HOUSTON, TX, USA
N650MS
MBB BO-105C
The pilot reported that during cruise flight at 800 feet MSL, a 'whine' was heard, which was 'slightly' different from the normal whine of the engines. A loud 'snap' or 'bang' was heard followed by severe vibrations. After determining a dual engine failure, the number one engine fire warning light illuminated, and the smell of smoke was noticed. An autorotation was initiated to a mall parking lot. During the autorotation, the pilot increased collective to clear trees bordering the parking lot. The helicopter touched down and slid approximately 200 feet to a stop. When the nurse opened the door to evacuate, smoke billowed into the cabin from the 'tunnel area.' After confirming that the flight crew was well clear, the pilot exited the helicopter. Examination of the helicopter revealed the drive shaft for the left engine had separated from the transmission input shaft flange. Examination of the main transmission lower housing revealed debris partially blocking the oil channel for the left side input pinion. Examination of both engines revealed internal thermal and foreign object damage.
HISTORY OF FLIGHT On September 14, 1995, at 1850 central daylight time, a MBB BO-105C, N650MS, registered to American Eurocopter, operated by Hermann Hospital Life Flight as a 14 CFR Part 91 positioning flight, was substantially damaged by an in-flight fire near Houston, Texas. The airline transport pilot and two medical technicians were not injured. The flight originated from the Hermann Hospital Helipad about 10 minutes prior to the accident and was en route to pick up a patient. Visual meteorological conditions prevailed and a company VFR flight plan was filed. The pilot reported that, during cruise flight at 800 feet MSL, a "whine" was heard, which was "slightly" different from the normal whine of the engines. Collective was reduced to determine if the whine would respond to a power change; however, there was no noticeable change. A loud "snap" or "bang" was heard followed by severe vibrations. Collective was increased slightly to determine if a single engine failure had occurred; however, neither engine responded and main rotor rpm decreased. The pilot added that the number one (left) engine fire warning light illuminated, and the smell of smoke was noticed. An autorotation was initiated to the Willowbrook Mall parking lot. During the autorotation the pilot increased collective to clear trees bordering the parking lot. The helicopter touched down and slid approximately 200 feet to a stop. The pilot told the crew to exit the helicopter and run. When the nurse opened the door to evacuate, smoke billowed into the cabin from the "tunnel area." After confirming that the flight crew was well clear, the pilot exited the helicopter. AIRCRAFT INFORMATION Historical records of the main transmission revealed two previous overhauls. Following the second overhaul, the transmission was installed in N650MS on February 25, 1994, at 5,409.9 hours total transmission time. At the time of the accident, the transmission had accumulated 724.5 hours since the last overhaul. A review of the airframe and engine records by the FAA inspector did not reveal evidence of any uncorrected maintenance defects prior to the flight; however, historical records revealed an unusual number of free wheel clutch replacements. WRECKAGE AND IMPACT INFORMATION The helicopter was removed from the parking lot and transported to Hermann Hospital Life Flight's hangar where it was secured for further examination. Examination of the helicopter revealed the drive shaft for the left engine was separated from the transmission input shaft flange. One bolt was sheared, and two bolt holes were elongated and torn out in the direction opposite of rotation. The main transmission oil line connector and pressure transducer were separated. The tail boom was covered with oil and was partially detached due to fire damage. Both engines were shipped to the engine manufacturer's facility in Indianapolis, Indiana, for further examination. The main transmission was removed and shipped to the aircraft manufacturer's facility in Grand Prairie, Texas, for further examination. TESTS AND RESEARCH An examination of the engines was accomplished on October 6, 1995. Both engines sustained internal thermal damage and foreign object damage (FOD) throughout. See enclosed excerpts of the manufacturer's teardown report. An examination of the main transmission was accomplished on October 2-4, 1995. During the borescoping inspection of one oil channel in the lower housing for the left side input pinion, a metallic object was found blocking the insertion of the borescope. This blockage appeared to obstruct about 50 percent of the passage. After several insertions of the borescope, the metallic object was bent downward, which allowed the borescope to bypass. Debris was found partially blocking the oil channel. See enclosed excerpts of the manufacturer's teardown report. Debris found blocking the oil passage and other debris found in the transmission was shipped to the National Transportation Safety Board, Office of Research and Engineering, Materials Laboratory, Washington, D.C., for further examination and evaluation. See enclosed report of their examination. ADDITIONAL INFORMATION The helicopter was released to the owner's representative.
the failure of the main transmission's #1 (left) input pinion assembly, due to the partially blocked oil passage which reduced oil flow, resulting in the disconnect of the drive shaft.
Source: NTSB Aviation Accident Database
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