A helicopter sustained substantial damage while making an emergency landing in a Waipahu field earlier this month, according to a federal report of the incident.

A 39-year-old flight instructor and her 45-year-old student pilot were the only people inside the Robinson R22 BETA helicopter when the two-seat craft landed near Kunia Road on Dec. 7, according to a National Transportation Safety Board accident report. Neither was injured.

The student was practicing autorotation — a flight maneuver that uses air rather than the engine to turn the rotors and descend — and had completed one iteration by 8:30 a.m. On a second attempt, the student was trying to recover from the autorotation about 1,000 feet in the air when the instructor felt the craft spin to the right. The instructor took over and noticed that the engine and rotor were working too hard and wouldn’t respond to her attempts to slow them down, the report said.

The helicopter continued descending, and the instructor chose a safe spot to land in a field below. During the landing, the low rotor rpm horn sounded and the helicopter touched down. After the occupants got out, they noticed the tail rotor had come off the tail boom and found it nearby.

The instructor said in a statement that she didn’t feel the impact was hard enough to cause the damage sustained by the helicopter. The helicopter, owned by Stasys Aviation Leasing of Anchorage, Alaska, was being operated by Mauna Loa Helicopters.

A cause of the mechanical malfunction was not determined in the accident report.