Accident Details
On September 15, 2018, at about 1233 Pacific daylight time, a Schweizer 300CBi helicopter, registration N2128T, sustained substantial damage in an accident near Granada Hills, California. The flight instructor and student pilot were uninjured. The helicopter, operated by Group 3 Aviation, Inc., was conducting a 14 Code of Federal Regulations Part 91 instructional flight.
The instructor reported that after departing from Van Nuys Airport (VNY) in Van Nuys, California, when the helicopter reached about 1,600 ft mean sea level (msl), it "experienced a significant and abrupt change in engine sound and power." Shortly after, the engine rpm rose to 3,000. The instructor initiated a partial-power emergency landing to a nearby golf course, rather than an autorotation, because the engine was still producing partial power. He stated that controlling the throttle was difficult as he lowered the collective and began descent. The instructor flared the helicopter at about 20 ft agl, and it landed hard on the golf course fairway.
Aircraft Damage
Examination of the accident site revealed that the tail rotor assembly, including horizontal and vertical stabilizers, was located forward of the main wreckage area. The tail rotor blades were undamaged. The tailboom remained attached to the fuselage but had separated near the tail rotor assembly. The upper tailboom support rod had separated in overload near its midsection. All three main rotor blades remained attached to the hub assembly; two exhibited impact damage, and the third was buckled near the hub. A section of the drive shaft assembly had separated from the transmission and tailboom.
Engine Examination
During engine examination, thumb compression tests on cylinders No. 1 and 4 could not obtain compression. Boroscopic examination of the No. 1 cylinder combustion chamber showed normal deposits on the piston head and chamber. The exhaust valve head exhibited thermal discoloration in three areas, consistent with combustion blowby; cracking near the valve head's edge and a small amount of missing material from the outer edges were noted. Examination of the No. 4 cylinder revealed mechanical damage on piston head and chamber surfaces, with the exhaust valve head missing about one-third of its material.
Maintenance Records
A review of the helicopter's maintenance records indicated that the engine was installed on July 31, 2014. The last annual inspection was conducted on August 17, 2018, when the engine had 1,571 total flight hours. Service Bulletin No. 388C, "Exhaust Valve and Guide Inspection," was complied with during that inspection. This bulletin describes procedures for checking exhaust valve guide condition; failure to comply could result in engine failure due to carbon buildup, sticking valves, or broken exhaust valves from excessive wear (bel-mouthing) of the exhaust valve guide. At that inspection, cylinder compressions for Nos. 1 through 4 were reported as 76/80, 77/80, 76/80, and 78/80, respectively. The helicopter flew an additional 31 hours before the accident.