Boeing says there have been no reports of lap joint fatigue cracks in sections 41, 43 and 44 of the in-service Boeing 777-200 and -300 fleet, though it agrees with a new US Federal Aviation Administration (FAA) airworthiness directive (AD) aimed at detecting potential cracks in those areas with repetitive ultrasonic inspections.
The FAA on 23 July finalised an AD that requires operators to perform the inspections on fuselage skin lap splices at 34,000 cycles and repeating every 4,200 additional cycles in an effort to identify cracks that could lead to sudden decompression in flight.
Boeing tells Flightglobal however that the cracks that spawned the AD were detected during full-scale fatigue testing of a 777 airframe at Boeing, not in the active fleet.
The airframer says the AD affects 104 airplanes in the global fleet of more than 1,000 aircraft. "More specifically, this is directed to 777-200 airplanes (line numbers 1-75) and 777-200 and 777-300 airplanes (line numbers 76-104). In the US, this potentially involves only 46 airplanes," says Boeing. The company issued a service bulletin in November 2011 recommending the inspections. Under the FAA AD, the inspections will now be mandatory for US-registered aircraft.
Boeing says 34,000 cycles, the threshold for the initial inspection, represents about 30 years in service. "No operators have airplanes of that age yet," the airfamer says.
Source: Air Transport Intelligence news
