AD 56-09-01

56-09-01 HILLER: Applies to All UH-12 Series Helicopters.

Compliance required as indicated.

Failures of tail rotor tension-torsion (T-T) bars, P/N 55003, have occurred in the threaded portion of the bars adjacent to the Rosan inserts in the yoke. These failures caused loss of the tail rotor and directional control of the helicopter. The failures were caused by excessive stresses in the T-T bars which resulted from adverse accumulation of machining and assembly tolerances and from unfavorable orientation of the T-T bar within the blade assembly. Excessive wear of the oilite bushings in the tail rotor blade root fitting may also cause high stresses in the bars.

1. To prevent recurrences of these T-T bar failures the following actions are required before the 100-hour replacement time of the T-T bars specified in telegraphic AD dated 11-1-55 has been accumulated.

(a) Disassemble the tail rotor blade sufficiently to determine the diametral clearance between the outside diameter of the tail rotor yoke and the inside diameter of the oilite bushings within the blade root fitting. If the differences in diameters exceed 0.005 inch, replace the bushings as necessary to obtain diametral clearances of 0.001 inch to 0.003 inch.

(b) Determine the run-out of the T-T bar at the Rosan insert in the yoke and at the end block in the blade root fitting. Methods of measuring the run-out are covered in Hiller Service Bulletin No. 53.

(c) Determine the orientation of the axes of the T-T bar as originally assembled. The major axis of the T-T bar elliptical section must be 90 degrees plus or minus 15 degrees to the blade chord.

(d) If the major axis of the T-T bar elliptical section is found to be within 15 degrees of the perpendicular to the blade chord upon initial inspection and if the run-out is not greater than tolerances specified in Hiller Service Bulletin No. 53, the T-T bar may be continued in further use as provided in item 3. If one or both of the above conditions are not satisfied at the initial inspection, the T-T bar shall be replaced with a new bar at the time of this inspection.

2. Repeat the inspection for diametral clearance specified in item 1(a) at not to exceed every 150 hours of operation.

3. Replace the T-T bars at not to exceed every 600 hours of operation regardless of the conditions found in the 150-hour inspection periods.

4. Replaced T-T bars shall be mutilated to preclude further use in this application.

This supersedes AD 53-02-02 and telegraphic AD dated November 1, 1955.