Incident Overview
On April 18, 2016, at approximately 1224 central daylight time, a Bell 206-L4 helicopter, registration N119SP, sustained substantial damage during a training accident near Arden Hills, Minnesota. The helicopter was registered to the Minnesota Department of Public Safety and operated as a public aircraft. The pilot reported minor injuries.
The accident occurred during a Minnesota Aviation Rescue Team training mission involving personnel from the St. Paul Fire Department and the Minnesota State Patrol. The training focused on pinnacle approach and landing techniques, with firefighters practicing mounting and dismounting the helicopter skids while positioned on top of Conex shipping containers.
Sequence of Events
According to the operator, the training involved multiple approaches to shipping containers. One firefighter wore a helmet camera that partially captured the accident sequence. The footage showed the helicopter approach and land on a container. Before firefighters could mount the skids, the helicopter lifted off, moved forward away from the container, then descended and impacted terrain between containers.
The pilot reported that earlier that morning, he completed an uneventful flight from St. Paul to the training facility in Arden Hills. After landing, he shut down and participated in a pretraining briefing. Following the briefing, he started the engine uneventfully and repositioned four teams of two firefighters each before beginning elevated close-quarter loading training. The accident occurred during his seventh approach and pickup of firefighters.
The pilot stated that he made a stable straight-in approach to a shipping container while maintaining visual contact with a firefighter on his right, who signaled a thumbs-up to continue. He noted that he lost visual contact with the firefighter shortly before the helicopter settled onto the container. He then heard or felt a "bang," which he initially thought was the tail contacting the container, but a tail strike was not possible due to the helicopter's attitude and position. He described the audible "thunk" as different from normal landing skid contact. The helicopter "felt loose," and he had difficulty keeping it positioned on the container. He decided to abort the landing and climb away.
Upon increasing collective, the pilot realized the helicopter was not climbing as intended. Another shipping container was ahead, so he maneuvered right, where a hard landing was completed between containers. He recalled hearing an engine-out warning and possibly a low rotor speed alarm, though he could not recall exactly when. He did not see any master warning lights, as his attention was focused outside. As the helicopter descended, he briefly lowered collective to gain main rotor RPM, then increased collective to reduce the rate of descent before the hard landing. After the accident, he shut off the fuel and battery switch and exited.
Damage and Video Analysis
The helicopter sustained substantial damage to the lower fuselage, tail boom, and tail rotor driveshaft. Review of the helmet camera footage showed that after aborting the landing, the helicopter rotated slightly left while climbing about 10–15 feet, then rotated about 90° to the right and descended to a dirt road between three shipping containers. An audible decrease in engine speed and main rotor speed accompanied the descent. The helicopter landed hard upright, deflecting the landing skids and fracturing the tail boom aft of the fuselage. The main rotor blades struck an adjacent shipping container, causing separation of 8–10 feet of one blade.
Postaccident Examination
Examination revealed that the engine, a Rolls Royce 250-C30P (serial number CAE 895853), remained in the engine compartment with undamaged mounts. Visual inspection showed no impact damage, fire, or internal failure. Engine control continuity was established from the collective twist grip to the engine, with no unusual resistance or excessive travel. All engine B-nut fittings were hand-tight and marked with torque paint. Both N1 and N2 rotors rotated by hand without anomalies. Continuity was established between the power turbine and the power output and tail rotor output shafts. The compressor inlet showed no damage to guide vanes or impeller. Continuity was confirmed through the N1 rotor system from impeller to starter-generator cooling fan. The 4th stage turbine blades and exhaust plenum appeared undamaged. Upper and lower magnetic chip detectors showed no metallic debris. The fuel cutoff control and valve were closed, and cycling the valve showed no anomalies. The cockpit fuel gauge indicated about 575 pounds (90 gallons) of fuel remaining. The inlet line to the fuel pump contained fuel, and a sample showed no contamination.
The engine was removed and shipped to the manufacturer in Indianapolis, Indiana, for a test run. Mounted in a test stand, it demonstrated the ability to develop maximum horsepower with no anomalies that would preclude normal operation. It successfully started, ran, and shut down five times at ground idle, flight idle, takeoff power, and maximum continuous power. The engine was rapidly cycled between flight idle and maximum power numerous times without hesitation or surging, and no excessive vibrations were noted. Corrected for standard atmospheric conditions, the engine exceeded expected power output at low cruise, normal cruise, and maximum takeoff settings.
The engine's fuel control unit (FCU) and power turbine governor (PTG) were removed and bench-tested at the manufacturer. Functional testing revealed no conditions that would have precluded normal engine operation.