56-13-02 SIKORSKY: Applies to All Model S-51 Helicopters.
Compliance required as indicated.
As a result of recent service experience, the main rotor hubs are to be retired from service at the times indicated.
1. Main rotor hubs, S10-10-1015 and S510117, are to be replaced every 480 hours. Magnaflux inspections are to be conducted at the 200-240 hour interval.
2. Main rotor hubs, S10-10-1033, with 1/16 inch or larger chamfer at the base on the outer edge of each arm, with 240 hours of service, must be replaced by September 15, 1956, or prior to the accumulation of 480 hours, whichever occurs first. The S510122 outer arm locknuts are to be checked for looseness every 30 hours on hubs continuing in service beyond 240 hours. With any indication of looseness, the hub is to be removed and a Magnaflux inspection for cracks at the root radius of the threads on the outer arm is to be conducted. Any hub with a crack is to be replaced. Hubs with 480 hours or more must be replaced prior to further service. After September 15, 1956, all hubs are to be replaced every 240 hours.
3. Main rotor hubs, S10-10-1033, with chamfer less than 1/16 inch at the base on the outer edge of each arm are to be replaced every 960 hours. Magnaflux inspections are to be conducted every 200-240 hours of service.
(Service Information Circular No. 76 Revisions D & F cover the same subject.)
This supersedes AD 49-44-01.