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On May 4, 2005, at 1145 Pacific daylight time, a Sikorsky S-70A, N160LA, encountered a vibration after an auxiliary power unit (APU) door separated in flight and struck a main rotor blade near Calabasas, California. The commercial pilot and two flight paramedics were not injured; the helicopter sustained substantial damage to one of the main rotor blades. The County of Los Angeles Fire Department, the registered owner, was operating the helicopter as a public-use flight under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed for the local flight, and no flight plan was in effect. The helicopter departed from the county's facility in Pacoima at 1130, and was destined for a private helicopter pad in Malibu, California.
According to the Director of Maintenance, the left APU door departed the helicopter during flight and impacted the main rotor blades. The APU door is located on the top of the helicopter, near the main rotor blades, and it is secured using two push-pin type hinges and two snap latches. Following the accident, the distorted latches were located separately in a field; each latch remained secured to portions of the door material. Both of the latches were found in the unlatched position. The forward latch was missing its outboard section, which locks the lever in the closed position; the aft latch was severely distorted. The sections of the hinges fastened to the door were not located, but both keepers remained secured to the aircraft. The APU door is opened and secured by maintenance daily and its security is to be verified by the pilot during the preflight. Although the helicopter was registered with the Federal Aviation Administration (FAA), it did not hold an FAA issued airworthiness certificate.
TESTS AND RESEARCH
The two fractured sections of left APU door were examined at Sikorsky Aircraft Company, a party to the investigation. Each section contained one of the two latches on the door. The two hinge keepers were sent to Sikorsky at a later date for examination. The remaining portions of the door and hinge assemblies were not located from the accident site.
Visual and optical examination disclosed evidence of fatigue cracks in the forward and aft keepers.
Since the pin retainers were missing, the orientation of the forward keeper could not be positively identified; however, damage sustained to the forward keeper indicated that the primary fatigue cracking likely occurred from the outboard side. The forward keeper fractured in a combination of 50 percent and 70 percent bending fatigue of the probable forward and aft pin enclosure, respectively, with the remainder of the fracture ductile overload.
The aft keeper, forward pin enclosure fatigue zone was .003-inch deep by approximately .085-inch wide from the outboard facing side of the forward fracture, and the remainder appeared to be overload. The aft keeper, aft pin retainer was cracked about 2/3 across with the pin retainer bent up 7 degrees clockwise looking aft and twisted about 66 degrees clockwise looking down.
Aluminum oxide particles were found embedded in the surface of the aft keeper, and embedded foreign material was found at two of the fatigue origins of the aft keeper's forward fracture. The fracture surfaces, and especially the fatigue origin sites, were smeared.
Hardness of the forward and aft keepers indicated they were in the TH1050 condition rather than the now required TH1100 condition. A micro hardness survey across the weld area showed that no heat treatment after welding had occurred and indicated that these hinges were procured to a previous design, not the new design, which incorporated welding and heat treating changes. Review of purchase records indicated that the helicopter was purchased with the previous hinge design and there was no recall by Sikorsky to replace the hinges with the new hinges first received by Sikorsky in December 2002.
The surface condition of the keepers appeared much rougher and textured with sharp indentations than PH 17-7 sheet stock obtained in the Sikorsky sheet metal department. That coupled with the embedded foreign material and chemical analysis which changes with depth below the surface, indicates that the processing of the subject keepers, and raw material they were produced from, probably introduced contamination which prompted fatigue initiation. Spectrographic Analysis indicated that the nickel content was above allowable to approximately 0.0015" below the surface, and the carbon and phosphorus were above maximum allowable near the surface.
Due to the damage sustained to the parts, Sikorsky was unable to confirm that the problem would have been detectable by flight crew or maintenance personnel prior to the accident flight.
According to Sikorsky, their records indicate that five helicopters designed with the subject hinges were sold for civilian use in the United States. The primary operator of the S70 (or similar versions) is the military, and they have determined that the problem is low risk and that current inspection methods are adequate for hinge flaw detection.
The general maintenance manual for the helicopter calls for a 10-hour/14-day inspection of the APU access doors. The following procedures are outlined:
1. Open APU access doors.
2. Inspect APU access doors for loose and missing fasteners, particularly at the door hinges and the center beam. Inspect latch mechanisms for cracks, wear, and security. Inspect latch mechanisms for worn or loose pins.
3. Inspect hinges on airframe and doors for cracks, wear, and security.
The military manual prescribes the same inspection procedures.
The examined parts were released to the Los Angeles County Fire Department on December 29, 2005. No parts or pieces were retained.