WINFIELD, KS, USA
N2198U
Brantly Helicopter B-2B
THE PILOT STATED THE HELICOPTER WAS IN CRUISE FLIGHT AT ABOUT 500 FEET AGL WHEN THE ENGINE RPM '...SUDDENLY AND RAPIDLY INCREASED AS IF THE ENGINE WAS ABRUPTLY UNLOADED...'. THE PILOT SAID HE AUTOROTATED TO A LANDING THAT WAS '...QUITE SMOOTH.' AFTER TOUCHING DOWN THE HELICOPTER SLID AND ROLLED OVER ON ITS RIGHT SIDE. THE PILOT SAID THE TOUCH DOWN FIELD WAS VERY MUDDY. AN EYE WITNESS SAID HE SAW THE HELICOPTER DESCEND VERY RAPIDLY THEN BOUNCE INTO THE AIR. AFTER TOUCHING DOWN A SECOND TIME IT ROLLED OVER ON ITS RIGHT SIDE. THE ON-SCENE INVESTIGATION REVEALED THAT THE HELICOPTER'S OVER-RUNNING CLUTCH HAD FAILED. EXAMINATION OF THE CLUTCH ASSEMBLY REVEALED ITS SPRAGS AND THE INNER AND OUTER RACES WERE WORN BEYOND THE MANUFACTURER'S DESIGN LIMITATIONS. THE WORN PARTS ALLOWED THE CLUTCH TO GO OVER CENTER AND BECOME DISENGAGED.
On April 19, 1995, at 1630 central daylight time (cdt), a Brantly B-2B, N2198U, piloted by a private pilot, was substantially damaged when it rolled over during an off airport landing on soft ground. The pilot stated the helicopter's power clutch failed during cruise requiring him to auto-rotate to a forced landing. Visual meteorological conditions prevailed at the time of the accident. The 14 CFR Part 91 ferry flight was not operating on a flight plan. The pilot and passenger reported no injuries. The flight had departed Strother, Kansas, 1610 cdt. According to the pilot's written statement attached to NTSB Form 6120.1/2, "All was normal. Engine RPM was 2900, altitude was approximately 500 feet AGL, and indicated airspeed was 80 MPH." During the flight the engine RPM "...suddenly and rapidly increased as if the engine was abruptly unloaded...", according to the pilot. He said he auto-rotated to a landing that was quite smooth. After touching down the pilot said the helicopter slid on the field's muddy surface and rolled over onto its right side. An eye witness said he observed the helicopter "...come down too fast, everything looked and sounded ok, just going too fast. The helicopter landed, bounced off the ground and rolled over on its side." Two other eye witnesses said they observed the helicopter descending very rapidly. The on-scene investigation revealed that the helicopter's over- running clutch unit had failed. A mechanic helping with the inspection of N2198U stated the sprag tangs inside the clutch assembly had failed. He said the wear pattern on the tangs appeared as though it had been under-greased early in its service life. The mechanic said there was adequate grease in the assembly when he looked at it. According to a March 29, 1994, airframe maintenance logbook entry, an over-running clutch assembly, serial number 93, was installed on N2198U. At that time, N2198U had a total airframe time of 833.8 hours. The clutch assembly had 353.2 hours on when it was installed on N2198U. The helicopter had operated about 129 hours with the clutch before it failed. The helicopter maintenance manual stated the clutch assembly "...must be returned to Brantly Helicopter Industries (BHI) for engineering evaluation, inspection, and overhaul as required at each 600-hour interval." The manual stated the clutch assembly's total accumulated service time is not to exceed 2,500 hours. N2198U's over-running clutch assembly (clutch assembly) was dismantled at the manufacturer. An FAA Principal Maintenance Inspector (PMI) represented the NTSB during the clutch assembly's inspection. N2198V's clutch assembly had a part number of 40237- 2 and had been rebuilt on October 13,1964. Examination of N2198U's assembly revealed the sprags in the clutch had been worn beyond the manufacturer's limits. The PMI said the assembly's inner and outer races were also worn beyond the manufacturer's limits. According to the PMI, the worn parts allowed the clutch to go over-center and become disengaged. The manufacturer of the clutch assembly said a new style clutch assembly had been produced and superseded the one found on N2198U. The new clutch assembly has a part number of 40237-3. The new clutch assembly is described as a positive, continuous, engagement over-running clutch, according to the PMI. The helicopter manufacturer was contacted and asked if they recommended the new clutch assembly be installed on the helicopter. A representative from the manufacturer said they did not, and advise those seeking a new clutch assembly that a new -3 model is what is available. She said the -3 clutch assembly is FAA approved. She said the older -2 type clutch assembly is still a usable item for the helicopter.
The total failure of the rotor drive system clutch assembly due to excessive wear of the clutch assembly. A factor in this accident was the soft ground in the emergency landing area.
Source: NTSB Aviation Accident Database
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