LAX94LA285

The pilot was orbiting the fishing vessel when he heard a loud 'bang.' He said the aircraft pitched forward and began an uncommanded spin to the right. He lowered collective and initiated an emergency autorotation to the water. He had difficulty in maintaining control of the aircraft and the aircraft landed hard. A postaccident inspection revealed the anti-torque rotor and hub were missing from the aircraft. Witnesses reported that they saw the anti-torque blades and hub separate as a unit just prior to the accident. The pilot reported that his previous inspections had not detected any indications of a void on either blade, and that the aircraft was in compliance with applicable airworthiness directives and service information notices. A laboratory inspection of the remaining portion of the anti-torque system gearbox failed to reveal any indications of preexisting cracks on the fracture.

On July 7, 1994, at 1630 local time, a Hughes 369GS, N9242F, sustained substantial damage during a forced water landing 300 miles south of the Gilbert Islands in the South Pacific Ocean. The aircraft was owned by the Hornet Corporation and was on an aerial observation flight. Visual meteorological conditions were prevalent at the time and no flight plan had been filed for the operation. The certificated commercial pilot and the observer sustained minor injuries. The flight originated from the deck of a fishing vessel at approximately 1600 on the day of the accident.

The pilot reported that he was at 1200 feet msl, orbiting the fishing vessel to the right, when he heard a loud "bang." He stated that the aircraft pitched forward and began an uncommanded spin to the right. He lowered collective and initiated an emergency autorotation to the water. The pilot said he had difficulty in maintaining control of the aircraft and reported the aircraft landed hard.

A preliminary inspection of the helicopter revealed a broken float and substantial fuselage damage. The anti-torque rotor and hub were missing from the aircraft. A large section of the vertical stabilizer was missing. The floats prevented the aircraft from sinking and it was later recovered by crewmembers from the vessel.

The anti-torque blades, model 369A1613-503, Serial Nos. 7541 and 7546, installed on this aircraft have had a history of leading edge abrasion strips separating in flight. Witnesses to the accident reported that they saw the anti-torque blades and hub separate as a unit, just prior to the accident.

The manufacturer published a mandatory service information notice dated March 23, 1987, in which inspection of tail rotor blade leading edge abrasion strip bonding was required. On September 27, 1991, the manufacturer published a mandatory service information notice in which one-time addition of rivets to tail rotor abrasion strips was required. Both notices were applicable to the 369A1613-503 tail rotor blades. According to the maintenance records accompanying the aircraft, the aircraft was in compliance with both service information notices.

The manufacturer's representative stated that testing and experience has shown that leading edge abrasion strips are still subject to separation even after the one-time addition of rivets was applied if a total de-bonding has occurred between the tail rotor blade and the abrasion strip. It is therefore necessary for the operator to maintain a continuous inspection schedule of the abrasion strips, to detect possible voids, for as long as the model tail rotor blades identified in service information notices remain in service.

Because he was aware of the prior history of the leading edge abrasion strips, the pilot stated that he routinely performed visual inspections and tap tests on the anti-torque blades. He reported that he had not previously detected any indications of a void on either blade.

The pilot, who is also a certified airframe and powerplant mechanic, stated that he had recently tracked and balanced the anti-torque system. He reported that he maintained the anti-torque system within .05 inch pounds (ips). The manufacturer recommends balancing the anti-torque system to within .2 ips.

An inspection of the remaining portion of the anti-torque system gearbox failed to reveal any indications of preexisting cracks on the fracture. The manufacturer reported that the time to ultimate failure of the tail rotor gear box, after the separation of a leading edge abrasion strip, is approximately 30 seconds and is preceded with noticeable vibrations resulting from the unbalanced condition.