AD 56-09-01
Applicability
| Type | Manufacturer | Model | Details |
|---|---|---|---|
| aircraft | Hiller Aviation | UH-12 | Airworthiness Directives; Hiller Model UH-12 Series Helicopters |
| aircraft | Hiller Aviation | UH-12A | Airworthiness Directives; Hiller Model UH-12 Series Helicopters |
| aircraft | Hiller Aviation | UH-12B | Airworthiness Directives; Hiller Model UH-12 Series Helicopters |
| aircraft | Hiller Aviation | UH-12C | Airworthiness Directives; Hiller Model UH-12 Series Helicopters |
Unsafe Condition
Failures of tail rotor tension-torsion (T-T) bars, P/N 55003, have occurred in the threaded portion adjacent to the Rosan inserts in the yoke. These failures caused loss of the tail rotor and directional control of the helicopter, resulting from excessive stresses due to adverse machining/assembly tolerances and unfavorable T-T bar orientation within the blade assembly. Excessive oilite bushing wear may also cause high stresses.
AI-generated summary from the source AD text. Verify against the official source before acting.
Required Actions
Disassemble the tail rotor blade to check diametral clearance between yoke and bushings; replace bushings if clearance exceeds 0.005 inch to achieve 0.001-0.003 inch. Measure T-T bar run-out at yoke and blade root per Hiller SB No. 53. Verify T-T bar major axis is 90±15 degrees to blade chord. Replace T-T bars if orientation or run-out exceeds tolerances. Repeat diametral clearance checks every 150 hours and replace T-T bars every 600 hours. Mutilate replaced bars to prevent reuse.
AI-generated summary from the source AD text. Verify against the official source before acting.
Compliance Time
before the 100-hour replacement time of the T-T bars specified in telegraphic AD dated 11-1-55 has been accumulated
AI-generated summary from the source AD text. Verify against the official source before acting.
Affected Aircraft
Hiller Aviation UH-12, UH-12A, UH-12B, UH-12C helicopters
AI-generated summary from the source AD text. Verify against the official source before acting.
Source: Official FAA Source ↗
Retrieved: Apr 8, 2026
Rights: U.S. Government Public Domain
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