What happened
On July 7, 2020, at approximately 12:13 MST, a Bell/Garlick UH-1H helicopter, registration N623PB, was destroyed during an accident near Payson, Arizona. The aircraft was being operated by Airwest Helicopters LLC on behalf of the United States Forest Service to support firefighting efforts for the Polles Fire in the Tonto National Forest. At the time of the accident, the helicopter was performing a Part 133 external load flight, transporting supplies via a 100-foot longline to reposition a ground crew.
Prior to the accident, the pilot had completed three successful loads between sites H2 and H5. During the fourth load, witnesses observed the helicopter flying erratically as it reached a plateau. The aircraft initially entered a high nose-up pitch attitude, causing the external payload to swing. After a brief period of stabilization, the helicopter exhibited another period of irregular movement, wobbling and banking errately before entering a steep nose-up attitude and descending rapidly to the ground. Witnesses noted the aircraft appeared to twist to the right during its descent. The pilot was fatally injured.
The investigation
Investigators examined the wreckage, which was highly fragmented and largely consumed by fire. The external payload was recovered 123 feet from the main wreckage. Examination of the engine showed no pre-existing conditions that would have prevented normal operation, though rotational scoring was found on the engine power output shaft. The tail rotor drive shafts No. 3 and 4 had been torsionally sheared.
Analysis of the hydraulic servos provided critical data. The left lateral servo was found at near full extension, and the right lateral servo was found slightly more than half extended. These positions corresponded to a cyclic position of full aft and either left or right lateral position, which matched the aircraft's extreme nose-up attitude before impact.
An examination of the cockpit annunciator panel revealed that the HYD PRESSURE warning light filament was not stretched, suggesting that hydraulic pressure had not dropped below 400 psi and the pilot had not turned off the hydraulic control switch prior to impact. While the internal configuration of the left lateral servo showed a bulging cap consistent with fire damage, the investigation could not definitively distinguish between a hydraulic hardover and a flight control stiffness event because the valve assemblies were destroyed.