On July 28, 2004, about 1610 eastern daylight time, a Robinson R22 Mariner, N516PB, registered to Sky Aviation LLC, operated by Palm Beach Helicopters, experienced a hard landing at the Palm Beach County Park, Lantana, Florida. Visual meteorological conditions prevailed at the time and no flight plan was filed for the 14 CFR Part 91 instructional, local, flight. The helicopter was substantially damaged and the certified flight instructor (CFI) was not injured while the pilot-rated student (student) sustained minor injuries. The flight originated about 36 minutes earlier from the Palm Beach County Park. The CFI stated that the student had completed six power-off autorotative landings; three without assistance in preparation for a CFI checkride that was planned. The CFI took control of the helicopter to demonstrate an autorotative descent and entered the maneuver while flying at 700 feet and an indicated airspeed of 85 knots. Upon entry, the CFI set the attitude of the helicopter to maintain 60-65 knots and turned 180 degrees from the entry heading without losing altitude. During the turn he cross checked the main rotor rpm and airspeed, both remained stable and the descent rate was 1,800 to 2,000 feet-per-minute (FPM). During the final 90 degrees of the turn, the descent angle appeared steep and it was apparent to him that the actual touchdown point would be short of the intended point of landing. The CFI verbally stated that he would recover at another location and when the flight was 75-100 feet, he leveled the helicopter by rolling out of the turn and lowered the collective to maintain main rotor rpm. At that point the indicated airspeed was 65 knots, and the main rotor rpm was 98-100 percent. The flight continued and 50 feet, he increased throttle with the intention of leveling off with power applied, and verified both needles were "joined." He raised the collective control to recover and the helicopter yawed sharply to the left with an immediate loss of main rotor rpm (approximately 93 percent). He immediately applied right anti-torque pedal input and lowered the collective control. At approximately 5 feet while in a 3-5 degree nose-up attitude he applied full up collective but the helicopter contacted the ground at an estimated indicated airspeed of approximately 45 knots slightly left skid low. The helicopter rolled onto the left side pinning the student's left arm; the helicopter was lifted and the student was freed. He further reported that the engine was not operating when the helicopter came to rest. NTSB review of the Robinson R22 Beta Flight Training Guide revealed that with respect to practice forced landings, prior to passing through 100 feet, the helicopter should be aligned with the touchdown area, at 60 to 70 knots, rotor in the green range (97% to 104%), and in trim. As previously reported by the CFI, at 40 feet the rotor rpm decreased to 93% following throttle application and resulting right yaw. Postaccident examination of the helicopter the day after the accident by an inspector from the Federal Aviation Administration revealed that the carburetor heat was full on, there was evidence of an exhaust leak at the carburetor heat intake, the bracket for the carburetor heat scoop was broken and "...appeared to have been that way for some time", and the engine air filter was burnt in one area. Following recovery of the helicopter, the engine was started and operated in the airframe with no discrepancies noted. The engine was then removed from the helicopter, placed in a test cell and operated with a test club installed. The engine was started and only operated to approximately 2,550 rpm instead of 2,700 rpm. Low differential compression (5 psi) of the No. 3 cylinder was noted when testing using 80 psi. Examination of the No. 3 cylinder, piston, and piston pin, was performed by personnel from Engine Components, Inc., with FAA oversight, which revealed discolorations which are typical of high temperature operations. Wear on the exhaust valve seating face and exhaust valve guide was noted. NTSB review of the maintenance records revealed all new cylinders were installed at the time the engine was overhauled on February 15, 2004; the engine had accumulated 186.6 hours at the time of the accident. The helicopter minus the retained No. 3 cylinder with piston and piston pin was released to Christopher Dannecker, of CTC Services Aviation (LAD, Inc.), on March 2, 2005. The retained components were released to Debra Sparks, of Palm Beach Helicopters, on March 11, 2005.