Aerospatiale AS350 N369BD accident near Starbuck, Washington

Probable cause

A total loss of engine power due to fuel exhaustion, which resulted from the pilot’s reliance on the fuel gauge that was displaying an incorrect fuel amount due to the separation of the fuel transmitter float pin guide from the float.

NTSB analysis

The commercial pilot reported that, during the agricultural application flight in the helicopter, the fuel pressure warning light illuminated. The pilot scanned the instruments and noted that the fuel pressure was low and that the fuel quantity was indicating between 40% and 50%. Subsequently, the engine started to lose power. The pilot entered an autorotation and set up the helicopter for landing in the flattest area. Subsequently, the helicopter landed hard, which resulted in substantial damage to the tailboom and main rotor assembly.

Postaccident examination of the helicopter revealed that the fuel tank was intact, undamaged, and empty of fuel. Given this evidence, the engine likely lost all power due to fuel exhaustion.

Despite the lack of fuel, the fuel quantity gauge indicated that 42% of the fuel was remaining (or about 60 gallons). Disassembly of the fuel transmitter revealed that the float guide pin was separated from the float assembly at the tack weld. The separation of the float guide pin allowed the float to slide down the center torque shaft and prevented the potentiometer drive plate at the bottom from rotating when the fuel quantity changed, which led to the fuel quantity gauge displaying an incorrect fuel level.

A review of the airplane's maintenance records revealed that the fuel transmitter was overhauled about 11 months before the accident. Given the evidence, it is likely that, during the overhaul, maintenance personnel improperly welded the float pin guide to its mount and subsequently did not adequately inspect it, which led to its eventual separation from the assembly at the welded area.

Text from the NTSB. Open the NTSB docket for WPR17LA014 for the full record.