On September 15, 2018, about 1233 Pacific daylight time, a Schweizer 300CBi helicopter, N2128T, sustained substantial damage when it was involved in an accident near Granada Hills, California. The flight instructor and the student pilot were uninjured. The helicopter was operated by Group 3 Aviation, Inc., as a 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor and pilot receiving instruction were conducting an instructional flight in the helicopter, which was primarily used for flight instruction. The instructor reported that, after departure from Van Nuys Airport (VNY), Van Nuys, California, when the helicopter was 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 immediately chose to initiate a partial-power, emergency landing to 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 began the descent. The instructor flared the helicopter when it was about 20 ft agl, and it subsequently landed hard on the golf course fairway. During examination of the accident site, the tail rotor assembly, including the horizontal and vertical stabilizers, was found 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 of the blades exhibited impact damage, and the third blade was buckled near the hub. A section of the drive shaft assembly had separated from the transmission and tailboom section. During the engine examination, thumb compression tests on the Nos.1 and 4 cylinders could not obtain compression. A boroscopic examination of the No. 1 cylinder combustion chamber revealed that the piston head and chamber exhibited normal deposits. 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. A boroscopic examination of the No. 4 cylinder combustion chamber revealed that the surfaces of the piston head and chamber exhibited mechanical damage. The exhaust valve head was missing about one-third of its material. Review of the helicopter’s maintenance records revealed 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 the inspection. The bulletin describes the approved procedures for checking exhaust valve guide condition. Failure to comply with 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 which result from excessive wear (bel-mouthing) of the exhaust valve guide. The Nos. 1 through 4-cylinder compressions were reported as 76/80, 77/80, 76/80, 78/80, respectively. The helicopter was subsequently flown another 31 hours before the time of the accident.