History of Flight
On 13 September 2011, a Wisk-Air Limited Bell 206B (registration C-GDPE, serial number 630) was conducting local training at Thunder Bay International Airport. The crew, consisting of a student pilot and an instructor, were performing 180° autorotations to a planned power recovery using Runway 30 threshold as the landing area. During the sixth autorotation, the student initiated power recovery at about 200 feet above ground level. The instructor observed rotor rpm decaying through 94% and took control. Despite flaring aggressively, the low-rotor-warning horn sounded and remained active. The helicopter landed firmly, sliding about 5 feet. A loud clattering noise preceded the rotor striking the tail boom; the mast separated below the rotor head. The crew shut down the engine and exited without injuries. No fire occurred.
Weather
At 1600 Eastern Daylight Time, Thunder Bay reported wind 250° at 12 knots, visibility 20 statute miles, sky mainly clear, temperature 18°C, altimeter 29 inHg. Tower reported wind 260° at 14 knots gusting 21 knots. Density altitude was about 2181 feet above sea level; airport elevation 654 feet asl. Day visual meteorological conditions prevailed.
Aircraft Information
The helicopter was equipped with a Rolls Royce/Allison Model 250 C20 turbine engine and a freewheel assembly driving the main rotor transmission. Since March 2004, annual utilization averaged about 145 hours with periods of inactivity. No preservation procedures were documented for storage intervals. An overhauled turbine module was installed on 3 September 2011, and a test flight was successful. The accident flight was the first since the test flight. No outstanding maintenance items were noted.
Findings
Investigation revealed moisture had entered the transmission oil system, causing corrosion of internal freewheel components and oil system fittings. Corrosion products blocked the oil restrictor fitting, severely reducing oil flow to the freewheel assembly. Operation without adequate lubrication damaged and overheated the freewheel, impairing its function. During the power recovery, the freewheel did not engage; engine power was not transmitted to the rotor. After touchdown, the freewheel suddenly engaged, causing a torque spike that severed the main rotor mast and damaged the drive train. The crew's autorotative landing was successful, with no injuries.