History of the Flight
Prior to departure on a scheduled passenger flight from Birmingham to Dubai, a routine runway debris inspection was underway. The inspection began at the 'A1' hold and progressed toward the threshold of Runway 33. Before completion, the driver was instructed to exit the runway at holding point 'F1' to allow aircraft A6-EMF ('MF) to be cleared for takeoff. The next aircraft received conditional clearance to line up after 'MF's departure but stopped short of the runway, as its crew observed a large piece of debris on the runway that they believed had fallen from the departing Boeing 777. The debris was recovered and confirmed as a metallic aircraft part (Figure 1).
After being informed by ATC, the flight crew decided to continue the flight at reduced speed until the debris could be verified as coming from their aircraft. However, while climbing through FL260 at 260 kt, a vibration developed that worsened with altitude. The climb was stopped at FL280, and a diversion to London Gatwick was initiated, requiring the jettisoning of 20 tons of fuel to stay within the maximum landing weight. The landing was uneventful. Inspection revealed that panel 427AL on the inboard side of the right engine pylon had lost its outer skin (Figure 2), with no other damage.
Panel 427AL Construction
The panel is typical of many on the Boeing 777, comprising an aluminum honeycomb core sandwiched between skins of 2024 T81 aluminum. The skins are bonded using BMS 5-137 primer and BMS 5-90 adhesive, which cure at 340–360°F (171–182°C) and have a design service temperature up to 350°F (177°C). The panel is attached to the pylon by 40 screws along its periphery. The pylon area was designed for a maximum exposure temperature of 300°F (149°C).
Further Investigation
The damaged panel was returned to the operator for repair and could not be examined by the AAIB, but photographs showed apparent heat damage to its forward edge (Figure 2). The aircraft manufacturer believed the most likely heat source was a leak of core air from the engine.
In early 2003, the same aircraft experienced delamination of a similar panel, 418AR, on the inboard side of the left engine pylon, also with heat damage at its forward edge. Examination led the manufacturer to conclude the panel had been exposed to temperatures up to 350°F (177°C). While within the adhesive's service temperature range, such exposure would reduce bond strength, and combined with aerodynamic loads, high sonic vibration, and cyclic thermal stresses, would increase the likelihood of delamination.
Following the 2005 incident, the operator inspected its Boeing 777 fleet but found no other aircraft with evidence of heat damage or delamination of pylon panels.
Further Panel Delamination Event
On 3 June 2005, 'MF experienced another pylon panel delamination during takeoff at Islamabad, Pakistan. This involved panel 418AR again, similar to the 2003 event. The panel was sent to the manufacturer for examination. The operator introduced a repetitive inspection of pylon panels on this aircraft until the cause of the failures is identified. The manufacturer is not aware of any reports of problems with these panels on any other Boeing 777 aircraft.