Warrenton, VA, USA
N3202D
BELL 206B
During a training flight in a Bell 206B helicopter, the certified flight instructor was demonstrating a rapid deceleration maneuver to a grass area at the airport. The helicopter touched down hard, buckling the tail boom downward. The subsequent misalignment fractured the tail rotor drive shaft into several sections, and deflected the tail boom to the right. Examination of the fractured tail boom and tail rotor segments revealed overload fractures consistent with a hard, vertical landing, near 70 percent main rotor rpm. Even though the pilots were not performing an autorotation, the conditions at touchdown were similar to procedures outlined in the Bell 206 Rotorcraft Flight Manual, which instructed pilots to adjust collective pitch during an autorotation maneuver as required to maintain 90 to 107 percent rpm. It was also recommended that a level touchdown be made prior to passing through 70 percent rotor rpm. The manufacturer had warned operators that the tail boom and aft fuselage could be damaged if during an autorotation landing the main rotor rpm was allowed to decay below 70 percent rpm.
On August 7, 2007, at 1130 eastern daylight time, a Bell 206B, N3202D, was substantially damaged after landing at Warrenton-Fauquier Airport (W66), Warrenton, Virginia. The certificated flight instructor (CFI) and certificated commercial pilot were not injured. Visual meteorological conditions prevailed for the local training flight, which was conducted under the provisions of 14 Code of Federal Regulations Part 91. According to the pilots, the CFI was demonstrating a rapid deceleration maneuver, in the grass area east of runway 33. Approximately 10 seconds after touchdown, as the pilots were briefing the maneuver in the helicopter, it yawed approximately 40 degrees to the left. The pilots performed an emergency shutdown and upon exiting the helicopter, they noticed that the tail section was damaged just aft of the fuselage, and canted approximately 20 degrees to the right. The pilots stated they did not hear the low rotor horn and were certain the rotor did not "droop," as they were emphasizing that during the maneuver. The CFI held a commercial pilot certificate with ratings for airplane single-engine land, rotorcraft-helicopter, instrument-airplane, and instrument-helicopter. His most recent FAA second-class medical certificate was issued February 12, 2007. The pilot receiving training held a commercial pilot certificate with ratings for airplane single-engine land, rotorcraft-helicopter, instrument-airplane, and instrument-helicopter. His most recent FAA second-class medical certificate was issued June 1, 2007. At the time of the accident, the CFI reported 2,520 hours of total flight experience, 1,170 hours in rotorcraft, and 11 hours in make and model. The commercial pilot reported 1,340 hours of total flight experience, 855 hours in rotorcraft, and 8 hours in make and model. Examination of the helicopter revealed no damage to the fuselage or skids. The tail boom was buckled and the tail rotor drive shaft was separated into several sections. The separation points displayed evidence of rotation. Tail rotor control continuity was confirmed from the cockpit to the tail rotor. The buckled section of the tail boom and fractured segments of the tail rotor drive shaft were retained and examined at the National Transportation Safety Board Metallurgical Laboratory in Washington D.C. The tail boom was buckled both downwards and to the right, and a smeared area was noted at the apex of the buckle. Examination of the fracture faces produced by the buckling revealed clean grainy surfaces that were inclined on a slant plane, consistent with an overload event. The drive shaft was comprised of a forward and aft section connected at a coupling. The forward drive shaft segment consisted of two pieces separated by a fracture. Both fracture faces displayed clean surfaces oriented on a slant plane, consistent with an overload event. The fractures also displayed a twist consistent with the normal direction of rotation of the tail rotor drive shaft. The coupling that connected the forward and aft sections of drive shaft was comprised of ten coupling discs. Examination of the discs revealed fracture features in the center of the discs consistent with reverse bending fatigue. Measurements of the fractures suggested the fatigue zone was generated during 48 revolutions of the misaligned drive shaft (or less than one second). Outside of the central area the fracture faces displayed features typical of an overload event. The aft section of the drive shaft contained circumferential smearing on the outer surface of the tubular portion. The smearing coincided with the location of the smearing observed on the tail boom. Examination of the fracture surface on the aft drive revealed severe smearing and mechanical damage. The winds, reported at an airport 12 miles to the northeast, at 1055, were calm. Information provided by Bell Helicopter revealed the tail rotor drive shaft rotated at 6,016 rpm at 100 percent main rotor rpm (or about 100 revolutions per second). The drive shaft rotated at 4,211 rpm at 70 percent main rotor rpm (or about 70 revolutions per second). Bell Helicopter issued an Operations Safety Notice (OSN) on September 13, 1984 to warn operators of reports of compression wrinkling of the tailboom skin. The OSN stated, "...the tailboom and aft fuselage can be damaged if during an autorotation landing the main rotor rpm is allowed to decay below 70 percent rpm. Applying collective pitch in excess of that required will, in some instances, result in excessive flapping of the main rotor during or after touchdown. This can cause a reaction that can damage the tailboom and/or aft fuselage." Examination of the Safety Board's database revealed a similar accident (SEA90FA079), during which a helicopter on a training flight touched down "firmly," and resulted in the same buckling of the tail boom. According to the FAA Rotorcraft Flying Handbook, "the rapid deceleration or quick stop maneuver is used to slow the helicopter rapidly and bring it to a stationary hover...After leveling off at an altitude between 25 and 40 feet, accelerate to the desired entry speed for the helicopter (approximately 45 knots)...Initiate the deceleration by applying aft cyclic to reduce forward speed, simultaneously lowering the collective as necessary to counteract any climbing tendency. The timing must be exact...After attaining the desired speed, initiate the recovery by lowering the nose and allowing the helicopter to descend to a normal hovering altitude, adjusting the throttle to maintain rpm, and applying proper pedal pressure, as necessary, to maintain heading. One of the common errors described in the handbook was "allowing the helicopter to stop forward motion in a tail-low attitude." According to the Bell 206 Rotorcraft Flight Manual, "Engine Failure and Autorotation" section, pilots were instructed to adjust collective pitch during the maneuver as required to maintain 90 - 107 percent rpm. It was also recommended that level touchdown be made prior to passing through 70 percent rotor rpm. Upon ground contact, collective pitch shall be reduced smoothly while maintaining cyclic in neutral or centered position.
The flight instructor's failure to maintain adequate rotor rpm during touchdown.
Source: NTSB Aviation Accident Database
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