Thursday, March 29, 2018

Bell UH-1H, N121PT: Accident occurred July 20, 2015 in Walla Walla County, Washington

Analysis

The commercial pilot reported that the helicopter was in a 165-ft hover in a narrow creek valley, over a shallow creek and that he was filling a fire bucket, which was attached to the helicopter via a long line. The terrain rose between 200 and 400 ft on either side of the creek. About 1 minute into the hover, the pilot felt 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 that the engine did not respond when he manipulated the collective. He immediately lowered the collective and performed a hovering autorotation toward an embankment. When the pilot raised the collective for the landing, the engine did not respond. The rotors struck several trees, then the helicopter spun 180°, landed hard, and rolled over onto its right side. After the accident, the engine continued to run until the pilot shut off the fuel and secured the battery. The helicopter was destroyed by postcrash fire.

Postaccident examination of the helicopter did not reveal any evidence of preimpact mechanical malfunctions or failures that would have precluded normal operation. Examination of the engine revealed compressor blade damage, rotational scoring witness marks, and metal splatter, consistent with the engine operating at the time of the accident.

An automated surface observation system (ASOS) located 10 miles west of the accident reported that, about 1 hour before the accident, the wind was from 250°, gusting to 22 knots. An unofficial weather station located 6 miles southwest of the accident site recorded that, about 3 minutes before the accident, the wind was from 239o at 20 knots, gusting to 30 knots. The ASOS reported that, about 13 minutes after the accident, the wind was from 239o at 14 knots. Given these conditions, orographic wind turbulence could not be ruled out as a factor. 

Probable Cause and Findings

The National Transportation Safety Board determines the probable cause(s) of this accident to be:
The helicopter's reduction in aerodynamic lift while operating in variable wind conditions for reasons that could not be determined because postaccident examination did not reveal any evidence of preimpact mechanical malfunctions or failures that would have precluded normal operation. 

Findings

Environmental issues
Variable wind - Effect on operation (Cause)
Tree(s) - Contributed to outcome
Mountainous/hilly terrain - Effect on operation

Not determined
Not determined - Unknown/Not determined (Cause)

Factual Information

History of Flight

Maneuvering-hover
Loss of lift (Defining event)
Attempted remediation/recovery

Autorotation
Off-field or emergency landing

Landing
Collision with terr/obj (non-CFIT)
Roll over

Post-impact
Fire/smoke (post-impact)

The National Transportation Safety Board did not travel to the scene of this accident.

Additional Participating Entity:
Federal Aviation Administration / Flight Standards District Office; Spokane, Washington

Aviation Accident Final Report - National Transportation Safety Board: https://app.ntsb.gov/pdf

Investigation Docket  - National Transportation Safety Board: https://dms.ntsb.gov/pubdms

http://registry.faa.gov/N121PT

National Transportation Safety Board - Aviation Accident Factual Report

Location: Walla Walla, WA
Accident Number: WPR15LA219
Date & Time: 07/20/2015, 1740 PDT
Registration: N121PT
Aircraft: GARLICK UH1H
Aircraft Damage: Destroyed
Defining Event: Loss of lift
Injuries: 1 Minor
Flight Conducted Under: Part 133: Rotorcraft Ext. Load 

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 it's 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.

Pilot Information

Certificate: Flight Instructor; Commercial
Age: 60, Male
Airplane Rating(s): Multi-engine Land; Single-engine Land
Seat Occupied: Left
Other Aircraft Rating(s): Helicopter
Restraint Used: 4-point
Instrument Rating(s): Airplane
Second Pilot Present: No
Instructor Rating(s): Airplane Single-engine
Toxicology Performed: No
Medical Certification: Class 2 With Waivers/Limitations
Last FAA Medical Exam: 07/10/2014
Occupational Pilot: Yes
Last Flight Review or Equivalent: 03/15/2014
Flight Time:  25200 hours (Total, all aircraft), 2280 hours (Total, this make and model), 25200 hours (Pilot In Command, all aircraft), 203 hours (Last 90 days, all aircraft), 67 hours (Last 30 days, all aircraft), 4 hours (Last 24 hours, all aircraft) 

Aircraft and Owner/Operator Information

Aircraft Manufacturer: GARLICK
Registration: N121PT
Model/Series: UH1H
Aircraft Category: Helicopter
Year of Manufacture: 1964
Amateur Built: No
Airworthiness Certificate: Restricted
Serial Number: 64-13711
Landing Gear Type: Skid;
Seats: 2
Date/Type of Last Inspection: 06/03/2015, Annual
Certified Max Gross Wt.: 9500 lbs
Time Since Last Inspection: 4 Hours
Engines: 1 Turbo Shaft
Airframe Total Time: 12168.9 Hours as of last inspection
Engine Manufacturer: LYCOMING
ELT: C91A installed, not activated
Engine Model/Series: T-53-L-703
Registered Owner: LRH EQUIPMENT LLC
Rated Power: 1800 hp
Operator: LRH EQUIPMENT LLC
Operating Certificate(s) Held: Rotorcraft External Load (133) 

Meteorological Information and Flight Plan

Conditions at Accident Site: Visual Conditions
Condition of Light: Day
Observation Facility, Elevation: KALW, 1194 ft msl
Observation Time: 1753 PDT
Distance from Accident Site: 10 Nautical Miles
Direction from Accident Site: 270°
Lowest Cloud Condition: Clear
Temperature/Dew Point: 36°C / -2°C
Lowest Ceiling: None
Visibility:  10 Miles
Wind Speed/Gusts, Direction: 14 knots, 250°
Visibility (RVR):
Altimeter Setting: 29.83 inches Hg
Visibility (RVV):
Precipitation and Obscuration: No Obscuration; No Precipitation
Departure Point: Walla Walla, WA
Type of Flight Plan Filed: Company VFR
Destination: Walla Walla, WA
Type of Clearance: None
Departure Time: 1710 PDT
Type of Airspace: Class G 

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. 

Wreckage and Impact Information

Crew Injuries: 1 Minor
Aircraft Damage: Destroyed
Passenger Injuries: N/A
Aircraft Fire: On-Ground
Ground Injuries: N/A
Aircraft Explosion: None
Total Injuries: 1 Minor
Latitude, Longitude:  46.051667, -118.090000 

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. 

Additional Information

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."

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