Accident Overview
On March 24, 2015, at approximately 1730 eastern daylight time, a Bell 206B helicopter, registration N43CM, was substantially damaged during an autorotation near Taylors Island, Maryland. The commercial pilot and one passenger were not injured. Visual meteorological conditions prevailed, and no flight plan was filed. The helicopter was operated by DC Helicopters Incorporated as a personal flight under Title 14 Code of Federal Regulations Part 91. The flight originated from Monmouth Executive Airport (BLM) in Farmingdale, New Jersey, at about 1530.
Pilot Report
The pilot reported that the helicopter was in cruise flight at 3,000 feet above ground level when there was a sudden drop in altitude followed by a 90-degree rotation to the left and a loud "bang." The pilot lowered the collective and observed that the torque gauge dropped below 10 percent. He then rolled the throttle to idle, entered an autorotation, and landed on a shore.
Postaccident Examination
A postaccident examination by a Federal Aviation Administration (FAA) inspector revealed that the helicopter's tailboom was buckled near the tail rotor gear box. Further examination showed that the tail rotor drive shaft was broken. Flight control continuity was confirmed by hand movement of the controls. The cyclic exhibited free movement in all quadrants, and the collective exhibited free up and down movement. Corresponding control movement was observed up to the main rotor hub assembly. Tail rotor pedals were moved, and control movement was seen at the tail rotor. Drive continuity was established through the drive system with movement observed to the main mast and to the tail rotor drive system. The tail rotor driveshaft on the tailboom exhibited a torsional crack. The tailboom showed evidence of tail rotor blade strikes. Both fuel boost pumps operated when the battery was turned on. The airframe fuel filter was removed and found to be filled with clean fuel, and the filter was also clean. The fuel valve was in the off position and motored on when the battery and fuel valve were switched on. The boost pumps were then energized, and a steady flow of fuel was present out the "IN" line to the airframe fuel filter.
Engine Examination and Testing
During a postaccident examination of the engine, all B-nuts and fittings were checked by hand for torque, and none were loose. The bleed valve was checked by hand for proper operation and operated smoothly with no lateral play of the poppet valve. The engine mounts were intact with no apparent deformities. The engine exhibited no evidence of having been damaged during the accident sequence. The engine cowlings were removed to facilitate examination of the compressor inlet. Examination of the compressor revealed no evidence of compressor damage or failure. Both the N1 (compressor) and N2 (power turbine) rotor systems were rotated by hand. The N1 rotated smoothly with no unusual noise or resistance. Continuity from the compressor through the auxiliary gearbox to the starter/generator was confirmed. Rotation of the N2 confirmed continuity to both the main rotor and tail rotor drives.
The engine was removed for test cell operation to attempt to replicate the reported loss of power. A pre-run examination of the engine found no anomalies or damage that might preclude running the engine on a test cell. The engine was installed on a test cell, started, ran normally, and met all serviceable engine standards. It produced normal power and responded properly to all power demands, including wave-off and sudden throttle and load changes.
Fuel Control Unit Investigation
After the engine test run, the helicopter owner sent the engine's fuel control unit (FCU) for testing. During testing, one technician found a "slight bend" in the FCU bellow's assembly. The condition of the bellows did not warrant rejection, and the FCU was reassembled and tested in accordance with manufacturer's specifications. During the test, the FCU performed erratically on the test stand and failed to meet serviceable standards. The bellows assembly was replaced with an overhauled bellows assembly, and the FCU then performed satisfactorily. The subject bellows assembly was then installed on a serviceable FCU, tested, and failed to pass serviceable standards.
Probable Cause
The investigation did not state a probable cause for the accident. The source noted that the FCU with the bent bellows failed to meet serviceable standards, but no causal link to the power loss was explicitly provided.