What happened
On September 24, 2022, an Agusta AW139 helicopter, registration N811TA, was conducting a Part 135 passenger flight near Houma, Louisiana. Approximately seven minutes before arriving at Houma-Tollbonne Airport (HUM), the flight crew and passengers detected a burning plastic odor. While there was no initial smoke or abnormal cockpit indications, the crew deactivated the air conditioning to investigate.
Shortly after, a loud sound was followed by smoke emerging from the aft portion of the overhead circuit breaker panel. Thick orange and brown smoke quickly filled the cockpit, reducing visibility to zero. The crew simultaneously experienced a rotor low audio warning, an engine overspeed, and uncommanded movement of the cyclic and collective controls.
To clear the smoke, the pilot removed the left-side cockpit window. Once visibility was restored, the crew found that the cyclic and collective controls required significant physical force to manipulate. Despite the crew applying full body weight to keep the collective down, the helicopter climbed between 3,500 and 4,000 feet. The crew eventually managed to descend by forcibly pushing the cyclic control forward at high airspeeds.
After arriving over the destination airport, the crew performed a high-speed descent and orbit to verify controllability and landing gear extension. Due to the inability to control engine power via the collective switches, the crew used engine mode switches on the lower console to alternate engines between flight and idle modes to manage airspeed. The crew ultimately performed an autorotative landing. The helicopter landed on the runway with forward airspeed and skidding off the right side into a grass area. The pilot, co-pilot, and four passengers were not injured.
The investigation
Investigators found that the aircraft's main landing gear had collapsed during the hard landing, causing substantial damage. Examination of the wreckage revealed that electrical wire P190A6-G had been chafed through its insulation due to contact with the C3 collective torque tube. This chafing caused an electrical short and a localized fire, which resulted in thermal damage and a longitudinal split of the C3 torque tube, leading to the loss of collective control continuity.
Further analysis determined that the left-side wiring support strip assembly was incorrectly manufactured. The plastic electrical mounts were installed on the upper side of the metal strip rather than the lower side, which caused the wiring to be routed above the strip instead of under it. This misrouting placed the wires in direct contact with the metal rivets on the torque tube.
Investigation of the manufacturing process revealed that the drawing for the left-side strip assembly was ambiguous, providing only a single view that did not clearly indicate the correct orientation of the mounts. Additionally, the manufacturer's assembly instructions lacked specific requirements to verify that the wiring was routed underneath the support strips.