On September 14, 2007, about 1115 Pacific daylight time, a Robinson R22 Beta, N877HF, rolled over during an autorotation forced landing after experiencing a drive belt failure near El Monte, California. Universal Air Academy was operating the helicopter under the provisions of 14 CFR Part 91. The helicopter sustained substantial damage. The certified flight instructor (CFI) and student pilot were not injured. The instructional flight departed El Monte Airport (EMT), El Monte, about 1100 for a local flight. Day visual meteorological conditions prevailed, and no flight plan had been filed. In a written report to the National Transportation Safety Board, the CFI stated he departed El Monte for an introductory flight. While in cruise flight about 4.5 nm northeast of El Monte and about 1,700 feet agl, he heard a loud "bang" from the back of the helicopter. The alternator light illuminated, and the CFI initiated the alternator emergency procedure. He noticed the engine's rpm was fluctuating with the rotor rpm indicated as high and the engine rpm as low. He entered an autorotation and picked a spot to land. The helicopter landed on uneven terrain, the main rotor struck the tail boom, and it rolled over onto its left side. The Safety Board investigator, the Federal Aviation Administration investigator, and a representative from the helicopter's manufacturer examined the helicopter at Robinson Helicopter Company, Torrance, California. The two main drive v-belts were located in the drive compartment. One v-belt was positioned on the drive pulleys in the middle slot, which is not the location for normal operation. The other v-belt was located off of the drive pulleys. Both belts exhibited damage on the inside including scratch marks, material and color transfers, and gouges that were inconsistent with the pulley geometry, but fully consistent with other foreign object damage. The clutch assembly was located in the up position with the actuator fully extended, and the safety stop activated. The oil cooler sustained damage to the outer fins. Rubber transfer was located on the oil cooler, starter, oil lines, and exhaust tube on a plane with the alternator belt's rotation. The alternator belt is located directly below the lower main drive pulley. The alternator belt was not recovered. No other preimpact anomalies with either the airframe or engine were discovered.