US FAA regulators have updated an earlier airworthiness directive instructing Boeing 767 operators to increase the frequency of inspections on upper wing skins to detect fatigue cracking.

The move is in response to reports of "cracking in the upper wing skin at the fastener holes common to the inboard and outboard pitch load fittings of the front spar", said the FAA.

If left unchecked, the FAA said the cracks could lead to "the loss of the strut-to-wing upper link load path and possible separation of a strut and engine from the airplane during flight".

According to its notice of proposed rule-making, 767-200, -300, -300F and -400ER aircraft would be subject to repetitive inspections every 2,000 cycles or 6,000h - down from 4,000 cycles or 12,000h.

The FAA said an operator reported finding a fastener hole with "significant crack sizes", measuring 13.4mm (0.53in) and 8mm (0.31in), on either side. The aircraft had accumulated 18,900 cycles and 89,500h, and the FAA emphasised that the cracks were found "sooner than expected".

Boeing issued a service bulletin on 2 March of this year recommending a reduced interval between inspections, which updated a 1 October 2009 bulletin recommending the first inspections on the upper wing skins.

"We support the rule proposed by FAA, which would require 767 operators to perform certain inspections of the upper surface of the wing skin," said Boeing, which added "The proposed rule essentially would mandate the recommendations that Boeing first made to operators in a service bulletin in 2009 and revised in March 2011."

As requested by Continental Airlines, the FAA clarified its inspection guidelines, recommending operators use ultrasonic inspections of both the upper surface of the upper wing skin, as opposed to the including the lower surface in its directive.

The FAA estimates that the proposed directive affects 417 N-registered aircraft, at a cost of nearly $30,000 for each inspection cycle, including labour costs. Replacement of the fastener, a hole repair or freeze plug repair will cost operators about $4,100 per repair.

Source: Air Transport Intelligence news