Bell UH1H N623PB accident near Payson, Arizona

Probable cause

A flight control malfunction due to either a hydraulic hardover or a flight control stiffness event, which resulted in a loss of control.

NTSB analysis

The helicopter was on its fourth short external load flight to reposition supplies for ground firefighting crews using a 100-ft-long line when the accident occurred. The pilot had flown a similar route for each of his three previous uneventful flights that morning. When the helicopter reached a plateau, about halfway through the flight, witnesses observed it begin to fly erratically, transitioning into a brief high nose-up pitch attitude and then wobbling and banking in a series of irregular movements before stabilizing. A few seconds later, the helicopter wobbled and banked erratically again and then entered another high nose-up attitude before it descended rapidly towards the ground. Witness statements indicated that the helicopter either banked left or right while in a high nose-up attitude just before it descended and impacted the ground.

A meteorology study found that at the time of the accident, the density altitude was about 7,980 ft; the wind was from the west-southwest about 10 knots; and sky conditions were clear. Comparison of the helicopter’s flight track at the time of the accident and the reported wind conditions indicated the helicopter likely encountered a right quartering tailwind during the first upset and a left quartering tailwind during the second (final) upset. Although the helicopter was operating in high density altitude conditions with a low forward groundspeed, the pilot had completed three successful external load lifts from the same departure point to the same destination before the accident with no reported helicopter instability. Further, although dust devils were reported by witnesses on multiple days before the accident, there were no indications from witnesses that any were present at the time of the accident. Therefore, it is unlikely that meteorological conditions contributed to the inflight loss of control.

The left lateral servo was found at near full extension, and the right lateral servo was found slightly more than half extended. These positions closely corresponded to a cyclic position of full aft and either right or left and were consistent with the extreme nose-up position of the helicopter immediately before its vertical descent to the ground. The helicopter would normally be controlled by the pilot via small fine cyclic inputs that would rarely exceed half of the full deflection of the cyclic control stick. An intentional full aft and either right or left deflection of the cyclic control by the pilot would not be consistent with the type of operation being performed with an external load, which requires smooth moderate control inputs and adjustments. The control servo positions were consistent with the preimpact attitude of the helicopter and likely indicate that the flight controls were positioned in a full aft and an undetermined lateral cyclic position and fixed there until impact.

Since it is unlikely the pilot intentionally manipulated the cyclic to produce the erratic movements displayed by the helicopter, these movements were likely the result of a hydraulic failure, flight control stiffness event, or hydraulic hardover. A light bulb analysis found that the HYD PRESSURE warning light filament was not stretched at the time of impact, which suggests that the hydraulic pressure had not dropped below 400 pounds per square inch and that the hydraulic control switch had not been turned off by the pilot; therefore, a hydraulic failure likely did not occur. A control stiffness or hydraulic hardover was likely as either could have been caused by a failure within the irreversible valve on either lateral servo. Either malfunction would have made it difficult for the pilot to operate the cyclic and maintain control of the helicopter. However, the investigation was unable to determine whether the loss of control was due to a hydraulic hardover or flight control stiffness event due to a lack of available evidence as the servo components were extensively damaged by impact and postcrash fire.

Text from the NTSB. Open the NTSB docket for WPR20LA211 for the full record.