N607RABell 206B · 1971

On August 7, 2018, about 1000 Pacific daylight time, a Bell 206B helicopter, N607RA, struck the ground during an agricultural spray run near Basin City Washington. The commercial pilot received minor injuries, and the helicopter was destroyed by a post-impact fire. The helicopter was registered to Ranger Aviation Leasing, and operated by Northwind Aviation as a Title 14 Code of Federal Regulations Part 137 aerial application flight. Visual meteorological conditions prevailed at the time of the accident and no flight plan was filed. The flight originated from a landing zone near the accident site.

The flight was a "rinse load" intended to clean the helicopter's chemical hopper with a load of water. According to the pilot, he took off from the loading spot, located on the southwest side of the field, with 60 gallons of clean water and about 25 gallons of fuel. He sprayed a portion of the north end of the field for about 3 to 4 seconds, turned off the spray, and proceeded to the east end of the field. He entered the field at a speed of about 50 to 55 mph, and activated the sprayer. He then turned off the spray and initiated a slight climb. When the helicopter was approximately midfield, at a speed of about 50 mph and an altitude of "about power pole height," the pilot applied left tail rotor pedal and slightly lowered the collective control, in order to assist him in looking to his left to view his spray pattern.

"Immediately after" those control inputs, the helicopter yawed quickly to the left, and began "shaking." The pilot pulled up on the collective control and applied right pedal to stop the left yaw. The helicopter continued to "shudder," and the pilot noticed that the helicopter was at a low airspeed and descending. He attempted to regain airspeed, and when the helicopter was about 5 to 10 ft above the ground, he attempted to level the helicopter in pitch before the skids contacted the ground. About that time the pilot observed that the torque gauge was reading about 108% to 110%. He did not recall hearing or seeing a low rotor RPM audio or visual alert.

With a pilot-reported forward speed of about 3 to 5 mph, the helicopter struck the ground, and fire erupted nearly immediately. The pilot exited the helicopter, but then reached in to close the throttle and shut off the fuel valve. He retrieved his mobile telephone and then moved away from the helicopter. The pilot estimated that from the time he climbed the helicopter until the accident was about 10 to 15 seconds. The pilot's loading assistant arrived on scene in the water truck within about 10 minutes of the accident. He and the pilot attempted to extinguish the fire, but were unsuccessful.

In his written account to the NTSB, the pilot stated that he did "not know with certainty if atmospheric conditions or mechanical" problems had caused the event. The wreckage was recovered a few days after the accident to a secure facility for subsequent examination.

Federal Aviation Administration (FAA) records indicated that the pilot held a commercial and flight instructor certificates with rotorcraft-helicopter and instrument-helicopter ratings. According to information provided by the pilot, he had approximately 950 total hours of flight experience, including about 900 hours in helicopters, and 600 hours in the accident helicopter make and model. His most recent flight review was completed in November 2017, and his most recent FAA second-class medical certificate was issued in January 2018.

The helicopter was manufactured in 1971. It was equipped with a Rolls-Royce 250-C20B turboshaft engine. The helicopter was registered to Ranger Aviation Leasing in March 2015.

The pilot reported that the maximum gross weight of the helicopter was 3,200 lbs, and that the helicopter weighed about 2,950 lbs at the time of the accident. When asked, the pilot stated that the power margins for the flight were 100% torque for 5 minutes, and 85% torque for continuous operation.

Helicopter Aerodynamics and Performance

When viewed from the top, the main rotor blades (MRB) rotate counter-clockwise, meaning that that the blades sweep right to left as viewed by the pilot. This requires the tail rotor to provide thrust to counter the nose-right yawing moment caused by MRB rotation; loss of tail rotor effectiveness (LTE) will result in a nose-right rotation of the helicopter.

When a helicopter accelerates from a hover to some forward airspeed, due to airflow changes through the rotor disc, a phenomenon known as translational lift will cause the helicopter to climb. The translational lift zone can occur at forward speeds as low as 10 to 20 knots, and the airflow changes can result in some aerodynamic buffeting. Likewise, when the helicopter decelerates, translational lift will decrease, and buffeting may occur.

The pilot stated that after he applied right pedal and lowered the collective to stop the left yawing, he saw that he was at a low airspeed and descending. These conditions are consistent with entering vortex ring state.

The Helicopter Flying Handbook (FAA-H-8083-21A) stated:

The following combination of conditions is likely to cause settling in a vortex ring state in any helicopter.

1. A vertical or nearly vertical descent of at least 300 fpm (Actual critical rate depends on the gross weight, rpm, density altitude, and other pertinent factors.)

2. The rotor system must be using some of the available engine power (20—100 percent)

3. The horizontal velocity must be slower than effective translational lift.

…Recovery is accomplished by increasing airspeed, and/or partially lowering collective pitch.

The 0953 automated weather observation at a Mesa, Washington, Weather Underground sensing station near the accident site included calm winds, temperature 27°C, and dew point 9°C. The operator's accident form reported that the wind, as listed on an unspecified internet source, was from 130° at 3-5 knots.

The helicopter was manufactured in 1971. It was equipped with a Rolls-Royce 250-C20B turboshaft engine. The helicopter was registered to Ranger Aviation Leasing in March 2015.

The pilot reported that the maximum gross weight of the helicopter was 3,200 lbs, and that the helicopter weighed about 2,950 lbs at the time of the accident. When asked, the pilot stated that the power margins for the flight were 100% torque for 5 minutes, and 85% torque for continuous operation.

Helicopter Aerodynamics and Performance

When viewed from the top, the main rotor blades (MRB) rotate counter-clockwise, meaning that that the blades sweep right to left as viewed by the pilot. This requires the tail rotor to provide thrust to counter the nose-right yawing moment caused by MRB rotation; loss of tail rotor effectiveness (LTE) will result in a nose-right rotation of the helicopter.

When a helicopter accelerates from a hover to some forward airspeed, due to airflow changes through the rotor disc, a phenomenon known as translational lift will cause the helicopter to climb. The translational lift zone can occur at forward speeds as low as 10 to 20 knots, and the airflow changes can result in some aerodynamic buffeting. Likewise, when the helicopter decelerates, translational lift will decrease, and buffeting may occur.

The pilot stated that after he applied right pedal and lowered the collective to stop the left yawing, he saw that he was at a low airspeed and descending. These conditions are consistent with entering vortex ring state.

The Helicopter Flying Handbook (FAA-H-8083-21A) stated:

The following combination of conditions is likely to cause settling in a vortex ring state in any helicopter.

1. A vertical or nearly vertical descent of at least 300 fpm (Actual critical rate depends on the gross weight, rpm, density altitude, and other pertinent factors.)

2. The rotor system must be using some of the available engine power (20—100 percent)

3. The horizontal velocity must be slower than effective translational lift.

…Recovery is accomplished by increasing airspeed, and/or partially lowering collective pitch.

The helicopter came to rest upright in the field, and was mostly consumed by fire. A detailed post-recovery examination was conducted in a secure hangar at the recovery facility. Components that were not consumed by fire typically exhibited thermal damage. In addition, fire-fighting water, and subsequent collateral exposure at the accident site by the irrigation system resulted in post-accident corrosion of some of the wreckage.

The majority of the fuselage, with the exception of most of the lower portion that remained attached to the landing skids, was consumed by fire. The main rotor blade assembly, tailboom, and tail rotor blade assembly were relatively free of thermal damage.

The main rotor mast was fractured immediately below the main rotor blade (MRB) hub, and both MRBs remained attached to the hub. One MRB was fractured at the outboard end of the blade doublers, with additional fracturing near the blade tip, and upward bending of the blade. The other MRB was missing its tip, and exhibited aft and upward bending. Both MRBs exhibited additional scrapes, dents, and gouges.

No evidence of any non-impact or operational thermal damage was observed on the transmission, but it did sustained thermal damage from the fire. Main drive continuity was demonstrated by manual rotation of the input drive quill, with resultant rotation of the rotor mast. No abnormal sounds were heard when the transmission was rotated manually. The engine to transmission driveshaft remained attached to the engine, but was separated from the transmission in a region of thermal damage. The two transmission chip detectors were removed and visually inspected, with no particulate matter observed. The mast chip detector could not be removed.

Both aftermarket composite tail rotor blades (TRBs) remained attached to the TRB hub. One TRB was missing its tip, with damage patterns consistent with ground impact. The other TRB and the TRB hub were intact and undamaged. Due to fragmentation or consumption by fire, tail rotor drive continuity could only be established from the tail rotor through the tail rotor gear box. Oil was present in the tail rotor gearbox; it appeared normal in color and no contaminates were present. The chip detector was removed and visually inspected, with no chips observed.

Most of the flight controls were consumed by the fire. The right collective stick was present, but the throttle twist grip was missing. The yoke mixing lever was present, with significant thermal damage. The right seat inboard anti-torque pedal remained in the wreckage. Only one of the three servo actuators was identified, and it was thermally damaged. No other flight control components were located in the wreckage, and were presumed consumed by fire.

The majority of the fuel and hydraulic system was consumed by the fire. The forward and aft fuel boost pumps, and one of the fuel bladder floats, were found in the wreckage.

The airframe fuel filter and the hydraulic oil reservoir were found detached in the wreckage. No other system components were identified.

The engine remained partially attached to the horizontal and vertical firewalls, and loosely attached to the main transmission via various lines and cables. The engine exhibited significant corrosion; this was attributed to the post accident exposure to firefighting and accident site irrigation water. The magnesium accessory gearbox had mostly burned away, revealing the interior gears; the magnesium fire also consumed or severely damaged the attached engine accessories.

All available/accessible engine B-nuts and fittings were checked by hand for proper torque, and all were found to be at least hand tight. The power turbine governor (PTG) was completely consumed by fire. The fuel control unit (FCU) was fire damaged, and the control arm could not be manipulated by hand.

The leading edges of numerous compressor blades exhibited evidence of small hard-body impacts. Neither the N1 nor N2 rotor systems could be rotated by hand. Visual inspection of the 4th stage (power turbine) revealed no evidence of foreign object damage (FOD), operational damage, or thermal degradation. No evidence of any pre-impact mechanical anomalies or failures of the compressor or turbine was observed.

Borescope examination revealed no evidence of FOD or thermal degradation of the 1st stage (gas generator) nozzle or turbine blades. A significant quantity of metal slivers and chips were found adhered to the leading edges of the 1st stage turbine nozzle, and shiny metal splatter was noted adhered to the turbine nozzle shield. The source of the metal slivers and chips was determined to be from the aluminum engine inlet plenum; this was consistent with engine flexure and component failure during impact, which permitted a rotating splined shaft to contact and grind the plenum, followed by engine ingestion and deposition of the ground material. This was consistent with continued engine operation during impact.

The lubrication system, including both magnetic chip detectors, was mostly consumed by fire. The engine-mounted oil filter was essentially intact, and the filter element was clear of debris.

Examination did not reveal evidence of any condition that would have prevented the engine from delivering power.

Maintenance Records

The operator reported that the most recent maintenance records were on board the helicopter at the time of the accident.

Some partially-burned and water-soaked maintenance records were found in the cabin area of the wreckage, and some intact duplicates were able to be provided by the operator. The latest dated records located bore a date of 3/3/18, with a Hobbs time of 972.9 hrs, and a total time in service of 17,687.7 hrs. An undated page bore a Hobbs time of 1,012.1 hrs. The investigation was unable to determine the in-service times on either the engine or airframe at the time of the accident.