Helicopter Self-Destructs Due To Ground Resonance
What You Haven't Seen
2018-04-28
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Note: There is no collision between the helicopter and the passing general aviation aircraft.
On June 2, 2012, about 1135 mountain daylight time, a Schweizer 269C helicopter, N2196F, entered ground resonance when it set down on the ramp at the Rocky Mountain Metropolitan Airport (BJC), near Denver, Colorado. The commercial pilot and the passenger were uninjured. The helicopter sustained substantial damage. The helicopter was registered to Top Flight Rotors LLC and operated by TYJ Global under the provisions of 14 Code of Federal Regulations Part 91 as an instructional flight. Day visual flight rules (VFR) conditions prevailed for the flight, which operated on a company VFR flight plan. The local flight was originating from BJC at the time of the accident.
The pilot reported that the flight was an introductory discovery flight for his student passenger. The pilot indicated that he picked the helicopter up into a two-foot skid height hover. An airplane started to taxi in front of the helicopter. The pilot, in part, stated:
"To avoid my downwash disturbing the airplane, or his prop wash from disturbing me, I set the helicopter back onto the ground as a courtesy. I set down the left skid first and then the right, utilizing a two point touch down. When the right skid made contact, the helicopter tried to drift to the right. I put slight left cyclic input in to keep the helicopter from drifting to the right as I lowered collective. As I lowered the collective fully and the skids started to settle apart, I noticed a slight low frequency vibration. Immediately, the vibration got worse, and I determined the helicopter was in the beginning phase of ground resonance. My engine RPM was too low, as was my collective, to pick the helicopter up off the ground to restore the rotor blades to their correct phase in time. With the collective full down, I rolled the throttle to idle to try and get rid of ground resonance. Within seconds, the helicopter had shaken itself apart."
Postaccident testing revealed that all four landing gear dampers failed to meet the manufacturer's serviceable testing specifications, and two dampers were overcharged.
Maintenance records indicated that the front dampers had been replaced with overhauled units about 10 months and about 534 flight hours before the accident. The aft dampers had been replaced with overhauled units about 7 months and about 280 flight hours before the accident.
A review of the manufacturer's overhaul procedure for the landing gear dampers revealed that the procedure used by the overhaul facility did not include functional testing of the dampers following overhaul, which would have identified the out-of-specification condition before installation. The manufacturer's instructions had been followed, however. (They simply did not include any functional or operational testing.)
As a result of this and a similar accident, the helicopter manufacturer has committed to revising its overhaul inspection guidelines.
On June 2, 2012, about 1135 mountain daylight time, a Schweizer 269C helicopter, N2196F, entered ground resonance when it set down on the ramp at the Rocky Mountain Metropolitan Airport (BJC), near Denver, Colorado. The commercial pilot and the passenger were uninjured. The helicopter sustained substantial damage. The helicopter was registered to Top Flight Rotors LLC and operated by TYJ Global under the provisions of 14 Code of Federal Regulations Part 91 as an instructional flight. Day visual flight rules (VFR) conditions prevailed for the flight, which operated on a company VFR flight plan. The local flight was originating from BJC at the time of the accident.
The pilot reported that the flight was an introductory discovery flight for his student passenger. The pilot indicated that he picked the helicopter up into a two-foot skid height hover. An airplane started to taxi in front of the helicopter. The pilot, in part, stated:
"To avoid my downwash disturbing the airplane, or his prop wash from disturbing me, I set the helicopter back onto the ground as a courtesy. I set down the left skid first and then the right, utilizing a two point touch down. When the right skid made contact, the helicopter tried to drift to the right. I put slight left cyclic input in to keep the helicopter from drifting to the right as I lowered collective. As I lowered the collective fully and the skids started to settle apart, I noticed a slight low frequency vibration. Immediately, the vibration got worse, and I determined the helicopter was in the beginning phase of ground resonance. My engine RPM was too low, as was my collective, to pick the helicopter up off the ground to restore the rotor blades to their correct phase in time. With the collective full down, I rolled the throttle to idle to try and get rid of ground resonance. Within seconds, the helicopter had shaken itself apart."
Postaccident testing revealed that all four landing gear dampers failed to meet the manufacturer's serviceable testing specifications, and two dampers were overcharged.
Maintenance records indicated that the front dampers had been replaced with overhauled units about 10 months and about 534 flight hours before the accident. The aft dampers had been replaced with overhauled units about 7 months and about 280 flight hours before the accident.
A review of the manufacturer's overhaul procedure for the landing gear dampers revealed that the procedure used by the overhaul facility did not include functional testing of the dampers following overhaul, which would have identified the out-of-specification condition before installation. The manufacturer's instructions had been followed, however. (They simply did not include any functional or operational testing.)
As a result of this and a similar accident, the helicopter manufacturer has committed to revising its overhaul inspection guidelines.
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