N144KWRobinson Helicopter R44 · 2009

On July 11, 2024, about 1305 Hawaii standard time, a Robinson R44 helicopter, N144KW, sustained substantial damage when it was involved in an accident near Kekaha, Hawaii. The pilot and two passengers were fatally injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 air tour flight.

The helicopter was owned by Aloha Helicopter Tours LLC and being operated by Ali’I Air Tours for air tours on the island of Kauai. The helicopter departed from the Lihue Airport (LIH) at about 1231 for a 1-hour tour of the island of Kauai. According to archived ADS-B data, after the helicopter departed from LIH, it initially proceeded west-southwest before turning to a northerly heading. As the helicopter neared the northern shoreline of the island of Kauai, archived ADS-B data ended. ADS-B coverage along the northern coast of Kauai is limited due to mountainous terrain.

Witnesses hiking on the Kalalua trail, along the Na Pali coast on the northern shore of Kauai, reported that during a brief rest stop overlooking the coast, strong gusting wind prevailed. They reported that, as they rested, a “huge” and sudden gust of wind blew over them, followed by a very loud “bang” noise. As their attention was drawn to the source of the loud noise, they saw the helicopter wreckage freefalling into the ocean in two sections: a main fuselage section and the tail boom section. The witnesses said that the helicopter wreckage sank moments after striking the water about one quarter of a mile offshore.

The U.S. Coast Guard located the wreckage in about 77 ft of water.

Examination of the wreckage revealed no evidence of any preexisting anomalies that would have precluded normal operation of the helicopter. There was evidence that a mast bumping event had occurred. There was no recorded information available that could be used to determine the helicopter's airspeed, altitude, or the pilot's control inputs.

A weather study conducted by the NTSB indicated that conditions for the area where the accident occurred were conducive to the development of strong downdraft winds.

Robinson Safety Notice SN-32 discusses flight in high winds and turbulence and explains how improper application of control inputs in response to turbulence can increase the likelihood of a mast bumping accident. It recommends that pilots reduce airspeed below normal cruise speed to 60 to 70 knots for flight in significant turbulence. It suggests techniques to avoid overcontrol of the helicopter and says to avoid flying on the downwind side of hills and ridges.

The pilot completed the Robinson Helicopter Pilot Safety Course on May 26, 2011.

The closest weather reporting station was from Barking Sands Pacific Missile Range Facility Airport (PHBK), Kekaha, Hawaii, located about 14 miles southwest of the accident site at an elevation of 23 ft. The military airport had an Automated Surface Observation System (ASOS), which was not augmented by any personnel during the period. The following conditions were reported around the time of the accident:

PHBK weather observation at 1251 HST, automated, wind from 020° at 11 knots gusting to 25 knots, visibility 10 miles or greater, ceiling broken at 5,500 ft agl, temperature 32°C (90°F), dew point temperature 21°C (70°F), altimeter setting 29.99 inches of mercury (inHg). Remarks: automated station with a precipitation discriminator, peak wind from 360° at 27 knots recorded at 1222 HST, sea-level pressure 1015.6-hPa, temperature 32.2°C, dew point 20.6°C.

PHBK special weather observation at 1308 HST, automated, wind from 340° at 10 knots gusting to 21 knots, visibility 10 miles or greater, ceiling overcast at 5,000 ft agl, temperature 31°C (88°F), dew point temperature 21°C (70°F), altimeter setting 29.98 inHg.

The next closest weather reporting station was Lihue Airport (PHLI), located about 24 miles southeast of the accident site at an elevation of 152 ft. The accident helicopter departed from PHLI at about 1230 HST.

A review of the National Weather Service Graphic Forecast for Aviation indicated that visual flight rules conditions with brief marginal visual flight rules conditions in light rain showers prevailed surrounding the period with generally northeasterly winds at 10 knots gusting upwards to 30 knots, and scattered to broken clouds between 2,500 ft to 5,500 ft agl.

A Weather Research and Forecast (WRF) model simulation surrounding the time of the accident was run using initialization data from the Global Forecast System (GFS) 0.5° grid. A west-to-east cross-section was made across the accident site, (Figure 1.) The two grid points were about 13 miles apart with the terrain reaching about 3,695 ft about 2.4 miles east of the accident site. The highest elevation on Kauai was recorded at 5,144 ft. The WRF model depicts the general easterly winds over the high terrain with maximum winds near 38 knots above the ridge and between 26 to 32 knots over the accident site and increasing further west to 34 knots offshore.

Figure 1- Plot of WRF model cross-section points used the accident site and general topography.

The WRF numerical model depicted upward vertical velocities east of the high terrain up to 600 feet per minute (fpm) (updrafts) and downward (downdrafts) of 500 fpm near the ridge and decreasing to 100 fpm over the accident site and then upward and downward further westward. This wave like pattern is common with mountain wave activity or gravity waves.

According to a Federal Aviation Administration inspector who interviewed other pilots that were flying in the area, it was a windy day, near the limits of when R44 pilots might start turning down flights. One pilot flying in the area stated it was a normal day with wind at 20-25 knots with a little turbulence. Another pilot reported the wind was 20-30 knots and he encountered some downdrafts that shook his aircraft pretty good.

A witness that was operating a charter boat and was located about 3 miles northeast of the accident and responded to the U.S. Coast Guard request for assistance. The ship’s captain reported east-northeast trade winds at a constant 20 to 25 mph (17 to 22 kts) with gusts to 40 mph (35 kts) and greater. In addition, he also observed waterspouts about 1/2 mile offshore, with a diameter of 50 yards and extended to about 1,000 ft.

The wreckage was examined in Honolulu. The wreckage consisted of four main sections; engine, cockpit/cabin, tail boom, and main gear box/mast/rotor. The engine had separated from the frame and the frame tubes were fractured at multiple locations. The main gear box (MGB) had fractured and separated from the airframe. The MGB, rotor mast, and main rotor remained together as an assembly. All four tail cone attach bolts remained attached to frame tubes with all four having tail cone skin fragments sandwiched in-between the bolts and frame. The tail cone had separated from the frame with skin folding and tearing on the left side tail cone attachments and buckling evident on the right side at the tail cone. The tail fin and tail rotor remained attached to the tail cone.

The airframe structural tubes, where the tail cone attaches, exhibited bending from right to left. Structural tubes were fractured in bending or deformation. No fractures or tube separations at the welded joints were observed. Tube fracture surface characteristics were deformation and 45° shear lips with the fracture surface having red oxidation deposits.

The fiberglass nose of the cockpit/cabin was broken in multiple pieces. Fragments of both front cockpit doors were present. The main cabin doors were missing. The forward chin area, on the left of the landing lights, exhibited an angled straight cut that pointed to a semicircular vertical floor crushed area under the copilot’s seat.

Cyclic flight controls were traced from the cockpit cyclic to the yoke assembly. The cyclic stick was fractured at the base. The left control tube up to the servo was fractured at the yoke assembly. The right control tube was fractured at the servo input rod end. The aft control rod was attached to the yoke and jackshaft. The control rod from the jackshaft to the aft servo fractured at the servo input rod end. The aft servo was connected to the stationary swashplate by a slightly bent control rod. The upper right lateral control rod was connected to the right servo and fractured just below the stationary swashplate rod end. The upper left lateral control rod was connected to the left servo and fractured at the stationary swashplate rod end. The stationary scissors and the rotating scissors were connected. Both pitch links were attached to the rotating swashplate and fractured at their respective blade pitch horn rod ends.

The right collective was attached to the torque tube assembly. The twist grip and governor linkages were connected. The twistgrip was seized and could not be rotated by hand. The collective push-pull tube was fractured between the torque tube and yoke. The antitorque pedals were not recovered. The first area of pedal control system identified was the fractured push-pull tube that connects to the lower aft bellcrank. The pedal control rods were traced via multiple breaks to the tail rotor pitch control bell crank. The control rod moved the tail rotor bellcrank in concert with movement of the tail rotor blades.

All three servo actuators were intact and had control rods or portions of control rods at each end. The pilot valve on each one moved freely. The hydraulic reservoir and all lines remained attached to the helicopter frame tube near the upper sheave.

The transmission/MGB, mast, and main rotor separated from the airframe as a single assembly. The ring gear could be inspected through the holes in the transmission case. No evidence of damage or unusual wear was identified on the ring gear. The magnetic drain plug was examined and had a single 1/4 inch piece of ferrous material. The flex coupling flanges were deformed and the two bolts connecting to the input pinion were connected. The drive shaft side of the flex coupling had one arm torsionally sheared off and the other had ripped away at the bolt hole. Both connecting bolts were present on the drive shaft side flange. The mast tube remained attached to the MGB base, with a slight bend (to the right) at the top. Both swashplates, rotor head, and main rotor blades were present on the drive shaft. The top of the drive shaft was bent above the swashplate.

The freewheel unit rotated freely in the clockwise direction and locked in the counterclockwise direction. All four drive belts were examined. All had overload separations and no evidence of belt degradation, excessive side wear, cracking, or splitting. The rotor head was attached to the drive shaft located inside the mast tube. Both main rotor blades pitch bolts were connected, and the teetering hinge moved in a rocking motion. The pitch links on each blade were fractured at each pitch horn connection rod end. The mast bump (teeter stop) on the red blade had the elastomeric block split in half horizontally, with a witness mark on the drive shaft. The blue blade teeter stop was missing the elastomeric block, but a horizontal witness mark was observed in the center of the block frame on the drive shaft. On each side of the rotor head there were contact witness marks that matched the curvature of each blades spindle housing/blade root.

Tail rotor drive continuity was established through a bending overload break of the drive shaft just aft of the sheave and freewheel unit flex coupling. A second drive shaft torsional fracture (opposite the direction of drive shaft rotation) was at the frame splice of tail boom bays 3 and 4. The tail rotor rotated freely by hand and the drive shaft moved in concert. The magnetic drain plug from the tail rotor gear box was examined and had no metallic particles.

The engine had separated from the airframe and was located by recovery divers about 40 ft from the fuselage wreckage. All 6 cylinders were attached to the engine case and there was no damage to the engine case halves. The engine drive belt sheave and fan were attached to the forward end of the engine. The valve covers on the left side of the engine (helicopter's right side) had been hydraulicly deformed into the valve rockers. The valve covers on the engine's right side were undamaged. Push rods for the Nos. 2, 4, and 6 cylinders were bent or broken, the push rod tubes on the Nos. 1, 3, and 5 cylinders were dented but remained relatively straight. All valves and rockers were intact. The starter ring had broken free, however, there was a starter ring grind mark on the engine oil cooler. The engine could not be rotated by hand. Holes were drilled into the engine case to examine the connecting rods, camshaft, and crank shaft.

The fuel pump was seized, however, upon disassembly, fluid with a strong fuel odor drained from the pump. All spark plugs had no mechanical damage and exhibited normal operational wear. The oil filter and oil suction screen were clear. Three of the six fuel nozzles were blocked with organic substance that was easily cleared with air pressure. The throttle body throttle plate was at idle and moved freely. The mixture lever was halfway between lean and rich and, by hand manipulation, a little movement from the mixture lever was achieved. Both throttle and mixture linkages were not connected to the airframe controls. One impact tube in the throttle venturi body was missing/fractured. The fuel distributor had a small amount of fluid with a fuel odor. The gascolator had fluid with a fuel odor in it. Both magnetos were seized but intact.

There were no devices recovered that could be used to determine the helicopter's airspeed, altitude, or the pilot's control inputs.

Robinson Safety Notice SN-32 discusses flight in high winds and turbulence and explains how improper application of control inputs in response to turbulence can increase the likelihood of a mast bumping accident. It recommends that pilots reduce airspeed below normal cruise speed to 60 to 70 knots for flight in significant turbulence. It suggests techniques to avoid overcontrol of the helicopter, and says to avoid flying on the downwind side of hills and ridges.