LEXINGTON, NE, USA
N132RS
Rotorway EXEC 90
According to the pilot/builder, the homebuilt helicopter was in cruise at 1000 feet agl when he initiated a 200-300 foot per minute descent. After approximately one minute, at an altitude of approximately 500-700 feet agl, the engine lost power. He executed an autorotation and forced landing into a corn field. The pilot stated that the helicopter had some forward velocity on landing, causing one of the skids to sink and get caught in the soft terrain. The helicopter subsequently nosed over and was substantially damaged. The temperature and dewpoint on the surface was reported to be 42 degrees F and 38 degrees F respectively. According to carburetor icing charts, the accident flight was operating in conditions favorable to carburetor icing. Post accident investigation revealed that the carburetor heat system was operating. The pilot stated that during the descent phase of flight, the engine temperatures were indicating in the low end of the green arc, but he did not notice the carburetor temperature gauge indication. The Vice President of RotorWay, International agreed to issue an Advisory Bulletin to all registered owners of RotorWay helicopters having the RI-162 engine, informing them of the background of this accident, as well as alternate way of carburetor icing prevention and avoidance.
On June 3, 1998, at 1900 central daylight time, a RotorWay Exec 90, N132RS, experienced a loss of engine power while descending for landing at Jim Kelly Field, near Lexington, Nebraska. The homebuilt helicopter was substantially damaged after nosing over during the subsequent forced landing. The pilot/builder reported no injuries. The personal solo flight was being conducted under 14 CFR Part 91. Visual meteorological conditions prevailed at the time of the accident and no flight plan was on file. The flight originated at Jim Kelly Field at 1800 cdt, and made one stop in a field at Loomis, Nebraska. The local flight was returning to Lexington when the accident occurred. In an interview, the pilot stated that he was in cruise flight approximately 1000 feet above the ground (agl), and had been airborne for approximately 15 minutes. He reduced throttle from 20 inches engine manifold pressure to 15 inches engine manifold pressure, and initiated a 200-300 foot per minute descent at a forward velocity of about 75 miles per hour (mph). After approximately one minute, and at an altitude of approximately 500-700 feet agl, the pilot noticed a decrease in rotor rpm. He responded by increasing the throttle, to which there was no response by the engine. He initiated an autorotation at a forward velocity of about 65 mph, and executed a forced landing into a corn field. The pilot stated that the helicopter touched down with some forward velocity. According to the pilot report, the "left skid dug into a corn row after sliding approx[imately] four feet." The helicopter subsequently rolled over, causing substantial damage. An inspector from the Federal Aviation Administration (FAA) examined the wreckage and observed damage to the main rotor blades, tail rotor blades, tail boom skin, canopy, and cabin shell. The owner started the engine for the first time since the accident, under the surveillance of the FAA inspector. According to the inspector, "The engine started immediately with no hesitation...did run somewhat rough initially, but smoothed out as the engine temperature rose." The engine was run for approximately 10 minutes. The ignition system was checked and was determined to have been operating properly. The engine instruments were monitored and found to show satisfactory indications. According to the inspector, "the engine did not show any evidence of internal failure." At the time of the accident, the National Weather Service (NWS) was reporting for Lexington a 1400 foot overcast ceiling with an air temperature and dewpoint of 42 degrees Fahrenheit (6 degrees Centigrade), and 36 degrees Fahrenheit (3 degrees Centigrade) respectively, on the field. According to the NWS, the relative humidity on the field was between 78% and 86%. In an interview, the pilot stated that the carburetor heating system had been turned on several flights before, and had remained on throughout the accident flight. The carburetor temperature gauge is located on the lower center pedestal in the cockpit. He stated during the descent phase of the accident flight, the engine instruments were indicating in the low end of the green arc; However, he stated that he did not observe the carburetor temperature gauge prior to, nor subsequent to, the engine losing power. The engine installed on the accident helicopter was a RotorWay RI-162. It is a 150 horsepower, opposed four cylinder, liquid- (water) cooled engine. It utilizes a single downdraft carburetor which is positioned above, and upstream, the engine intake manifold. The carburetor heating system is designed to utilize circulating water that had been heated after passing through the cylinder heads, and then fed into the manifold midsection. According to the FAA inspector who observed the system, the heated water is used to "warm the intake manifold below the carburetor throttle plate." In order for the system to operate, a valve located near the manifold midsection within the engine compartment must be manually turned to the 'ON' position. The FAA inspector indicated that the valve was set to the 'ON' position upon post accident examination. In an interview, the Vice President of RotorWay, International, told a Safety Board representative that there is an increased risk of encountering a carburetor icing condition when the engine power is reduced to the low end of the green arc, even with the carburetor heating system operating. He stated that a bulletin had been issued to owners of RotorWay Exec models that have a dual-carburetor system. He said that the bulletin contained a suggestion that the helicopter's "belly scoop" be covered, so that heat could be maintained within the engine compartment. When the Safety Board representative asked whether or not this procedure would assist in the prevention of carburetor icing in helicopters with the single carburetor system, he stated that it would. When the Safety Board representative asked whether or not safety would be compromised by performing this procedure on helicopters with the single carburetor system, he stated that it would not. In a telephone conversation on July 10, 1998, the vice-president agreed to have RotorWay, International, draft and distribute a safety bulletin to all registered owners of RotorWay helicopters having the RI-162 engine, informing them of the background of this accident, and alternate ways of carburetor icing prevention and avoidance.
An improper autorotation by the pilot in command. Factors contributing to this accident were the accumulation of ice in the carburetor resulting in a total loss of engine power, and the soft terrain.
Source: NTSB Aviation Accident Database
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