Sikorsky S-55, S-58 and S-61Main Rotor Blades
SPAR CORROSION
The blades installed on aging aircraft will be a concern for operators.
All operators shop for the best price and many of the replacement blades purchased may visually appear to be in good condition. Some of these replacement blades end up in our shop for a complete inspection while others are installed on helicopters as serviceable.
There are several conditions we think all operators need to be aware of.
When inspecting blades for possible purchase, note the way that they are stored. The reusable aluminum containers that most surplus blades are stored in 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 in these blades and it does work very well. If the blade has been properly serviced, the spar has been purged with nitrogen which 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 which leaves the spar bare in some areas. This can be a very deleterious situation if not addressed right away. Many of these blades can be salvaged if the damage is not left until the last minute.
The above scenario is one of the reasons we X-ray the main spar which will show any internal corrosion. We also X-ray the pockets for the same reason, to view the presence of internal corrosion or water. If water has been absorbed in the pockets, it will sit on the spar back wall and will render the blade unserviceable. This problem of water contamination in the pockets will only occur on the S-55 and S-58 main rotor blades with honeycomb pockets installed, while the S-61 and S-64 still have a definite problem with internal corrosion and corrosion on the back wall. Many of the blades we have seen have the X-ray stamp on the spar, but this inspection may have been completed many years ago. We recommend that a current X-ray be completed on any surplus blade that is planned to be put into service.
The next area of concern we would like to address are 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. If the seals were installed years ago some may 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.
