N6197N

On April 25, 1995, at 1950 hours Pacific daylight time, a Bell 206-L4 helicopter, N6197N, sustained a tailboom deformation after completing a simulated autorotation to touch down at Thermal Airport, Thermal, California. The pilots were conducting a visual flight rules instructional flight. The helicopter, registered to and operated by the California Highway Patrol (CHP), sustained substantial damage. Neither the certificated commercial pilot/certified flight instructor (CFI), nor the certificated commercial pilot trainee was injured. Visual meteorological conditions prevailed. The flight originated at Thermal Airport at 1920 hours.

The CHP chief helicopter pilot reported that the CFI was preparing the dual student for a progress evaluation flight to be conducted the following day. The CFI demonstrated a right 180-degree autorotation on runway 30. The CFI terminated the autorotation with ". . . full throttle at a 3-foot hover with a slight forward ground speed . . . ." While in the hover, the CFI decided to demonstrate a hovering autorotation to a landing.

The CFI retarded the throttle and cushioned the landing by applying up collective. The skid heels initially contacted the runway and were followed by the remainder of the skids. The flight crew described the touchdown as ". . . smooth and unremarkable . . . ." The helicopter, however, came to an unusually quick stop.

The crew then monitored a CHP law enforcement call and air-taxied the helicopter to the CHP pad to pick up a flight officer to respond to the call. After arriving at the pad, the CFI noticed that the tailboom was wrinkled between tailboom stations 24 and 26 and instructed the pilot trainee to shutdown the helicopter.

The CHP reported that the surface winds were from 280 degrees at 15 knots, gusting to 30 knots. They also reported that runway 30 is an asphalt surfaced runway that had been resurfaced on March 2, 1995.

The CHP performed a hard landing inspection following the accident. The inspection revealed that the helicopter did not sustain any damage, other than the wrinkled tailboom, which would be consistent with a hard landing.

The inspection showed, however, a 5-inch by 5-inch metal sheet patch in the interior of the tailboom assembly. Subsequent investigation showed that the patch was installed by the factory during manufacture of the helicopter.

The helicopter was equipped with skid shoes in accordance with a supplementary type certificate (STC) No. SR00313AT. According to a Bell Helicopter representative, the skid shoes would have a higher traction value than the original skids.

Bell Helicopter provided the NTSB with a copy of Operations Safety Notice (OSN) No. 206L-84-10, dated September 5, 1984. This notice addresses compression wrinkling of the tailboom skin near station 45. The OSN states, in part:

Past 206 experience and flight tests with an OH-58A (A military helicopter with similar tailboom and aft fuselage structure) have revealed 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 instance's result in excessive flapping of the main rotor during or after touch down. This can cause a resonant response that can damage the tailboom and/or aft fuselage.

Touch down rotor RPM above 70 percent RPM is preferred. Upon ground contact collective pitch should be reduced smoothly without delay while maintaining cyclic pitch near center position.

Caution: Long ground runs with the collective up, or any tendency to float for long distance prior to skid contact should be avoided.

The OSN information was incorporated in the helicopter flight manual. The CHP helicopter chief pilot told Safety Board investigators that the CFI was aware of the OSN.

The CHP sent the tailboom buckled area to California Polytechnic Institute, San Luis Obispo, California, for analysis. The CHP reported that the analysis indicated the ". . . the failure (buckling) was from inertial forces created during the autorotational landing . . . ."

The CHP sent a section of the tailboom skin which encompassed the patch to the National Transportation Safety Board Materials Laboratory for examination. The examination disclosed no evidence of any buckling or other type of deformation.

A Bell Helicopter Product Support representative reported that the conditions relating to the OSN were well known and are taught during training. He said that the phenomenon (a.k.a. pylon whirl) " . . . occurs after touchdown from an autorotation. Elements that contribute to the whipping are low main rotor RPM, substantial ground run, and collective being maintained or raised after touchdown . . . ."