Metro Aviation American Eurocopter LLC AS350B2 accident

As the helicopter departed the helipad, a witness saw dark black smoke coming out of the engine exhaust. (This smoke would not have been visible to the pilot.) The pilot reported that, about 10 minutes after departure, he noticed the illumination of the low engine oil pressure warning light. He noted that the oil pressure gauge was also showing low pressure but that there was no rise in the oil temperature or the exhaust gas temperature. Because the pilot had recently experienced an intermittent oil pressure gauge problem in another helicopter, he assumed the accident helicopter was experiencing a similar problem and continued toward the intended destination while attempting to verify the loss of engine oil pressure. Several minutes later, the pilot noticed a low-torque indication but no rise in the oil temperature or exhaust gas temperature. About 25 minutes after departure, the pilot saw a rise in the exhaust gas temperature and lowered the collective to reduce power. After briefly decreasing, the exhaust gas temperature began rising again, and the pilot located an open field to perform an off-airport landing. The engine experienced a total loss of power before the helicopter reached the open field; the pilot performed an autorotation, and the helicopter landed hard.

Postaccident examination revealed that the engine oil tank was empty and that the engine had sustained heat damage consistent with overtemperature due to oil exhaustion. An obstruction of solidified oil carbon was found in the rear bearing chip detector housing union, which would have restricted the oil scavenge of the rear bearing. According to the engine manufacturer, failure to scavenge the oil in the rear bearing would cause overpressurization of the rear bearing housing area, which would force engine oil out of the rear bearing vent line. In addition, oil overpressurization could overcome the labyrinth seal of the piston shaft, which would allow oil to flow into the gas exhaust path. It is likely that this occurred because oil flowing into the exhaust path would result in black smoke coming from the engine exhaust as was observed by the witness. The continued operation of the engine in this condition likely resulted in the depletion of the oil reservoir, the illumination of the low engine oil pressure warning light, and, subsequently, as the flight continued, a total loss of engine power.

A review of data recovered from a flight recorder installed in the helicopter indicated that, about 2 minutes after departure (about 26 minutes before the accident), the oil pressure dropped from about 4 bars to about 0.2 bar, which would have illuminated the low engine oil pressure warning light. The flight manual and the cockpit emergency checklist both directed that, when the low engine oil pressure warning light was illuminated and the engine oil pressure gauge indicated low oil pressure (as reported by the pilot), the procedure was to land immediately. If the pilot had followed this procedure instead of continuing the flight with the warning light illuminated, it is likely that the accident would have been prevented.

Review of the helicopter's maintenance records indicated that, about 12 flight hours before the accident flight, the engine's 2nd-stage turbine disk (T2) was replaced. During the replacement of the T2, the maintenance technician determined that the rear bearing was "clean"; therefore, he did not clean the rear bearing even though the maintenance procedures required a systematic cleaning when the T2 was replaced regardless of the condition of the bearing. If the cleaning had been performed, as called for in the maintenance procedures, it is likely that the oil carbon deposit in the rear bearing chip detector housing union would have been discovered and removed, thus averting the accident.

Verbatim NTSB analysis and probable cause from the NTSB dataset