July 22, 2013
As investigations continue into the transmitter fire which severely damaged an Ethiopian Airlines 787 at London Heathrow Airport July 12, Boeing is gearing up for the most challenging aircraft repair and recovery operation it has yet faced on the new twinjet—and perhaps one of the most complex ever undertaken on a commercial composite aerostructure.
Questions remain as to how Boeing, regulators and the aircraft's owner, Ethiopian Airlines, will approach the rework. The main choices include installing a complex composite “patch” on the damaged area, replacing the entire tail section of the aircraft or even writing off the aircraft as a hull loss. According to Aviation Week's Fleets database, the damaged 787 (ET-AOP) was delivered in November 2012 and is one of four aircraft the African carrier is flying. It has six additional 787s on order.
External evidence of heat damage to the crown of the aft fuselage extends over at least 16 ft. from the base of the vertical fin forward to near the body join between the major Section 47 tail unit and the main center fuselage section. However, the charring and discoloration on the outer carbon laminate skin only hints at what the U.K. Aircraft Accident Investigation Board (AAIB) describes as “extensive heat damage” to the interior structure throughout the aft crown from the two aft doors, R4 and L4, forward.
In its July 18 report, the AAIB says the fire caused “significant thermal effects on aircraft insulation and structure. Surveying and detailed examinations of the damaged areas revealed that the greatest heat damage and highest temperatures were centered on the rear fuselage close to the crown and displaced to the left of the aircraft centerline.” It also confirms that the source of the fire coincides with the location of the aircraft's aft Honeywell emergency locator transmitter (ELT), which is mounted internally on structure close to the skin.
It is unclear whether Boeing will attempt to install an external patch in order to obtain a ferry permit and fly the aircraft to Seattle or some other location with the types of equipment needed for a final “engineered” repair, whatever that may be. Despite concerns that the fire could have led to the aircraft becoming an insurance write-off, company officials indicate that plans are underway to recover the 787. If sanctioned, these repairs will involve replacing the damaged skin section with a large composite panel or panels, already containing the co-bonded fore-and-aft stiffeners that are built into the one-piece barrel sections used in the standard fuselage assemblies. Bonded and hybrid repair schemes, which combine bonded skins and stringers with bolted sub-structures such as fuselage frames, will also be used.
Boeing developed large-scale composite skin repair techniques as a key support element of the 787 program, though likely did not anticipate dealing with such a major repair task so early in the airliner's operational life. Repair technology was a major focus during development, as composites make up 50% of the 787's structure (including the fuselage barrel sections), compared to 11% on the 777 and 3% on the 767.
Insiders also quietly note that the composite structure held up well to the fire. Compared to conventional aircraft-thickness aluminum, for which FAA tests have shown burn-through times of 30-60 sec. in intense fires, Boeing flame tests exhibited longer burn-through times for sections representative of the 787's composite laminate skin. The aircraft maker also points out that although the fire was severe enough to visibly char the exterior of the skin, the fire did not penetrate the surface.