Mt. Baldy, CA, USA
N324RS
ROBINSON HELICOPTER COMPANY R44 II
The commercial pilot planned to land the helicopter at the top of a high mountain peak. Three passengers were onboard, and the helicopter was loaded just below its maximum gross weight. The enroute climb was uneventful, and after reaching the peak, the pilot performed an orbit over the landing site. While surveying his landing options, the helicopter's low rotor rpm warning horn sounded, and he corrected by lowering the collective and increasing engine power. The horn stopped, and he continued to circle the area while reviewing an engine limit manifold pressure chart and determining that he had enough engine power available to proceed. Hikers were close to his intended landing area, so he initiated an approach with a tailwind toward the lower face of a ridgeline with the intention of turning back to land into the wind. As he approached the landing spot, the low rotor rpm horn sounded again, and he lowered the collective. The helicopter descended rapidly, and due to terrain, there was no escape path, so he warned the passengers to brace for impact. The helicopter landed hard and sustained substantial damage. The helicopter was operating at the upper limit of its performance capabilities at the reported gross weight and calculated density altitude. Performance charts indicated that the helicopter would not have been able to hover at that altitude, and as such, a successful landing, while not impossible, would have been challenging. The activation of the warning horn during the initial orbit would have been another indication to the pilot that there was limited engine power available for landing. The pilot reported no preimpact mechanical malfunctions or failures with the airframe or engine that would have precluded normal operation, and the accident is consistent with the pilot's failure to maintain rotor rpm while operating near the helicopter's performance limits.
HISTORY OF FLIGHTOn December 29, 2016, at 1131 Pacific standard time, a Robinson Helicopter Company R44 II, N324RS, landed hard near the peak of Mount San Antonio, Mount Baldy, California. The flight instructor sustained serious injuries, and the three passengers received minor injuries. The helicopter sustained substantial damage during the impact. The helicopter was registered to November Alpha LLC., and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan had been filed. The personal flight departed Fullerton Municipal Airport, Fullerton, California at 1100 with a planned destination of Mount Baldy. The pilot reported that prior to flight he performed a weight and balance check, and based on the results, he filled the main fuel tank to half of its capacity, and the auxiliary tank to one quarter. The departure and flight to Mount Baldy were uneventful (Image 1). As the helicopter approached the summit from the south, the pilot noticed a group of hikers on a ridgeline at the top of the mountain. While executing a right turn orbit around the summit the low rotor RPM warning horn sounded. To recover, the pilot lowered the collective and increased engine power with the throttle, and as a result the warning horn sound ceased. He then checked the helicopter's limit manifold pressure placard and, "determined we could pull 19.6 inches of power". The pilot stated that as he started to descend from the south, he felt the helicopter encounter a tailwind. He decided to change the approach to a low pass in order to circle back to the north and land with a headwind (Image 2). He made the pass at a speed of between 35 to 40 knots however a few seconds later the low rotor RPM horn sounded again. He lowered the collective, but the helicopter rapidly descended towards the mountain face. He reported that he did not have enough altitude to recover, or an escape path to guide the helicopter away from terrain. He warned everyone that a crash was imminent, and just before colliding with the ground, he applied full left foot pedal and the helicopter landed sideways rather than nose-down. After impact, everyone was able to exit unimpeded and he set the transponder to 7700 and turned the fuel valve to the "off" position. Image 1 - Approximate Route of Flight (Data Provided by Harris Corporation) Image 2 - Approximate Approach Path (Data Provided by Harris Corporation) The helicopter came to rest just below a ridge, at an elevation of 9,860 ft, about 1,500 ft west of the summit of Mount Baldy. The aft fuselage was partially submerged in the snow and had sustained buckling damage through to the tailboom. (Photo 1,2) The pilot reported no preimpact mechanical malfunctions or failures with the airframe or engine that would have precluded normal operation. Photo 1 – Helicopter at the Accident Site Photo 2 – Helicopter at the Accident Site AIRCRAFT INFORMATIONThe helicopter was manufactured in 2003 and was equipped with a Lycoming IO-540-AE1A5 engine, serial number RL-26858-48E. On July 6, 2015, both a helicopter overhaul and engine rebuild were completed. The most recent maintenance event was for a 50/100-hour inspection, which was completed on December 10, 2016, 28 flight-hours before the accident. At the time of inspection, the airframe and engine had accumulated 789.4 flight hours since overhaul/rebuild. METEOROLOGICAL INFORMATIONAn upper air sounding for the accident site was created utilizing the Weather Research and Forecasting (WRF) computer model. The data indicated that at 1100, at an elevation of 9,920 ft mean sea level, the temperature would have been about 5.4 °C, and dew point -13.9 °C. It indicated wind from 147° at 12 knots. Utilizing these values and the 1053 pressure reading (30.15 inches of mercury) at Ontario International Airport, Ontario, California, the density altitude at the accident site would have been about 10,900 ft. A Los Angeles County Sheriff's deputy, who arrived at the accident site via helicopter at 1330, reported that the temperature was 10 °C, with wind from the east at 10 to 15 knots. AIRPORT INFORMATIONThe helicopter was manufactured in 2003 and was equipped with a Lycoming IO-540-AE1A5 engine, serial number RL-26858-48E. On July 6, 2015, both a helicopter overhaul and engine rebuild were completed. The most recent maintenance event was for a 50/100-hour inspection, which was completed on December 10, 2016, 28 flight-hours before the accident. At the time of inspection, the airframe and engine had accumulated 789.4 flight hours since overhaul/rebuild. ADDITIONAL INFORMATIONWeight The pilot and front seat passenger weighed 145 and 235 pounds respectively. The total weight of the rear seat occupants and their baggage was 399 pounds. Based on the pilot's statement regarding fuel quantity, the main tank would have contained 15.25 gallons (91.5 pounds) and the auxiliary tank 4.25 gallons (25.5 pounds) of fuel at takeoff. According to the helicopters most recent weight and balance sheet, the maximum gross weight was 2,500 pounds, and the helicopters basic empty weight was 1,565.2 pounds. Utilizing these values, the helicopter was about 38.8 pounds short of its maximum gross weight at takeoff. Performance The performance section of the R44 II Pilot Operating Handbook (POH), stated that the in ground effect (IGE) hover ceiling, based on full throttle, zero wind, and a gross weight of 2,460 pounds was about 8,800 ft at 5 °C, and 9,400 ft at -5 °C. The out of ground effect (OGE) hover ceiling for the same parameters was about 5,400 ft, and 6,600 ft respectively. The limitations section of the POH stated a maximum operating density altitude of 14,000 feet. Manifold Pressure Chart The helicopter was equipped with a manifold pressure chart, which was required to be in full view and readable by the pilot in flight. The intent of the chart was to provide the pilot with the engines maximum allowable continuous manifold pressure at varying temperatures and pressure altitudes. The chart was not intended for determining power available. The pilots reference to "19.6 inches of power" corresponded to a pressure altitude of 10,000 ft, and an outside temperature of –10°C.
The pilot's failure to maintain main rotor speed during a low orbiting approach over a confined mountainous area. Contributing to the accident was the pilot's decision to attempt to land in an area that did not allow for a safe exit path under conditions near the limit of the helicopter’s performance capability.
Source: NTSB Aviation Accident Database
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