NYC93GA082

The pilot was on approach at about 150-200 ft AGL when the helicopter began rotating clockwise about the nose in a nose-down attitude. It descended to the ground and landed hard in a level attitude collapsing the landing skids. The helicopter then rolled over on its right side, and the main rotor blades struck the ground. According to the pilot, he made a wide right turn to align with the landing area when the nose yawed right about 45 deg. The helicopter rotated twice despite the pilot's attempt to correct with full left pedal and less collective. He stated the engine appeared to be operating at full power after the collision. The investigation did not disclose evidence of mechanical malfunction. Based on army and bell helicopter data, the airspeed and wind combination were conditions conducive for the occurence of the loss of tail rotor effectiveness.

History of flight

On Wednesday, April 14, 1993, at 0347 eastern daylight time, a Bell 206B helicopter, N72HP, piloted by Mr. Edgar Clevinger, and operated by the Ohio State Police, collided with the terrain during the descent for a landing at a remote site near Lucasville, Ohio. The helicopter was destroyed. The pilot and the one passenger received minor injuries. Visual meteorological conditions prevailed at the time and no flight plan had been filed. The flight was being conducted under 14 CFR 91.

The helicopter was on a right base turning to final, at an altitude of approximately 150 to 200 feet above the ground, with slow forward airspeed, when it started to rotate clockwise around the nose, in a nose low attitude descending to the ground. The helicopter landed hard on the skids, breaking the skids, and the main rotor blades struck the ground.

According to the pilot's statement on the NTSB Form 6120.1/2:

...after take off I began to search the facility...after five to seven minutes the search was completed and I was returning to land. I made a wide right turn to line up for a straight in approach to the same location I had just left. After lining up [heading towards the south] I began to descend and slow down at est. [sic] approx. [sic] 30-40 mph, approximately 150-200' agl when the helicopter's nose yawed right approximately 45 degrees. I immediately used left pedal to correct. The nose started back as though to recover then started back right and began to rotate. Full left pedal was used and collective reduced. The a/c [sic] made approximately 2 rotations then struck the ground in a relatively level attitude. After impact the helicopter rolled on it's right side and both rotors struck the ground...after the crash engine still appeared to be operating at full power...the throttle had not been reduced...

The accident occurred during the hours of daylight at approximately 38 degrees 08 minutes north and 85 degrees 01 minutes west.

Personnel information

Mr. Edgar L. Clevinger was born on October 8, 1944. He held a Private Pilot Certificate, No. 299389267, with helicopter and single engine land ratings. Mr. Clevinger was issued a Second Class Airman Medical Certificate on January 29, 1993, with limitations and waiver number 10158425, (hearing).

At the time of the accident Mr. Clevinger had recorded a total of 13,344 flight hours, of which 441 hours were in Bell 206B, helicopters.

Wreckage and impact information

The wreckage was examined at the accident site on April 15, 1993. The helicopter impacted in a grass field about one-quarter mile north of the main entrance to the State Prison. The entire helicopter was within the confines of the wreckage site. The nose of the helicopter was heading in an easterly direction. Control continuity was established to all the flight controls, and the helicopter was removed from the crash site for further examination.

Test and research

The following parts were removed from N72HP and examined under the supervision of the NTSB at Bell's facilities, Fort Worth, Texas, on May 10 and 11, 1993. The examined parts included; swashplate and support assembly; main transmission and mast; fuel boost pumps; main driveshaft; free wheeling assembly; ninety degree tail rotor box.

The examination of the main rotor pitch link tubes revealed that they had fractured by overstress. The swashplate collective sleeve was found to be binding with the swashplate support. The binding resulted from deformation which occurred when the swashplate inner ring over traveled.

The main transmission assembly could not be easily rotated at the main input driveshaft quill, due to corrosion. The oil in the transmission was analyzed, and globules of free water were visible.

Functional testing was conducted on the forward and aft boost pumps and no discrepancies were found. The Bell report did say the pumps, "...did exceed the amperage maximum of 4.0..."

The main driveshaft, aft female coupling was fractured in two parts.

The free wheeling assembly, inner race shaft was fractured by torsional overstress.

The tail rotor 90 degree gearbox was disassembled and no discrepancies were found.

The engine was examined on June 2, 1993, at Allison's facilities, Indianapolis, Indiana, under the supervision of the FAA at the request of the NTSB, Investigator-In-Charge. The results of the examination as per Allison's report were:

...the engine successfully started and achieved takeoff power when tested in an as received condition. The takeoff power available was within 2.9% of new engine specifications...

Additional information

According to Bell Helicopter Textron Inc's, Information Letters, 206-84-41 and 206L-84-27 dated July 6, 1984, a series of tests conducted for the US Army by Bell on OH-58 [206] series helicopters revealed:

...the occurrence of an unanticipated right yaw under certain low speed mission conditions. The Army referred to the right yaw characteristic as loss of tail rotor effectiveness (LTE)...these characteristics are present only at airspeeds less than 30 knots and apply to all single rotor helicopters...

Bell's definition of unanticipated right yaw is:

...the occurrence of an uncommanded right yaw rate which does not subside of its own accord and which, if not corrected can result in the loss of aircraft control. The term "loss of tail rotor effectiveness" is misleading. The tail rotor of the...206 series helicopters has exhibited the capability to produce thrust during all approved flight regimes...

Bell's tests revealed four characteristics during low speed flight identified as factors in unanticipated right yaw. Certain wind velocities and direction of relative wind must be present. Bell lists these wind conditions as;

...1) weathercock stability (120 to 240 degrees;

2) tailrotor vortex ring state (210 to 330 degrees);

3) main rotor disc vortex interference (285 to 315 degrees;

4) loss of translational lift (all azimuths)...if the pilot is inattentive for some reason and a right yaw rate is initiated in one of the above relative wind regions, the yaw rate may increase unless suitable corrective action is taken...

At the time the accident occurred the winds were 090 degrees at 12 knots.

The Bell Information Letter said that the loss of translational lift:

...results in increase power demand and additional anti-torque requirements. If the loss of translational lift occurs when the aircraft is experiencing a right turn rate, the right turn will be accelerated as power is increased, unless corrective action is taken by the pilot. When operating at or near maximum power, this increase in power demand could result in rotor rpm decay...this characteristic is...associated with unanticipated right yaw for two reasons. First, if the pilot's attention is diverted as a result of increasing right yaw rate, he may not recognize that he is losing relative wind hence losing translational lift. Second, if the pilot does not maintain airspeed while making a right downwind turn, the aircraft can experience an increasing right yaw rate as the power demand increases and the aircra