Arrow-Falcon Exporters, Inc. UH-1H N711GH accident near Coalinga, California

Probable cause

The pilot’s inability to maintain control of the helicopter due to a hydraulic system failure for reasons that could not be fully determined due to the thermal destruction from the postimpact fire.

NTSB analysis

The pilot was engaged in external load fire-fighting operations when the accident occurred. After dropping several loads of water, the pilot reported abnormal noises and vibrations and stated "it's my hydraulics" before he lost airspeed and control of the helicopter. The helicopter made several uncontrolled rotations before colliding with terrain and erupting in flames. The wreckage was highly fragmented and most consumed by the postimpact fire.

The hydraulic system continuity could not be confirmed due to fire damage. The servos exhibited exposure to fire, including an oxide layer on the outer surface, micro-cracks, resolidified metal, and deformation damage. Foreign high-density particles were found in the hydraulic system. Hydraulic flight control stiffness and hard-over conditions are most likely a result of a malfunction of the servo irreversible valve.

Only one of the three irreversible hydraulic valves was not destroyed in the postcrash fire. Examination of this valve found high-density metal particles deep within the valve assembly and preexisting damage to the seat of a check valve. Since all the servos were operated from a single hydraulic pressure source, it is likely that the other two irreversible valves also had similar particles within their assemblies.

Foreign high-density particles and a worn check valve within the irreversible valve could have interfered with the valves’ proper operation and resulted in control stiffness or a hard-over condition. There was no evidence of preexisting mechanical damage on the three servo actuator pistons or rods as the internal moving parts within the servo actuators showed no evidence of gouging. There was no wear damage in the bushing areas or around the housing ball assembly.

Toxicological tests detected the antidepressant medication citalopram and an augmenting agent buspirone in the pilot’s system. Given that the pilot was performing complex firefighting tasks when the helicopter developed mechanical problems and he made reasonable decisions to attempt to land the helicopter, he appears to have been functioning at a high level. It appears unlikely that the pilot’s use of the antidepressants or his depression were factors in the accident.

The pilot’s declaration that he had a hydraulic problem and the final uncorrected right hand descending spiral turn flightpath are consistent with what would be expected if the helicopter experienced control stiffness or a hard-over condition. However, due to the extent of the impact and postimpact fire damage, the reason for the loss of control could not be conclusively determined.

Text from the NTSB. Open the NTSB docket for WPR20LA280 for the full record.