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On May 26, 2012, about 1220 central daylight time, a Schweizer 269C-1 helicopter, N345MH, was substantially damaged during takeoff at Spirit of St. Louis Airport (SUS), Chesterfield, Missouri. The pilot was not injured. The aircraft was registered to and operated by MySky LLC, doing business as Midwest Helicopter, under the provisions of 14 Code of Federal Regulations Part 91 as a solo instructional flight. Visual meteorological conditions prevailed for the flight, which was not operated on a flight plan. The local solo flight was originating at the time of the accident.
The pilot noted that immediately before the accident flight, he completed a dual instructional flight with his flight instructor. That flight consisted of a run-on landing and pedal turns. They subsequently hover taxied to the north practice area and set down. The pilot stated that when they touched down, the helicopter entered some ground resonance. He reduced engine power to idle and the oscillations stopped. His flight instructor briefed him on the plan for the solo flight and exited the helicopter.
The pilot reported that the helicopter started to shake as he increased power for takeoff on the solo flight. He reduced engine power, but the oscillations became stronger. He reported that the oscillations became uncontrollable and some components began to separate for the helicopter. He subsequently shutdown the engine and exited the helicopter once the oscillations had stopped.
The pilot held an Airline Transport Pilot certificate with single and multi-engine land airplane ratings. He also held a Flight Instructor certificate with single and multi-engine airplane, and instrument airplane ratings. He was receiving flight training for the addition of a helicopter rating and was operating the accident helicopter as authorized under a logbook solo endorsement. The pilot was issued a first class airman medical certificate in July 2011.
Helicopter maintenance records indicated that an annual inspection was completed on November 29, 2011, at 4,000.0 hours total airframe time. The most recent 100-hour inspection was completed on April 11, 2012, at 4,199.5 hours. The maintenance records noted that overhauled landing gear dampers were installed at the aft positions, left and right, on April 23, 2012, at 4,200 hours total airframe time. The operator stated that the periodic inspection procedure specified in section 12/14 of the maintenance manual was completed after installation of the overhauled dampers. In addition, no further maintenance had been performed on the dampers prior to the accident. The records did not include any entry indicating that the forward landing gear dampers had been replaced.
Weather conditions recorded by the SUS Automated Surface Observing System (ASOS), at 1154, were: wind from 190 degrees at 4 knots; 10 miles visibility; clear sky; temperature 32 degrees Celsius; dew point 21 degrees Celsius; and altimeter 30.03 inches of mercury. At 1243, the weather conditions were: wind calm; 10 miles visibility; few clouds at 4,000 feet above ground level; temperature 32 degrees Celsius; dew point 21 degrees Celsius; and altimeter 30.01 inches of mercury.
WRECKAGE AND IMPACT INFORMATION
The helicopter came to rest upright, located partially on a concrete landing pad and partially in the surrounding grass area. The helicopter sustained damage to the main rotor blades and tail rotor blades. The tail boom partially separated from the fuselage. The main rotor mast partially separated from the aft fuselage structure. A postaccident examination conducted by Federal Aviation Administration inspectors did not reveal any anomalies related to a mechanical failure or malfunction occurring prior to the accident.
TESTS AND RESEARCH
Testing was conducted on the landing skid dampers at the manufacturer’s facilities under supervision of the Federal Aviation Administration. During testing, a specified load profile is applied to the subject damper and the resulting displacement (stoke) is measured. The load vs. displacement profile is compared to specification in order to determine the status of the damper. None of the four accident dampers met the manufacturer’s specifications.
The forward dampers were undercharged. Specifically, the forward dampers met the load vs. displacement (stroke) requirement at the low pressure test point. However, the ultimate load provided by the forward dampers did not meet the specification. The dampers tested at 2,500 lbs. and 2,520 lbs. for the left and right dampers, respectively. The specification required 2,900 lbs. to 3,600 lbs.
The aft dampers were overcharged. Specifically, the aft dampers met the load vs. displacement (stroke) requirement at the low pressure test point. However, the ultimate load required to meet the required displacement exceeded the specification. The dampers tested at 4,400 lbs. and 4,550 lbs. The specification required 3,200 lbs. to 3,900 lbs. (Note: The aft dampers were not identified as left or right.)
The Federal Aviation Administration Rotorcraft Flying Handbook noted that ground resonance develops when the rotor blades move out of phase with each other and cause the rotor disc to become unbalanced. This can occur when a helicopter touches down firmly on one corner of the landing gear (skid). This condition can lead to a violent, uncontrollable oscillation. In the event that the rotor speed 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. On the other hand, if the rotor speed is in the normal operating range, the helicopter should be brought off the ground into a hover to allow the blades to automatically realign.
The helicopter manufacturer issued a safety advisory, dated February 22, 2012, regarding ground resonance events. The advisory noted that landing gear dampers act to slow airframe rocking motion, which may be initiated by a pronounced ground contact on one landing skid. The advisory warned that ground resonance can occur when landing dampers do not meet design specifications.
The helicopter manufacturer’s maintenance procedures require that the forward and aft landing gear dampers are checked for operation, condition, and extension at each 100 hour inspection. In the event that the dampers do not meet the extension requirements, the dampers must be repaired or recharged.
The overhaul procedure for the landing gear dampers provided for the inspection, cleaning, reassembly and recharging of the components. The overhaul procedure did not provide for functional/operational testing of the dampers following overhaul. The procedure also noted that incorrect fluid levels, improper pressure, or inoperable valve function will reduce the effectiveness of the dampers and may lead to ground resonance. A representative of the repair station noted that the damper overhaul process is closely monitored to insure compliance with the approved overhaul procedure, resulting in a properly functioning overhauled damper.