheel hub, bearings and brake pack strewn along the runway from point P to an area where most of the debris was found, abeam Taxiway BD.
The AAIB analysed:
The right outer mainwheel (No 4) detached from its axle as a result of a failure of its wheel bearings.
Bearing operating conditions
During start up, pushback and taxi, the loaded bearing rotates slowly and gradually increases speed during the takeoff until eventually it slows and stops as the landing gear is retracted. During this period, the bearing gently warms and settles into running surrounded by a compliant lubricant.
The conditions during the landing are very different. The bearing hub and wheel assembly will have been ‘cold soaked’ at altitude and at touchdown the wheels and bearings will accelerate to landing speed with increasing load as the aircraft’s aerodynamic lift reduces.
These conditions do not normally present a problem; however, the landing conditions can exacerbate any faults or wear and lead to premature failure of the bearing assembly.
Sequence of events leading to the wheel detaching
The inboard bearing was more severely damaged than the outboard bearing indicating that the inboard bearing failed first, causing secondary damage to the outboard bearing. The severity of the damage to the inboard bearing indicates that it may have been running for a period of time in a distressed condition during which large amounts of heat were generated.
The elliptical damage to the hub, and the area where the inboard bearing cup is located, is consistent with the inboard bearing becoming loose allowing the wheel to wobble about its axle.
With a loss of wheel alignment, the outboard bearing would have deteriorated until the rollers and cage were released. Damage to the brake components show that as the bearings failed, the wheel loads were imparted into the brake components leading to the torque tube failure. The brake pistons then had nothing to react against, so fully extended under hydraulic brake system pressure. The damage to the hub and bearings was such that the wheel was able to move outwards over the outboard bearing cone, washer and nut until the wheel eventually came off the axle.
The bearings were too badly damaged to determine why they failed, but damage can be caused by:
- Insufficient or poorly applied grease
- Incorrect handling
- Incorrect type of grease
- Break down of the grease properties
- Excessive grease (not a common occurrence)
- Ingress of water, debris or other contaminants
- Incorrect assembly, by misalignment or by incorrect preload
- Sudden shock loading whilst stationary
- Overload during rotation
- Wear and gradual degradation of the rolling surfaces over time
The following aspects were considered during this investigation.
Damaged during the landing
The aircraft had not been subjected to a heavy landing since the wheel had last been fitted. Therefore, the possibility that the bearing assembly failed as a result of excessive landing loads was discounted.
Poorly prepared bearing
Pre-installation inspection of the wheel and bearing assembly, prior to being fitted to the aircraft, was carried out by the engineer. The engineer identified nothing abnormal and reported that the bearings had been greased correctly.
The severity of the damage to the bearing components make it impossible to identify any pre-existing faults, or damage, or the presence of excessive moisture in the grease. Water ingress is known to cause a rapid degradation of bearings of this type.
The inboard and outboard bearing assembly was too badly damaged to establish if any debris (metallic, grit or dust) had initiated their failure.
The importance of applying the correct preload while rotating the wheel is emphasised by the bearing and aircraft manufacturer. There was no evidence that the correct preload procedure was not applied when the wheel was fitted.
The torque wrench used (#WS3045) was calibrated and certified by the manufacturer and was required to be tested annually. It was last calibrated in January 2019 and was found to be out of calibration five weeks after the event. It is good practice to check the torque set on the torque wrench prior to use, but this is not a mandatory requirement and, as in this case, it may not always be possible to ensure that a suitable test set is readily available.
It is not known how long the torque wrench was over reading or the likely error when the wheel was fitted. If it was over reading by 11% to 12% then the actual torque applied could have been as low as 484 to 489 lbf-ft, which is slightly below the minimum requirement of 500 lbf-ft. Consequently, the compressive force applied to seat the bearings might have been 3,900 kg, which is 100 kg below the minimum requirement of 4,000 kg. However, this relatively small reduction in force is not considered sufficient on its own to have caused the bearing to fail.
This article is published under license from Avherald.com. © of text by Avherald.com.
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