What happened
On July 27, 2015, a Cathay Pacific Airways Boeing 747-400F, registered B-HUL, was operating a cargo flight from Hong Kong to New Delhi. The aircraft landed on runway 10 at 1240 UTC without any immediate indication of trouble in the cockpit. However, while taxiing toward the bay, air traffic controllers notified the flight crew that a part had detached from the aircraft during the landing sequence.
Upon reaching the parking stand, the crew requested a walk-around inspection. While the initial check by ground engineers showed no obvious missing parts, a subsequent inspection with the wing flaps fully extended revealed that the left wing inboard trailing edge fore-flap was missing. The separated component was later recovered from the runway, having broken into three pieces upon impact with the ground. \n## The investigation
An inquiry by the AAIB examined the structural integrity of the flap assembly and the maintenance history of the aircraft. Investigators found that the aircraft's airworthiness certificates and maintenance records were valid, and the crew was fully qualified for the operation.
Technical examination of the remaining flap structure showed significant damage, including punctures to the honeycomb skin and a deformed linkage bracket. While some critical components intended for laboratory analysis by the NTSB in the USA were lost during transit due to being mistakenly identified as scrap, investigators were able to analyze the remaining mid-flap track. This analysis revealed bending overstress but no evidence of pre-existing corrosion or fatigue cracking.
Findings
- The separation of the fore-flap was likely triggered by the failure of the outboard fore-flap inner attachment link or the inboard carriage attachment lug.
- The investigation noted that the inboard sequence carriage may have become out of phase due to a sheared detent roller.
- A significant contributing factor was the operator's non-compliance with Service Bulletin (SB) 747-27-2366. Because this SB was not mandated by the FAA or HKCAD, it had not been incorporated into the airline's maintenance program.
- The failure to implement the SB may have allowed for skewed operation of the flap assembly, potentially leading to the accumulation of debris from broken rollers, which subsequently caused the attachment fittings to fracture.