Manhattan, KS, USA
N400PD
Schweizer 269C
During a public use solo instructional flight the helicopter sustained substantial damage when it nosed over in a rocky, uneven field during a precautionary landing following an observed split in rotor/engine tachometer readings and an audible low rotor rpm alarm in cruise flight. The pilot perceived a mechanical emergency and performed a precautionary landing. The control box, tachometer cable, and tachometer were examined. The control box warning circuitry was not operational when first checked and operated after repair of an open circuit found on the circuit board. No anomalies were detected with the tachometer. Visual examination of the cable revealed indications of excessive wear and possible binding with windup of the outer strands. Binding of the cable is consistent with a bouncing tachometer needle.
HISTORY OF FLIGHT On November 18, 2005, about 1430 central standard time, a Schweizer 269C helicopter, N400PD, operated by the City of Topeka Police Department, sustained substantial damage when it nosed over during a precautionary landing near Manhattan, Kansas, following an observed split in rotor/engine tachometer readings and an audible low rotor rpm alarm during cruise flight. Visual meteorological conditions prevailed at the time of the accident. The solo instructional flight was being conducted as public use under 14 Code of Federal Regulations Part 91. A visual flight rules flight plan was on file. The pilot was taken to the hospital for observation and he reported no injuries. The flight originated from the Philip Billard Municipal Airport, near Topeka, Kansas, about 1330 and was en route to the Emporia Municipal Airport, near Emporia, Kansas, at the time of the accident. The pilot's accident report, in part, stated: At or near 15 miles East of Manhattan, I descended to approx 2500 [feet above mean sea level] MSL after listening to [Automated Surface Observing Systems] ASOS for current winds and runway information for landing traffic at Manhattan airport. I recall the surface winds were within 30 degrees of Runway 210 with winds within my ability and not a concern. Pattern altitude at Manhattan for light aircraft is 1900 MSL putting my position about 600 ft. [above ground level] AGL directly East of the field inbound landing. The traffic pattern was for left traffic with a restricted (hot) zone to the North of the field. I expected Tower instructions to have me either report 45 degrees for left downwind runway 21, or left base runway 21. Having flown to Manhattan in the past I recall the Kansas River being just South of Runway 13/31 near the field. When I passed 8 miles I was preparing to announce my position and intensions e.g. (Manhattan Tower 400PD eight miles East inbound landing, Stop and Go). The collective friction was not on, I maintained level flight and I was enjoying the flight. I had crosschecked my GPS position and heading and was on coarse 273 degrees as desired with airspeed of 75 [knots] Kts. As I was looking for ground reference landmarks to the airport, I heard the low rotor horn/alarm. I did not have any unusual attitude or noted decrease in airspeed. When this occurred I checked the RPM gage and expected it to be at the top of the green arc with a minor power increase necessary to resolve the alarm (3,000 RPM to 3,100 expected). What was indicated however was a large split in the needles (engine rpm vs. rotor speed) with the rotor rpm at the bottom of the green arc. I did not understand why I had this reading but knew if correct, there was a mechanical emergency. At this time I noted three indications of a problem. The large split in needles engine rpm vs. rotor rpm), the audible horn heard that alerted me initially, and now a high-low engine sound that I had not heard in normal flight (Similar to the throttle corelator sound at the end of the day after the rotor system has been disengaged, prior to the engine mixture being pulled at 100 rpm). I did not induce throttle and take it away and I could not explain what had happened causing the readings and alarm. I recall lots of movement in the shared rotor/engine gages, with the needles vibrating. At the same time I entered autorotation with a full down collective and then concentrated outside of the aircraft. I had Manhattan Tower in my #1 radio Manhattan ASOS in radio #2 and was able to send a distress call to the control tower. I recall saying something similar to "Mayday, Mayday, 400 Papa Delta 8 miles West (meaning to say East)-unknown what direction was said to the tower. The tower responded to me but I do not recall my reply. Days later Manhattan Tower stated I repeated my distress and reported a mechanical. The previous seconds and those following felt very fast with many things happening at once. I recall them in fragments. Attitude ok. Pick a spot, ok. Winds at Manhattan southwest, Airspeed (55-60kts ideal), ok. On short final the terrain was not as flat as I had thought but I was committed and did not feel I had the inertia (stored energy) or skill to try for an alternate (more flat) spot South and West before touchdown. I was mentally preparing for landing reminding myself to be ready on the anti-torque pedals, that this was not a power recovery maneuver (which we recover from at 50 ft during training) and my flare would be much closer to the ground. The engine was making the same high low rpm sound as I stated earlier and my landing point was a downhill slope. I started my flare. The cyclic was brought back and slightly left into my Left thigh. I remember the spot where the flight control touched my leg. I also remember thinking my forward ground speed did not slow as much as I expected but now it was time to pull all available power (raise the collective) and contact was made. Once I had touched the helicopter began to slide. The contact was hard but not much harder than a high hovering autorotation. The ship was intact. The terrain was rough, uneven, and rocky. My forward slide felt fast. I felt an impact with something, slowed rapidly and flipped forward. I do not remember seeing the limestone that caught the searchlight or skid and could not have avoided it if I had. During the forward roll/flip of the helicopter, it disintegrated. The next thing I recall was being inverted in my harness and needing to get free and away fearing a potential fire. PERSONNEL INFORMATION The pilot held a private pilot certificate with an airplane single engine land rating. His most recent Federal Aviation Administration (FAA) second-class medical certificate was issued on October 19, 2004, without limitations. The pilot reported that he had logged a total time of 236 flight hours. He indicated 56 of those hours were in the same make and model as the accident helicopter and of which 8 were as pilot-in-command of the same make and model as the accident helicopter. AIRCRAFT INFORMATION N400PD was a Schweizer 296C, serial number S1833, helicopter. The helicopter had a three-bladed, fully articulated single main rotor system and was powered by a Lycoming HIO-360-D1A engine rated at 190 hp at 3,200 rpm. Power was transmitted from the engine through a V-belt drive system and a tail rotor drive shaft transmitting power to two variable pitch tail rotor blades through a tail rotor transmission. The fuselage had a central, tubular steel, open frame construction. The canopy and door transparent areas were of cast acrylic construction. The seat structure was an assembly that included riveted aluminum beams, frames, bulkheads, and supports. The helicopter was reported to be configured with two seats. The landing gear was a non-retractable skid type with its crossbeams providing attachment for struts and oleo-type shock-absorbing dampers. The aircraft's maintenance instructions, in part, stated: The AES/STAR system consists of a solid state electronic control unit, a special dual tachometer with electrical encoders, a special clutch engagement switch, a weight on skids switch, a red ROTOR LOW RPM Light, a warning horn, a TEST/MUTE HORN button, an AUTO ENGAGE function switch, an AES/ROTOR WARN circuit breaker and associated wiring. The system monitors engine and rotor speed (RPM) at the dual tachometer and sends signals to the control unit. Depending on operational mode the system provides three separate functions. During engine startup, the limiter will ground the magnetos at approximately 1800 RPM, to prevent an overspeed condition that may damage the lower pulley coupling shaft. After engine startup, the pilot may elect to initiate and monitor an automatic rotor engagement. If rotor speed drops below approved RPM during flight, the ROTOR LOW RPM light will illuminate and the warning horn will sound, to alert the pilot. Pressing the TEST/MUTE HORN button will silence the horn in flight and sound it on the ground. If the system is disabled, if the control unit is removed, or if a standard tachometer is installed, the aircraft may be operated normally but without the features described above. The system monitors engine and rotor RPM through two encoders in the dual tachometer. During automatic engagement, the control unit senses the acceleration of the rotor RPM and the deceleration of the engine RPM, to cycle electrical power ON and OFF to the linear actuator, until the rotor and engine tachometer needles are superimposed and the rotors are engaged. This system automatically performs the pilot's manual task of moving the CLUTCH switch from ENGAGE to HOLD and back to ENGAGE, during rotor engagement. During startup, the engine RPM signal is used to trigger transistors, in the control unit, that ground (disable) the magnetos when the RPM is at or above the set point (approximately 1800 RPM). The magnetos are released to normal condition, as soon as the RPM falls below the set point. Pilot action to retard the throttle is required to prevent repeated cycling of the magnetos. During flight operations, the rotor encoder signal is used to illuminate the flashing red ROTOR LOW RPM light and sound the pulsing horn when the RPM is below the bottom red line of the engine green arc. The rotor encoder signal is used to activate the same light and horn to a steady light and continuously sounding horn, when RPM drops to the bottom of the rotor green arc (approximately 390 RPM). Pilot action to increase throttle or reduce collective pitch and increase RPM above the set points, will extinguish the light and silence the horn and is required to return to safe flight conditions. The last inspection of the helicopter was a 100 hour inspection on November 18, 2005. The aircraft's total time was reported as 2,253.4 hours. The operator had reported that approximately 15 to 20 flight hours prior to the accident flight, the helicopter experienced a problem with the tachometer reading jumping in-flight. A precautionary landing was performed. The tachometer cable was inspected, lubricated, and reinstalled. METEOROLOGICAL INFORMATION At 1452, the recorded weather at the Manhattan Regional Airport, near Manhattan, Kansas, was: Wind 220 degrees at 12 knots; visibility 10 statute miles; sky condition clear; temperature 13 degrees C; dew point 0 degrees C; altimeter 30.14 inches of mercury. WRECKAGE AND IMPACT INFORMATION The helicopter came to rest about seven miles east of the Manhattan Regional Airport in a rough uneven field nearly inverted on its nose at latitude 39 degrees 6.451 minutes N and longitude 96 degrees 31.118 minutes W. Federal Aviation Administration inspectors examined the wreckage. No engine or airframe anomalies were detected. The wreckage was recovered back to the police helicopter unit. An end of the tachometer cable was rotated mechanically with a drill and the other end of the cable did not rotate at the same rpm. TESTS AND RESEARCH The operator reported that the tachometer cable was damaged and was cut to facilitate its removal. The airplane's tachometer, tachometer cable, and AES control box were shipped to Schweizer Aircraft Corporation for examination under FAA supervision. The Schweizer report, in part, stated: The AES control box warning circuitry was not operational when first checked at Schweizer, but operated after repair of an open found on the circuit board. ... The pilot reported a warning light prior to autorotation. The Flight Manual Supplement CSP-C-1 W requires operational check of the warning system prior to flight and there was no report the system was inop prior to flight. The tachometer appears to be mechanically functional for needle operation and electrically functional for warning light actuation. The square drive bushing was compromised ... . Whether there was an oversize drive bushing or under size cable prior to the failure could not be established. The system is designed so that no malfunction that may have been sensed by optical encoders in the dual tachometer, nor any malfunction of the electrical system, could have had a mechanical effect on the tachometer. Visual exam of the cable exhibited indications of excessive wear and possible binding with windup of the outer strands. ADDITIONAL INFORMATION The FAA and Schweizer Aircraft Corporation were parties to the investigation. The aircraft wreckage was released to a representative of the Topeka Police Department.
The binding of the main rotor blade's tachometer cable during cruise flight resulting in an observed split tachometer indication/aural alarm and perceived mechanical emergency and the unsuitable terrain the pilot encountered during the emergency landing. A factor was the rocky terrain.
Source: NTSB Aviation Accident Database
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