Schweizer 300Cbi 269C1 accident near Granada Hills, California, September 15, 2018

The NTSB's narrative final · quoted from the NTSB record

The flight instructor reported that during the instructional flight, when the helicopter was about 1,600 ft mean sea level (msl) and about 65 knots, the engine experienced “a significant and abrupt change in engine sound and power.” Shortly after, the engine rpm rose to 3,000. He immediately chose to initiate a partial-power, emergency landing on a nearby golf course instead of an autorotation because the engine was still producing partial power. He stated that it was difficult to control the throttle as he was lowering the collective and beginning the descent. He flared the helicopter when it was about 20 ft agl, and it subsequently landed hard on the level golf course fairway. During the examination of the wreckage, the tail rotor assembly, including the horizontal and vertical stabilizers, was found separated. The tail rotor blades were undamaged. The tailboom remained attached to the fuselage but had separated near the tail rotor assembly due to the upper tailboom support rod separating in overload near its midsection. All three main rotor blades remained attached to the hub assembly. Two of the blades exhibited impact damage, consistent with the blades contacting the tailboom. A section of the drive shaft had separated from the transmission and tailboom. During the engine examination, thumb compression tests, the Nos. 1 and 4 cylinders could not obtain compression. Examination of the No. 1 cylinder with a borescope revealed that the exhaust valve head exhibited thermal discoloration in three areas, consistent with combustion blowby. Cracking was visible near the valve head’s edge, and a small amount of material was missing from the head’s outer edges. Additionally, the No. 4 cylinder piston head and combustion chamber exhibited mechanical damage. The exhaust valve head was missing about one-third of its material. The last annual inspection was accomplished one month before the accident. The maintenance records indicated that a service bulletin that directed the inspection of the exhaust valves’ condition had been complied with. The bulletin indicated that failure to comply with the provisions of the bulletin could result in engine failure due to excessive carbon build up between the valve guide and valve stem resulting in sticking valves or broken exhaust valves that result from excessive wear (bell-mouthing) of the exhaust valve guide. Given the condition of the No. 1 and 4 exhaust valves, it is likely that the inspection that had been accomplished in accordance with the service bulletin was not adequate and that the damage and material loss from the valve heads resulted in the loss of compression in those cylinders and partial loss of engine power.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.