History of Flight
On May 26, 2012, about 1220 central daylight time, a Schweizer 269C-1 helicopter, registration 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 14 Code of Federal Regulations Part 91 as a solo instructional flight. Visual meteorological conditions prevailed, and the flight was not operated on a flight plan.
The pilot reported that before the accident flight, he completed a dual instructional flight with his flight instructor, which included a run-on landing and pedal turns. They then hover taxied to the north practice area and set down. Upon touchdown, the helicopter experienced ground resonance. The pilot reduced engine power to idle, and the oscillations stopped. The flight instructor briefed him on the solo flight plan and exited.
For the solo takeoff, as the pilot increased power, the helicopter began to shake. He reduced engine power, but oscillations intensified, becoming uncontrollable, and components began to separate. He shut down the engine and exited after oscillations ceased.
Aircraft Information
Helicopter maintenance records indicated an annual inspection 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. Overhauled landing gear dampers were installed at the aft positions (left and right) on April 23, 2012, at 4,200 hours. The operator stated that the periodic inspection per section 12/14 of the maintenance manual was completed after installation. No further maintenance was performed on the dampers prior to the accident. The records did not indicate replacement of the forward landing gear dampers.
Meteorological Information
At 1154, the SUS Automated Surface Observing System (ASOS) reported wind from 190 degrees at 4 knots, 10 miles visibility, clear sky, temperature 32°C, dew point 21°C, altimeter 30.03 inHg. At 1243, conditions were wind calm, 10 miles visibility, few clouds at 4,000 feet, temperature 32°C, dew point 21°C, altimeter 30.01 inHg.
Wreckage and Impact
The helicopter came to rest upright, partially on a concrete landing pad and partially on grass. Damage included main rotor blades, tail rotor blades, partial separation of the tail boom from the fuselage, and partial separation of the main rotor mast from the aft fuselage structure. A postaccident examination by Federal Aviation Administration (FAA) inspectors revealed no anomalies related to a mechanical failure or malfunction prior to the accident.
Tests and Research
Testing of the landing skid dampers at the manufacturer’s facilities under FAA supervision applied a specified load profile, measuring displacement (stroke) and comparing to specifications. None of the four accident dampers met specifications. The forward dampers were undercharged: they met the low-pressure test point but not the ultimate load requirement (tested at 2,500 lbs and 2,520 lbs vs. specification of 2,900–3,600 lbs). The aft dampers were overcharged: they met the low-pressure test point but exceeded the ultimate load specification (tested at 4,400 lbs and 4,550 lbs vs. specification of 3,200–3,900 lbs). The aft dampers were not identified as left or right.
Additional Information
The FAA Rotorcraft Flying Handbook notes that ground resonance occurs when rotor blades move out of phase, causing rotor disc imbalance, often triggered by a firm touchdown on one corner of the landing gear. This can lead to violent, uncontrollable oscillations. Corrective actions depend on rotor speed: at low rotor speed, close throttle and lower collective; at normal rotor speed, lift off into a hover to allow blades to realign.
The helicopter manufacturer issued a safety advisory on February 22, 2012, regarding ground resonance events. It stated that landing gear dampers slow airframe rocking motion, which may be initiated by pronounced ground contact on one skid. The advisory warned that ground resonance can occur when dampers do not meet design specifications.
Manufacturer maintenance procedures require checking forward and aft landing gear dampers for operation, condition, and extension at each 100-hour inspection; if extension requirements are not met, dampers must be repaired or recharged.
The overhaul procedure for landing gear dampers includes inspection, cleaning, reassembly, and recharging but does not provide for functional/operational testing after overhaul. The procedure notes that incorrect fluid levels, improper pressure, or inoperable valve function will reduce damper effectiveness and may lead to ground resonance. A repair station representative stated that the damper overhaul process is closely monitored to ensure compliance with the approved procedure, resulting in properly functioning overhauled dampers.