Airbus Helicopters Deutschland MBB-BK 117 B-2 N238BK accident near Perryville, Missouri

Probable cause

Fuel starvation due to the pilot’s failure to turn on the fuel transfer pump switches during takeoff, which led to a total loss of engine power. Contributing to the accident was the pilot's improper decision to activate the annunciator panel’s dimming function during dusk, which prevented him from seeing the illuminated fuel transfer pump caution light indicating that the pumps were off and the illuminated caution lights for low fuel in the supply tanks.

NTSB analysis

The airline transport pilot was operating an emergency medical services flight. He reported that, about 17 minutes after takeoff while in cruise flight at dusk, the helicopter "experienced a sharp change in attitude yawing to the left with a hard-upward bump," followed by a change in engine noise. He saw that the engine gas generator speed (N1) gauges for each engine were indicating below 40% and decreasing and that the No. 1 engine low warning light, the No. 1 generator light, and the battery discharge warning lights were illuminated. The pilot stated that the helicopter suddenly "pitched nose up and rolled to the right" and that he then heard the rotor speed begin to deteriorate. He entered an autorotation by applying right forward cyclic and lowering the collective to full down. During the autorotative descent, he saw power lines and a ditch, which required him to change the helicopter's flightpath and land on the far side of the ditch. He flared the helicopter about 100 ft above ground level, and the rotor speed began to decay rapidly. He stated that he attempted to level the helicopter "as it began to fall," but the helicopter landed right skid low and then skidded for about 100 ft. The main rotor blades hit the ground as the helicopter rolled onto its right side. The pilot reported that, after exiting the helicopter, he observed fuel draining in a solid stream from a fuel vent port on the helicopter's belly and that he then re-entered the cockpit and turned off all electrical and fuel switches to minimize the risk of fire.

The pilot's recollection of the accident circumstances was consistent with a dual-engine loss of power. Additionally, the damage observed on the rotor system was consistent with the engine not having power at the time of impact. The fuel transfer system between the main tanks and the supply tanks and from the supply tanks to their respective engines and the fuel delivery system functioned normally during operational testing. No residual fuel was found within the engine fuel filter bowl, indicative of no fuel reaching the engines. Based on the pilot's statement that he saw a steady stream of fuel leaking from a fuel vent port on the helicopter's belly shortly after the accident, fuel was likely present within the main fuel tanks.

Therefore, based on the evidence, it is likely that the pilot did not activate the fuel transfer pumps, which resulted in no fuel transferring between the main fuel tanks and the supply tanks and led to eventual fuel starvation. Thus, when the engines consumed all available fuel from their respective supply tanks, the dual-engine loss of power occurred.

The advisory, caution, and warning annunciator panel functioned normally during operational testing. However, postaccident examination of the helicopter revealed that the dimming function was activated, and the pilot confirmed that he dimmed the panel before takeoff. The annunciator panel contains caution lights for when the fuel transfer pumps are off and for when the fuel quantity in each supply tank is low. Illumination of these caution lights leads to the illumination of the master warning light but generates no aural tones. The ambient light at the time of the flight and the pilot's activation of the dimming function in conjunction with the night vision imaging system filters likely precluded the pilot from being able to see the illuminated caution lights on the annunciator panel and an illuminated master warning light.

Text from the NTSB. Open the NTSB docket for CEN17FA252 for the full record.