N911GV

On June 24, 2019, about 2200 eastern daylight time, a Bell OH-58C, N911GV, was substantially damaged when it was involved in an accident at Gainesville Regional Airport (GNV), Florida. The pilot receiving instruction and flight instructor were not injured. The helicopter was operated as a public aircraft instructional flight.

The pilot receiving instruction and instructor planned to perform standard maneuvers in unaided and aided (with night vision goggles [NVG]) night visual meteorological conditions. According to the pilot receiving instruction, both pilots were wearing NVGs when the instructor took control of the helicopter to demonstrate a standard, straight-in autorotation for landing on the parallel taxiway. After entry, the instructor made trim and alignment adjustments during the descent and announced “100 feet” above ground level prior to initiating the decelerative flare. The pilot receiving instruction said the radar altimeter displayed 75 feet at the announcement and described the flare, the remainder of the descent, and the leveling of the helicopter prior to touchdown.

The pilot receiving instruction stated that, at touchdown, “I immediately felt like we were driving down a pothole filled dirt road and after several quick bumps, up and down while skidding across the taxiway, I felt a massive whip forward and to the left followed by several more throws in a counterclockwise motion. The throws became more intense and violent and … braced myself for the aircraft to come apart or tip over.” During the ground run, the instructor announced the helicopter was experiencing “pylon whirl.” The instructor continued to adjust the flight controls until the ground run ended and the “throws” subsided.

The instructor described the descent and the “initial pitch pull” to slow the descent and level the helicopter for touchdown. At touchdown, the heels of the landing gear skids touched “first”, and the helicopter “rocked” forward, “grabbed,” and began to oscillate “uncontrollably.” The instructor stated that he adjusted the flight controls to keep the helicopter “straight and level” and lowered the collective control “to prevent pylon whirl.”

The instructor repositioned, shut down, and inspected the helicopter after the landing and said that the “effects of pylon whirl were readily apparent.”

A Federal Aviation Administration (FAA) examined the helicopter after the accident and noted damage to the tail boom, isolation mount, and the upper cowling around the rotor mast of helicopter.

According to the OH-58A/C Technical Manual, “Pylon whirl is a condition which occurs after blade flapping and mast bumping. The resultant motion of the pylon is elliptical, and spike knock is apt to occur. If the frequency of motion coincides with a particular natural frequency of the helicopter, and the amplitude and direction of the force is large enough, damaging vibrations can occur in the aft section tailboom of the helicopter. Motion of this type could occur during touchdown autorotations, if operational limits are exceeded.

The Transportation Safety Board of Canada cited in Aviation Investigation Report A16P0161:

The fuselage of the Bell 206B is suspended from the main-rotor transmission by 2 A-frame pylons. Spherical bearings allow the pylons, and hence the main-rotor transmission and mast, to move in relation to the fuselage, with the motion dampened by the isolation mount. Main-rotor thrust and dynamic forces normally maintain the main-rotor transmission and fuselage in alignment; however, certain maneuvers can cause unbalanced pylon motion. This phenomenon, known as pylon whirl, can lead to spike knock, which is described as follows:

Spike knock is the contact of the transmission drag pin with the roof mounted static stop plate as a result of pylon motion or pylon whirl. This results in an audible knocking noise. Low rotor RPM, extreme asymmetric loading, poor execution of an auto-rotational landing, and low G maneuvers are factors that may contribute to the occurrence of spike knock.