BOEING IS notifying operators of a fleet-wide structural strengthening programme for 757 and 767 engine mounts, following reports of cracking in strut boxes and fuse pins.
The programme, which will affect more than 1,200 aircraft in service, will be explained to operators on 11/12 July and is likely to be implemented over more than ten years. Modification kits for 757s are expected to be available in early 1996, slightly ahead of kits for the 767.
"It's all part of a structural re-assessment, a lot of it energised in 1990 after we had fatigue cracking on a [Rolls-Royce] RB.211-powered 767 strut, and afterwards on the 747," says Boeing structures engineering director, Jack McGuire.
Boeing still has to work out details of the programme, including differences between changes to the production line and design of upgrade kits.
"Problems then started to show up on the 757 and 767, with cracking of fuse pins and some other components, so we decided we needed these modifications," says McGuire. Changes will involve strengthening the existing structure with thicker gauge materials, rather than adding more structure, as in the 747 modification. He says that the move is aimed at reducing stress levels in the struts, to avoid cracking, and to remove the need for Boeing to make additional supplementary inspections as aircraft age. "It will also improve the damage tolerance of the structure," he adds.
The changes will also be incorporated into new aircraft on the production line from 1996 onwards. In most cases, the modifications concentrate on strengthening of the box beam and fittings on the back of the strut, which form the lugs for the fuse pins. The diagonal strut or drag brace (which takes the thrust load) will also be strengthened, as will the upper link connecting the topside of the strut to the wing surface. Some parts made from titanium will be replaced with steel.
Cracking has been reported on 757 and 767 fuse pins, 757 strut boxes and 767 mid-spar fittings. The 757 cracking is "...mainly because of inertia factors caused by gusts acting on the aircraft, and mainly by landings. The 767 is similar, except we haven't noticed so much inertia on landings," says McGuire.
Although the cost of the programme to Boeing remains undisclosed, the manufacturer says that it will pay for the upgrade kits, and share labour costs of the modifications which will be performed during scheduled "D" checks.
"We're getting way ahead of the game," says McGuire, who notes that the changes will supersede seven 757 service bulletins, including two airworthiness directives (ADs) and four 767 service bulletins, one of which is an AD.