Hampton, GA, USA
N9422F
Hughes 269B
While hovering, the pilot attempted to set the helicopter down. During the transition from hover to landing, the pilot noticed a vibration in the helicopter. He decided to “pull more power” in an attempt to regain a hover condition. When power was applied, the helicopter began to shake violently. He then “rolled the power off and secured the landing.” The helicopter sustained substantial damage. A postaccident examination of the helicopter revealed that the four oleo landing struts were sufficiently extended; however, damage to the helicopter precluded measurement of the struts in the properly loaded condition. No maintenance was performed on the struts and no annotations about their condition were recorded during the most recent annual inspection. Although the pilot did not initially report any anomalies with the engine, he subsequently stated that the engine had “sputtered” during the hover and later reported that he had noticed a vibration and loss of power during the hover. He advised that he was a student helicopter pilot and was not completely familiar with the types of vibrations that were “normal” versus those that were not, and he had discussed this with his instructor on previous flights. The examination of the engine revealed no evidence of pre-accident mechanical malfunctions or failures that would have precluded normal operation. The investigation was unable to determine if the ground resonance was caused by improper maintenance of the oleo landing gear struts, the pilot’s improper landing technique, or a combination of both.
On May 15, 2019, about 1600 eastern daylight time, a Hughes 269B helicopter, N244F, was substantially damaged when it was involved in an accident at Henry County Airport (HMP), Hampton, Georgia. The pilot was not injured. The helicopter was operated under Title 14 Code of Federal Regulations Part 91 as a personal flight. The pilot reported that he had purchased the helicopter about a week prior to the accident. He said that an annual inspection was performed shortly after the purchase, with no issues noted. According to his recollection, no maintenance was required or performed on any of the four landing skid damper “oleo” units. In an interview with a Federal Aviation Administration (FAA) inspector on the day of the accident, the pilot reported that during hover practice, he decided to set the helicopter down on the ramp because an aircraft was crossing in front of him. During the transition from hover to landing, the pilot noticed a vibration in the helicopter. He decided to “pull more power” and regain a hover condition. When power was applied, the helicopter began to shake violently. He then “rolled the power off and secured the landing.” During a subsequent telephone interview with the NTSB duty officer, the pilot reported that while he was hover taxiing, the engine “sputtered” and as he landed on the taxiway, the helicopter experienced ground resonance. During a follow-up telephone interview with an NTSB investigator, the pilot reported that while hover taxiing to his hangar after refueling, he noticed a vibration and loss of power. He advised that he was a student helicopter pilot and was not completely familiar with the types of vibrations that were “normal” versus those that were not, and he had discussed this with his instructor on previous flights. As he set the helicopter down, it “experienced ground resonance.” He attempted to lift the helicopter back into a hover but it “did not have enough power to get airborne again.” He then elected to shut the helicopter down while it continued in ground resonance. An examination of the wreckage by a Federal Aviation Administration (FAA) inspector revealed that the tail boom was fractured and separated aft of the tail boom support brackets. The tail rotor blades were largely undamaged. The “blue” main rotor blade damper body attachment bracket was damaged, and the damper arm attachment bracket was fractured. All flight controls appeared intact. The engine’s No. 3 cylinder exhaust valve rocker arm and pushrod were removed, and the pushrod was found slightly bent. A follow up examination of the helicopter by a National Transportation Safety Board Investigator revealed that the collective and cyclic controls remained intact, and when manipulated manually operated normally with no binding from the cockpit controls to the main rotor head. All three main rotor blades showed buckling damage on the trailing edge just outboard of the blade roots, consistent with bending in the aft direction. The tail rotor drive shaft and tail rotor pitch control tube were fractured at the tail boom fracture area. The pitch control tube was continuous to the cockpit anti-torque pedals. The four landing gear oleo struts were examined. The rubber boots were attached, but not secured. The pressure in the struts could not be measured due to the lack of a suitable pressure gauge. Measurement of the strut extension length were within the maintenance manual limits, however the helicopter could not be properly configured for the prescribed measurement technique (the fuel tank could not be filled, the tail boom was no longer attached to the fuselage, and the main rotor blades had been previously removed). The eight engine to transmission V-belts were in position and slack. The bottom spark plugs were removed; all plugs exhibited normal wear and color when compared to a Champion Check-a-Plug chart. The engine was rotated manually at the crankshaft extension at the V-belt drive section. With the exception of the No. 3 cylinder (for which the rocker arm and pushrod had been removed during the previous inspection), suction and pressure were observed on all cylinders when the engine crankshaft was rotated. No internal binding or unusual noise was noted. According to the pilot/owner, the engine had been operated about 184 hours since overhaul. The most recent annual inspection was performed on May 10, 2019, at which time the helicopter had accumulated a total of 5,249 hours. A safety advisory from the helicopter manufacturer recommends that the oleo struts be examined, and their extensions measured, during all periodic inspections to reduce the likelihood of ground resonance. A review of the available maintenance records revealed one entry that specifically referred to the oleo struts: on March 28, 2018, at an aircraft total time of 5,241 hours an entry read “checked Oleo pressure – OK.” The FAA Rotorcraft Flying Handbook publication FAA-H-8083-21 states: Ground resonance is an aerodynamic phenomenon associated with fully-articulated rotor systems. It develops when the rotor blades move out of phase with each other and cause the rotor disc to become unbalanced. This condition can cause a helicopter to self-destruct in a matter of seconds. However, for this condition to occur, the helicopter must be in contact with the ground. If you allow your helicopter to touch down firmly on one corner (wheel type landing gear is most conducive for this) the shock is transmitted to the main rotor system. This may cause the blades to move out of their normal relationship with each other. This movement occurs along the drag hinge. If the r.p.m. is low, the corrective action to stop ground resonance is to close the throttle immediately and fully lower the collective to place the blades in low pitch. If the r.p.m. is in the normal operating range, you should fly the helicopter off the ground, and allow the blades to automatically realign themselves. You can then make a normal touchdown. If you lift off and allow the helicopter to firmly re-contact the surface before the blades are realigned, a second shock could move the blades again and aggravate the already unbalanced condition. This could lead to a violent, uncontrollable oscillation. According to FAA airman records, the pilot held an airline transport pilot certificate with ratings for airplane single-engine and multiengine land. He reported 5,200 hours of total flight experience the day after the accident, when he renewed his FAA first-class medical certificate. Regarding his helicopter flight experience, the pilot indicated that he had a student certificate for rotorcraft and had accumulated a total of 27 hours of helicopter flight time, of which 17 hours were in the same make and model as the accident helicopter. A review of excerpts from his logbook revealed that all 17 flight hours in make/model were dual instruction received.
Ground resonance during landing for reasons that could not be determined based on the available information.
Source: NTSB Aviation Accident Database
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