MATHER, CA, USA
N33CN
McDonnell Douglas 369D
The left seat pilot was practicing night autorotations to runway 22 to maintain his currency. After the left seat pilot completed six hovering autorotations to the ground and eight autorotations to a power recovery, the right seat pilot snapped the right collective throttle to the ground idle position at 250 feet in a climb, to simulate a loss of engine power and announced 'power failure.' The left seat pilot initiated an autorotation; neither pilot was aware of an actual loss of power. When the left seat pilot applied throttle to initiate a power recovery, the engine's compressor stalled and the 'engine out' and 'auto re-ignite' lights illuminated. The left seat pilot continued with the autorotation, landed on runway 22 with the tail-first, broke the skids, and the aircraft rolled over. The investigation revealed that the right seat pilot did not perform the throttle rigging check before departing on the accident flight as recommended by the 369D pilot handbook. Several postaccident ground runs of the engine were conducted. Based upon those, it was determined that rapid roll off of the right seat throttle could retard the throttle below ground idle and flameout the engine. The throttle misrigging had not been detected by maintenance personnel.
On November 10, 1995, at 1745 hours Pacific standard time, a McDonnell Douglas 369D helicopter, N33CN, collided with the terrain and rolled over on runway 22 at Mather Airport, Mather, California. The pilots were conducting a local visual flight rules instructional flight. The helicopter, operated by the Sacramento Sheriff's Department, Sacramento, California, sustained substantial damage. The certificated commercial pilots received minor injuries. Visual meteorological conditions prevailed. The flight originated at Mather Airport at 1715 hours. The investigating Sacramento Sheriff's Department submitted the required Pilot/Operator Aircraft Accident Report, National Transportation Safety Board form 6120.1. The reporting sergeant reported that the first pilot was practicing night autorotations to runway 22 in order to maintain currency in accordance with the Sheriff's Department Air Operations Guidelines. The right seat pilot was a performing the instructor pilot duties. He said that the first pilot performed about six hovering autorotations to the ground. The first pilot then executed four straight-in and four 180-degree autorotations with a power recovery. When flying at 250 feet above the ground, the second pilot rolled the throttle to the ground idle position while simultaneously saying "power failure." The first pilot lowered the collective and set up the autorotation profile. When the first pilot applied throttle for a power recovery, the pilots heard a loud bang and then observed that the engine-out and the auto reignite warning lights illuminated. The first pilot continued the autorotation to a landing on runway 22 that, at this time, was slightly to his left. The reporting sergeant said that the ground scars revealed the helicopter tail stinger initially touched the ground followed by the right and left skids. The left skid broke at touchdown and the helicopter rolled over on its left side. The fuselage came to rest about 100 feet from the initial touchdown point and rotated 180 degrees. The main rotor blades and tail boom assembly separated from their respective attach fittings. According to the Allison representative, the engine was operated during the course of the investigation and functioned normally. The airframe and fuel system examination disclosed no evidence of any preexisting malfunctions or failures. The rigging and entire throttle system were found to be without any obvious defects. The representative reported that the engine would flame out when the right throttle twist grip was snapped to the ground idle position. The fuel control pointer showed that the flameout occurred about 12 to 15 degrees. This condition was repeated several times with the same results. This anomaly would not occur if the right twist grip was slowly twisted to the ground idle position. The Allison representative also said both collectives, together with their associated torque tubes and bevel/sector gear boxes, were removed for examination. He said that the examination disclosed "nothing of an obvious nature was identified." According to the McDonnell-Douglas representative, the pilots told him that they did not perform the engine run-up/throttle rigging check for both throttles. These checks are required by the flight manual and are listed in the before takeoff checklist. The sergeant said that the left seat pilot performed the throttle rigging check. The second pilot told the sergeant that he could not recall if he performed the throttle rigging check. The mechanic told the sergeant that the second pilot told him that he did not perform the throttle rigging check. The flight manual states, in part: * Throttle rigging check * N2 102 percent Recheck * Pilot's twist grip Snap to Idle * If the engine flames out, refer to the HMI for proper throttle control rigging * If dual controls are installed, repeat procedure using copilot's twist grip The representative said that McDonnell-Douglas engineers consider the term "Snap to Idle" to be rapidly closing the twist grip using two fingers and a thumb. This definition is not stated in the flight manual. The representative said that disassembly and examination of the throttle assembly showed that the throttle control mechanism shaft, tube, spring pin and the pinion gear ". . .indicated wear which permitted play/slop in the throttle control system. . . ." The representative did not quantify the amount of wear, but said that could only be determined at the factory. He said that the Sheriff's Department did not give him permission to take the assembly for testing. The sergeant said that he could not allow the parts to be removed from his facility until the insurer's inspected the parts. The left collective has a mechanical stop that prevents the throttle twist grip from positioning the fuel control pointer to a point where it would shut down the engine. The right throttle twist grip does not have a mechanical stop installed.
Fuel starvation due to the right seat safety pilot's inadvertent engine shutdown when he closed the throttle to simulate an engine failure, and his failure to perform the throttle rigging check before takeoff. Contributing to the accident was the improper rigging of the throttle system coupled with the pilot's improper recovery from the autorotation.
Source: NTSB Aviation Accident Database
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