AD 55-19-01

55-19-01 HILLER: Applies to All UH-12, UH-12A, and UH-12B Model Helicopters.

Compliance required as indicated.

There have been several instances of cracking and failure of the tail boom rear bulkhead casting adjacent to the gearbox attachment holes. Cause of the cracking is not definitely known, but may be caused by flight loads or by damage incurred when the bulkhead is riveted to the tail boom. The following inspections are required:

(1) Prior to the next flight and every 25 hours operating time thereafter, remove the tail rotor gearbox in accordance with the pertinent Hiller Service Manual. Inspect the tail boom rear bulkhead P/N 62201 for evidence of cracking, using dye or fluorescent penetrant methods.

(2) Daily inspect the visible portions of the tail boom rear bulkhead P/N 62201 for visible evidence of cracking or failure.

(3) If cracks or other damage are found during the above inspections, the P/N 62201 (magnesium) bulkhead must be scrapped andreplaced with the P/N 62202 (aluminum) bulkhead in accordance with Hiller Service Bulletin No. 49, revised August 23, 1955, prior to further flight. If a P/N 62202 bulkhead is not available, a new P/N 62201 bulkhead may be installed provided the inspections called for in paragraphs (1) and (2) are continued.