Incident Overview
On May 15, 2019, at about 1600 eastern daylight time, a Hughes 269B helicopter, registration N244F, was substantially damaged during an accident at Henry County Airport (HMP), Hampton, Georgia. The pilot, who was the sole occupant and operator of the personal flight conducted under 14 CFR Part 91, was not injured.
Pilot Account
The pilot reported that he had purchased the helicopter approximately one week prior to the accident. 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 initial interview with an FAA inspector, the pilot stated 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, he noticed a vibration. He attempted to regain a hover by applying more power, but the helicopter began to shake violently. He then reduced power and secured the landing.
Subsequently, in a telephone interview with the NTSB duty officer, the pilot reported that while hover taxiing, the engine “sputtered” and as he landed on the taxiway, the helicopter experienced ground resonance.
During a follow-up interview with an NTSB investigator, the pilot stated that while hover taxiing to his hangar after refueling, he noticed a vibration and loss of power. He mentioned 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 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.
Wreckage Examination
An examination by an 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 by an NTSB investigator showed 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 exhibited 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.
Landing Gear and Engine Inspection
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 was within the maintenance manual limits; however, the helicopter could not be properly configured for the prescribed measurement technique because the fuel tank could not be filled, the tail boom was no longer attached, 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 and 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, suction and pressure were observed on all cylinders. No internal binding or unusual noise was noted.
Maintenance History
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 available maintenance records revealed one entry specifically referring to the oleo struts: on March 28, 2018, at an aircraft total time of 5,241 hours, an entry read “checked Oleo pressure – OK.”
Additional Information
The FAA Rotorcraft Flying Handbook describes ground resonance as 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, potentially leading to a violent, uncontrollable oscillation if the helicopter is in contact with the ground.
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 helicopter flight experience, the pilot indicated 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.