On September 17, 2004, about 1445 Hawaiian standard time, a Robinson R22 Beta, N22XM, experienced a hard landing at the Kona International Airport, Kona, Hawaii. Hawaii Pacific Aviation was operating the helicopter under the provisions of 14 CFR Part 91. The certified flight instructor (CFI) and student pilot were not injured; the helicopter sustained substantial damage. The local instructional flight departed Kona about 1330. Visual meteorological conditions prevailed, and a flight plan had not been filed. In a written statement, the CFI stated that he and the student were both on the controls while executing a practice autorotation. As the helicopter's main rotor revolutions per minute (rpm) began to decrease, the CFI raised the collective in an effort to show the student a gentle and anticipated rpm drop. While the rpm needle moved to the bottom of the green arc, with the helicopter descending through about 200 feet above ground level (agl), he noticed that the airspeed had decreased to about 55 knots. He informed the student that they were going to execute a go-around procedure, because the airspeed should be at 65 knots when performing autorotations. The CFI further stated that the low rpm horn began to sound and he lowered the collective while pushing the carburetor heat control knob in the "off" position. Realizing that he did not adequately push the carburetor heat control knob down, the CFI leaned forward to look down at the exact knob position. While assessing the knob position, he felt the helicopter begin to settle and looked up at the instrument panel; he noted the rpm at 94 percent with the airspeed at 53 knots. With the ground approaching quickly, he flared the helicopter and subsequently opted to configure it in a level attitude because he feared that the tail boom would contact the surface first. The helicopter hit the surface hard and the main rotor severed the tail boom. The CFI stated that he did not know of any mechanical malfunctions or failures with the helicopter prior to impact.