N121PTBell UH-1H · 1964

On July 20, 2015, about 1740 Pacific daylight time, a Garlick Helicopters, UH-1H, N121PT, landed hard after the pilot sensed that the helicopter was settling while in a high hover, 10 miles east of Walla Walla, Washington. The commercial pilot received minor injuries and the helicopter was destroyed by a postaccident fire. The helicopter was registered to LRH Equipment LLC, and operated under the provisions of 14 Code of Federal Regulations Part 133, under contract to the Oregon State Department of Forestry for firefighting services. Visual meteorological conditions prevailed for the flight, which operated on a company visual flight rules flight plan.

The pilot stated to the NTSB investigator-in-charge (IIC) that he was in a 165-foot hover over a shallow creek, filling a fire bucket which was attached to the helicopter via a long line. Filling the fire bucket from the creek takes about 2 minutes. The creek bed was situated in a narrow valley with terrain rising between 200 feet and 400 feet either side of the creek. About 1 minute into the hover, the pilot sensed the helicopter start to settle. He applied collective to arrest the settling but the helicopter continued to settle. The pilot said that there was no yaw, or unusual vibration or noise during the event, only a smooth settling of the helicopter, and the engine system didn't respond when he manipulated the collective. He immediately lowered the collective, and performed a hovering autorotation toward the stream embankment. When he applied up collective for the landing, the engine did not respond. The helicopter's rotors struck a number of trees, the helicopter then turned 180o, landed hard, and rolled over onto its right side. The engine continued to run until the pilot shut off the fuel and secured the battery after the accident. The pilot egressed the helicopter and hiked out of the area.

Weather conditions at Walla Walla Regional Airport, Walla Walla, WA, which was 10 miles west of the accident site were reported by the airport operated ASOS (automated surface observation system) at 1753 PDT as wind from 250o at 14 knots, 10 statute miles visibility, clear sky, temperature was 36o C, dew point was -02o C, and atmospheric pressure was 29.83 inHg. About one hour prior to the accident the Walla Walla ASOS recorded wind from 250o degrees, at 13 knots gusting to 22 knots. An unofficial weather station located 6 miles to the southwest of the accident site recorded wind from 239o at 20 knots with gusts of 30 knots at 1743. The pilot stated that the winds in the canyon area were light and variable, and above the canyon the wind was driving the smoke from the fire to the north.

The helicopter wreckage was recovered and transported to a storage facility in Auburn, Washington. On October 1, 2015, the wreckage was examined by the NTSB IIC, the pilot, a FAA Inspector, and a technical representative from the engine manufacturer (Honeywell). The helicopter had been exposed to an extreme postcrash fire which destroyed the entire fuselage, engine case, and main transmission case. All major components of the helicopter were laid out, engine, transmission, main rotor hub & blades, tail boom, and tail rotor hub & blades. The remaining wreckage was contained in large agricultural grain bags. The grain bags were emptied one at a time and sorted through by hand. The exam verified that the majority of the helicopter was accounted for, however, the fuel control, droop compensator and half of the KaFlex shaft were not accounted for and presumed to either have been destroyed by the fire or remnants remained at the accident site. Complete accounting of all elements and/or components of the helicopter was not possible. There was no evidence of mechanical malfunctions or failures of the flight control system, main transmission, or rotor systems that would have precluded normal helicopter operation. The engine components were sent to Honeywell, Phoenix, AZ, for further examination.

On December 9, 2015, a tear down and inspection of the engine components was conducted at the Honeywell facility in Phoenix, AZ, under the oversight of the NTSB IIC. Honeywell determined that the gas producer section of the engine blade failures were the result of short term overheating beyond the capacity of the blade material accompanied with high rotational speeds resulting in stress rupture of the blades. Metallic splatter was adhered to the combustion chamber deflector. Examination of the compressor section revealed that all of the blades of the axial compressor rotor section were either separated at the platform or were missing from the assembly. The axial compressor vane assemblies were fractured and bent. There was brown earthen debris adhered to the flow path surfaces and vanes of the centrifugal compressor impeller. Several vanes of the centrifugal compressor were bent at the impeller inlet opposite the direction of rotation. There was leading edge damage of all of the vanes at the inlet to the centrifugal compressor impeller. The shroud line edge of all of the centrifugal compressor impeller vanes displayed rotational scoring at the exducer end with corresponding rotational scoring to the impeller shroud. There was earthen debris adhered to the centrifugal compressor shroud. Honeywell concluded that the type and degree of damage was indicative of an engine that was rotating and operating at the time of impact. No pre-existing condition was found that would have prevented normal operation.

Orographic wind affects are discussed in AC-00-57, Hazardous Mountainous Winds and Their Visual Indicators. The following was extracted from Section 3.3 Low-Level Mountain Flying, " Aircraft engaged in low-level flight operations over mountainous terrain in the presence of strong winds (20 kt or greater at ridge level) can expect to encounter moderate or greater turbulence, strong up-and-down drafts, and very strong rotor and shear zones. This is particularly true for general aviation aircraft."