GALLIPOLIS, OH, USA
N51786
ENSTROM F-28C
THE PILOT ATTEMPTED A PINNACLE TAKEOFF FROM A GRASSY KNOLL. DURING THE TAKEOFF, HE ENCOUNTERED A LOW ROTOR RPM CONDITION IN WHICH THE ENGINE RPM AND MAIN ROTOR RPM DECREASED TOGETHER. THE PILOT SAID HE LOWERED THE COLLECTIVE; HOWEVER, HE WAS UNABLE TO RECOVER THE MAIN ROTOR RPM IN TIME TO PREVENT A FORCED LANDING. HE MADE A RUN ON LANDING IN A GRASSY AREA AND THE MAIN ROTOR AND TAIL ROTOR BLADES STRUCK THE GROUND. THE PASSENGER EXITED THE HELICOPTER WHILE THE MAIN ROTOR WAS STILL TURNING AND HE WAS STRUCK IN THE HEAD BY A BLADE. ACCORDING TO THE FAA BASIC HELICOPTER HANDBOOK, A CONDITION OF LOW ROTOR CAN BE CORRECTED BY REDUCING THE COLLECTIVE WHICH WILL DECREASE THE DRAG ON THE ROTOR, AND THEREFORE, INCREASE THE RPM. ACCORDING TO THE HELICOPTER MANUFACTURER, ONCE THE RPM DROPS BELOW THE NORMAL OPERATING RANGE IT COULD TAKE SEVERAL SECONDS FOR THE RPM TO RECOVER TO THE NORMAL OPERATING RANGE.
On Thursday, August 12, 1993, at 1230 eastern standard time, an Enstrom F28C, N51786, operated by the State of Ohio, lost power on takeoff, and made a hard landing. The helicopter received substantial damage and the passenger received serious injuries. Visual meteorological conditions prevailed and the pilot was flight following on a company flight plan. In the NTSB Accident report, the pilot stated: ...Perry made a takeoff [from a grassy knoll] upwind to a three foot hover...Forward cyclic was applied and after moving approximately ten feet toward Gallia Centerpoint road Perry noted a rapid loss of RPM. Perry lowered the collective but still experienced a rapid loss of RPM...Perry attempted to slow the helicopter and ground contact was made at approximately 10-15 mph...The nose pitched forward and one main rotor made a ground strike. The tail then came down and made contact with the ground. Salisbury was yelling "Get out", "Get out, its going to catch on fire." Perry was attempting to shut down the helicopter, turn off fuel etc. Perry yelled at Salisbury to remain in the helicopter numerous times...and...did not hear Perry. Salisbury exited ...and was struck on the head by a main rotor blade.... When interviewed, after the accident, Mr. Perry said the main rotor and engine RPM needles came down together and did not split. In a telephone interview, Mr. Taylor, an accident investigator for Enstrom Helicopters said that for the main rotor RPM and engine RPM to decrease together as reported by the pilot, is an indication of too much pitch (collective) being applied for the engine power (throttle) available. Mr. Taylor said that once the RPM drops below the normal operating range, it could take several seconds for the RPM to recover to the normal operating range. According to the FAA Publication, Basic Helicopter Handbook, AC61-13B, page 25, Collective Pitch-throttle coordination: Collective pitch is the primary control for manifold pressure; the throttle is the primary control for rpm.... Problem: RPM low, manifold pressure high Solution: Lowering the collective pitch will reduce the manifold pressure, decrease drag on the rotor, and therefore, increase the RPM. On page 103, General rules for pinnacle and ridgeline operations, item 5: Since pinnacles and ridgelines are generally higher than the immediate surrounding terrain, gaining airspeed on the takeoff is more important than gaining altitude. The airspeed gained will cause a more rapid departure from the area. In addition to covering unfavorable terrain rapidly, a higher airspeed affords a more favorable glide angle and thus contributes to the changes of reaching a safe area in the event of a forced landing. If no suitable area is available, a higher airspeed will permit a more effective flare prior to making an autorotative landing. According to the report from the Ohio State Police, "...the knoll [where the helicopter departed] is elevated several feet above [the] farm fence. There is an abrupt downward slope from the knoll to the fence and the hayfield continues on a downward slope until it reaches the fence line at CR60 [County Road 60]." According to FAA Operations Inspector, Mr. Kevin M. Kelly, of the Cincinnati Flight Standards District Office, the engine was run satisfactorly and no problems were noted.
The pilot's improper use of the collective and throttle which resulted in a low rotor rpm condition and a loss of control, and an inflight collision with the terrain.
Source: NTSB Aviation Accident Database
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