AD 56-13-02
Applicability
| Type | Manufacturer | Model | Details |
|---|---|---|---|
| aircraft | Sikorsky Aircraft Corporation | S-51 | Airworthiness Directives; Sikorsky Model S-51 Helicopters |
Unsafe Condition
Main rotor hubs may develop cracks at the root radius of the threads on the outer arm due to fatigue, leading to failure of the main rotor hub assembly.
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Required Actions
Replace main rotor hubs S10-10-1015 and S510117 every 480 hours. Conduct Magnaflux inspections at 200-240 hour intervals. Replace S10-10-1033 hubs with 1/16 inch or larger chamfer by September 15, 1956, or before 480 hours, whichever comes first. Check S510122 locknuts every 30 hours for looseness; inspect for cracks if loose. Replace hubs with cracks or exceeding 480 hours. After September 15, 1956, replace all S10-10-1033 hubs every 240 hours. Hubs with chamfer less than 1/16 inch must be replaced every 960 hours with inspections every 200-240 hours.
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Compliance Time
Compliance required as indicated in the directive (e.g., by September 15, 1956, or prior to specified flight hour limits).
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Affected Aircraft
Sikorsky Model S-51 Helicopters with main rotor hubs S10-10-1015, S510117, S10-10-1033, and S510122 outer arm locknuts.
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Source: Official FAA Source ↗
Retrieved: Apr 8, 2026
Rights: U.S. Government Public Domain
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