Eurocopter 350/550 airframe problems!!!

[blakmax]

Repair of delaminations

there is still little or no bond with the underlying structure

You are dead set right 500e. It is actually possible to repair cracks using bonded patches, but such designs rely on the bond being effective both sides of the crack. In this case as I understand the descriptions of the structure the frame has disbonded and separated from the underlying structure. (Correct me if I am wrong. A photo would help.) In that case, the repair would be bonded on one side of the crack and the frame would still remain disbonded.

There is an issues that must be addressed: Why did the structure disbond in the first place? To answer that would require examination of the disbond surfaces. There are three possibilities:

1. The adhesive has fractured through the body of the adhesive layer. This is a design/load driven failure, in which case the discussion by others about the FLIR attachment may be relevant. It also raises the question of the original certification process.
2. The composite layer has pulled off the structure or the frame. This again is a design/load driven failure, in which case the discussion by others about the FLIR attachment may be relevant. It again raises the question of the original certification process.
3. The failure is through the interface between the adhesive and the substrate (either the structure ir the frame). This is a far more serious issue that is 100% related to processing at the time of manufacture.

Many manufacturers rely on the use of sacrificial layers known as "peel plies" to remove contamination prior to bonding and that is the only preparation specified for the bonding surfaces. To enable the peel ply to be removed, the fibres in the cloth must be treated by the cloth manufacturer to provide a weak bond, otherwise the process of removing the peel ply would reult in undesirable peeling off the first ply of the composite. There are two broad methods for treating the cloth: heat scouring which creates a glazed surface on the fibres which is slick and does not bond, or coating the fibres with a release material such as silicone. Unfortunately the glazed fibres create a cast impression in the resin of a glazed surface which is also slick and will not bond. The coated fibres transfer the release material onto the bonding surfaces. (see Hart-Smith, L.J., Redmond, G, Davis, M.J., The Curse of the Nylon Peel Ply, 41st. Int SAMPE Symp. and Exhib., Anaheim, 25-28 Mar 1996.)

In both cases, unless there has been further treatment of the bonding surface, the result will be disbonds in service.
The questions are, in the manufacture of these aircraft, a. were peel plies used? b. was there any secondary bond preparation if peel plies were used?
Anyone know the answers to these questions?

While we are on the subject, Scomp, what was the preparation used before bonding the fibregalss patch?

Blakmax