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INFORMATION LETTER #1997-1

Sikorsky S-55, S-58 and S-61Main Rotor Blades

SPAR CORROSION

With the problem of aging aircraft that we all face at one time or another, the blades installed on our aircraft are now giving us reason for concern.

We all shop for the best price we can and many of the blades seem to be in good condition visually. Some of these blades end up in our shop for a complete inspection but some are installed on helicopters as serviceable.

There are several conditions we think all operators need to concern themselves with and that is the reason of this INFORMATION LETTER.

When inspecting the blades for possible purchase, note the way that they are stored. The reusable aluminum containers that most of the surplus blades are stored in are about as good as it gets but it may still need very careful scrutiny. Make sure the B.I.M. system is still intact and that it has a serviceable indicator installed with a good indication. As most inspectors know, this is a very critical system installed in these blades and they do work very well. If the blade has been properly serviced, the spar has been purged with nitrogen and as most of us know, nitrogen is used to eliminate moisture, which is critical in a closed system such as this. These blades have nothing more than a thin coat of primer on the inside of the spar and some of the primer deteriorates with time and flakes off. This leaves the spar bare in some areas. As you can see from the attached illustration, this can be a very bad situation if not handled right away. Many of these blades can be salvaged if the damage is not left until the last minute.

The above is one of the reasons we X-ray the main spar, it will show internal corrosion. We also X-ray the pockets for the same reason, internal corrosion and the presence of water. If water has been absorbed in the pockets, this water will sit on the spar back wall and will render the blade unserviceable. This problem, water contamination in the pockets, will only occur on the S-55 and S-58 main rotor blades with honeycomb pockets installed, but the S-61 and S-64 still have a definite problem with internal corrosion and corrosion on the back wall. Many of the blades found have the X-ray stamp on the spar, but remember, this inspection may have been completed many years ago. We recommend that a current X-ray be completed on any surplus blade that you plan to put into service.

The next area of concern we would like to address is the pocket seals. The seals are fabricated from fully encapsulated nitrated foam. These foam seals are then sealed with a fuel resistant coating prior to installation. With the seals installed so many years ago, some have deteriorated to such a state that they do nothing more than trap moisture in these areas. If this problem is not rectified in a timely manner, corrosion will soon scrap your blade. So make sure your pocket joints are properly sealed.

David M. Stutesman
President