AIRCRAFT INVESTIGATION BOARD
HURLBURT FIELD, FLORIDA
07 SEPTEMBER 2007
On 07 September 2007, at 2349 local time, and MH-53M helicopter, serial number (SN) 69-05794, sustained extensive damage while making an emergency landing to Landing Zone (LZ) X-ray, Eglin Range Complex, Florida, while conducting unilateral night vision goggle (NVC) alternate insertion/extraction (AIE) training. The MH-53 was assigned to the 20th Special Operations Squadron, 1st Special Operations Wing, Hurlburt Field, Florida. None of the seven crewmembers were seriously injured, the aircraft sustained $8.6M in damage, and there were no damage to private property.
Following 1.2 hours of flying training, the mishap aircraft (MA) departed Hurlburt Field at 2336 local time to conduct night vision goggle (NVC) hoist training at LZ X-ray. Twelve minutes after takeoff, the mishap crew (MC) was in a high hover at approximately 75-150 ft east of the LZ, well above all trees in the vicinity. The mishap tail scanner (MTS) sensed a worsening vibration and called for a “go around”. The mishap copilot (MCP) initiated a takeoff from the hover, and within seconds, every crewmember sensed an obvious malfunction. The mishap pilot (MP) assumed control of MA and initiated a right turn back to the LZ for an immediate landing.
It took approximately 45 seconds to return to the LZ. In that time, the MC observed abnormal oscillation in the engine and rotor instrument, commensurate with audible changes in rotor and engine speed, progressively increasing in magnitude. As the MA cleared the trees, the rotor speed had decreased to 70%. The MP lowered the collective to regain rotor speed, which put the MA into a rapid decent. At 20-25 feet above the LZ, the MP pulled up full collective to cushion the landing, which induced a rapid right yaw due to loss of anti torque effectiveness and drive. The MA was damaged extensively when it contacted the ground.
There is clear and convincing evidence this mishap was caused by the material failure of some of the mounting nuts and studs which secure the intermediate gear box (IGB) to the tail pylon of the aircraft. The loose gear box allowed the IGB internal gears to cyclically become loosely meshed (possibly unmeshed) and re-meshed, inducing an abnormal oscillation in the speed of one or both engines, which forced the crew to make an emergency landing.
Under 10 U.S.C 2254(d) any opinion of the accident investigators as to the cause of, or the factors contributing to, the accident set forth in the accident investigation report may not be considered as evidence in any civil or criminal proceeding arising from an aircraft accident, nor may such information be considered an admission of liability of the United States or by any person referred to in those conclusions or statements.