Ian Sheppard/LONDON

An aircraft-repair technique developed by the Sandia National Laboratories in New Mexico together with composites specialists from Textron Systems, has gained initial approval from the US Federal Aviation Administration as an alternative to riveted aluminium. The bonded composite "doubler" is the result of a three-year FAA-sponsored research and validation project by Sandia's Airworthiness Assurance Nondestructive-Inspection/Validation Center (AANC).

Traditional metal patches are relatively stiff, and riveting can further magnify stresses, while moisture can become trapped and lead to corrosion. The composite doubler, however, is flexible and adheres evenly to the aircraft skin surrounding the damage, to form a watertight repair.

A 0.25mm-thick tape of parallel boron fibre in an epoxy resin is repeatedly applied, pressed and heated to ensure adhesion as a multi-layer, laminated patch. Sandia claims that, with a uniform stress field, there is less risk of stress concentration, resulting in a repair three times tougher than an aluminium patch of equivalent thickness. In addition, it can be easily formed into whatever shape the repair demands without machining, and can be applied according to the damage with more plies (layers) in a particular direction - perpendicular to a crack, for example.

The composite doubler is still in the proof-of-concept stage, having undergone loading and harsh environment tests at the Sandia laboratory. In December 1996 its application to the door-corner of an in-service Delta Air Lines Lockheed L-1011 TriStar gained FAA approval following non-destructive testing (NDT), and the aircraft has since crossed the Atlantic several times in a 45-day period with no flaws found. It continues to fly, to ensure integrity over time.

According to the project team, the application took less than half the time of a conventional repair, and will bring many repairs within the overnight-stop timeframe for the first time, promising significant savings for airlines.

The AANC hopes eventually to test the technique for repairs to fuselage joints, landing-gear bay-doors and cargo doors to build up a generic set of applications. Sandia project engineer Dennis Roach says that a further NDT inspection and a "tap" test to check for delamination will be conducted on the L-1011 at the end of this month. Lockheed Martin is planning a service bulletin allowing other maintainers to undertake the L-1011 repair shortly, says Roach.

Source: Flight International