Jetstar B788 at Singapore on May 13th 2017, flaps asymmetry as result of damaged tyre
Last Update: August 30, 2017 / 14:23:16 GMT/Zulu time
Australia's Transportation Safety Board (ATSB) rated the occurrence an incident and opened an investigation into the occurrence reporting the aircraft sustained minor damage as result of the occurrence.
On Aug 30th 2017 the ATSB released their final report concluding the probable cause of the serious incident was:
- The number 6 wheel tyre experienced shoulder step-wear, which led to cracking and undercutting of the tyre tread and a subsequent delamination of the number 6 tyre, which occurred in less than the normal average life cycles.
- Debris from the delaminated tyre penetrated the left under wing panel and damaged the flap torque tube, resulting in an asymmetric flap condition when the flaps were commanded to retract.
- When retracting the wing flaps, the crew received a flap drive fault indication, which resulted in a return to the departure airport and a high-speed overweight landing.
- Following the damage to the flap torque tube, the aircraft protective systems operated as designed and the flight crew completed the checklist as published.
The ATSB reported the crew had set flaps 5 for departure. Climbing through 3000 feet the first officer called for flaps 1, the captain set the flaps lever to 1, however, an EICAS message "FLAPS DRIVE" occurred. The captain advised ATC, the aircraft levelled off at 6000 feet. The crew worked the checklists and decided to return to Singapore. The flight crew instructed cabin crew to prepare for the return, computed their landing performance (Vref at 195 KIAS with flaps selected to position 1). The captain decided to not dump fuel due to the proximity to other aircraft, no requirement by the checklists and the available landing distance providing sufficient margin. The aircraft landed safely back. While taxiing to the apron the crew noticed a high brakes temperature indication for the right hand brakes.
After landing ground staff informed the flight crew of damage to the left hand wing, which was further corroborated by other ground staff reporting rubber debris on the landing runway.
The tyre manufacturer reported the tyre showed evidence of shoulder step wear and a crack along the outboard serial side shoulder, that had propagated through the thread reinforcing ply.
The ATSB summarized the manufacturer's conclusions: "the shoulder step-wear allowed for a raised tread rib, which was subjected to a lateral force strong enough to tear the tread rubber with continued use. The cracking ‘propagated through the tread reinforcing ply and generated the thrown tread as the consequence’ (Figure 7 left). The number 5 wheel tyre was found with cracking on the opposite serial side shoulder (inboard of main landing gear truck), which had started to propagate through the tread reinforcing ply (Figure 7 right). They concluded that chevron cutting around the tread of both tyres indicated that they had been operated on an aggressive runway surface. When asked to clarify the reference to an ‘aggressive runway surface’, the manufacturer indicated that chevron cutting is linked to grooved runways. The forces required to accelerate the tyre to ground speed during the touchdown phase generate a tearing action, which results in the chevron cutting damage."
The ATSB analysed:
The flap drive fault the flight crew received on departure from Singapore Changi Airport was the result of the delamination of the number 6 wheel tyre tread during take-off. The airport operator found two debris fields on runway 20C, which was used for the take-off and landing. The delamination on take-off likely occurred at the southern end of the runway, when the tyre was at high speed, which provided sufficient energy for the tread to penetrate the left under wing panel and break a flap torque tube. The runway 20C northern debris field was likely the result of further tyre delamination on landing as a result of the touchdown and wheel acceleration.
The tyre manufacturer concluded that the tyre had operated over its life on a grooved runway surface, which generated chevron cutting damage. Before departure the aircraft tyres were certificated as inspected in accordance with the aircraft manufacturer’s general visual inspection requirements. While no faults were recorded for the arrival and pre-departure service checks, it was possible that the tyre shoulder was already subject to undercutting of the tread before departure. The arrival inspection likely occurred during daylight hours, but the aircraft may have been parked with the tyre tread positioned such that the initiation site was not visible to the inspector. However, this was not confirmed by the ATSB. However, the certification for the pre-departure service check was likely an acknowledgement that the tyre check was not required, rather than that it was performed.
When the flight crew moved the flap selector from the flaps setting of 5 to 1, the left inboard and outboard flaps were unable to move, due to the broken torque tube. The right flaps started to move, which generated a flap misalignment signal. The flight control electronics cabinets shut down the flap drive system in response to the misalignment and generated the FLAPS DRIVE fault message. The flight crew then completed the FLAPS DRIVE checklist. While the crew did not know about the tyre damage, the aircraft protective systems and crew actions allowed for a safe return and landing, despite the aircraft being overweight and at a higher than normal landing speed.
This article is published under license from Avherald.com. © of text by Avherald.com.
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