What happened
During a heli-logging operation in the Comox Lake area of Vancouver Island, a Boeing Vertol BV-234 helicopter crashed while attempting to lift a heavy load. At approximately 1615 PST, ground crews attached a log, weighing an estimated 16,000 pounds, to a 250-foot long-line suspended from the aircraft. As the helicopter lifted the log two-thirds of the way off the steep terrain, the aircraft suddenly began a rapid right turn.
Over the following 5 to 10 seconds, the helicopter performed several rapid right turns and moved laterally before descending in a nearly level attitude and striking the ground. The impact, which occurred on a 60-degree slope, caused the aircraft to break into three sections. The two pilots on board sustained fatal injuries.
The investigation
Investigators examined the aircraft's flight control and hydraulic systems, as well as the engines and drive train. The investigation found no issues with the engines, the total weight of the aircraft, or the external load. The weather and terrain were also ruled out as contributing factors.
Technical examination focused on the automatic flight control system (AFCS). Investigators discovered that a specific electrical component, the U12 analog switch in the yaw axis of the No. 2 AFCS computer, had failed. This failure caused an electrical overload that sent an instantaneous signal to the No. 2 yaw extensible link actuator (ELA), causing it to extend rapidly and fully.
This sudden movement created enough force to burst the No. 2 lower stick-boost actuator (LBA) cylinder and cause the yaw connecting link to buckle and break. Because the link was fractured, the pilots were unable to use their pedals to provide corrective left-yaw input to counter the uncontrolled right turn.
Findings
- The failure of the U12 analog switch in the yaw axis of the No. 2 AFCS computer due to electrical overload initiated the sequence of events.
- The rapid extension of the yaw ELA caused the bursting of the yaw LBA cylinder and the breakage of the yaw connecting link.
- The loss of the connecting link prevented the crew from countering the right-yaw condition, leading to pilot disorientation and the subsequent impact with the terrain.
- The pilots were not wearing shoulder harnesses at the time of the accident, a practice that can increase risk during in-flight emergencies.