Incident Overview
On May 31, 2014, at approximately 1015 central standard time, a Schweizer 269C helicopter, registration N902CP, sustained substantial damage due to ground resonance at Draughon-Miller Central Texas Regional Airport (TPL) in Temple, Texas. The commercial pilot and one passenger were not injured. The helicopter was operated by Kachemak Bay Flying Service Inc. under 14 CFR Part 91 as a demonstration flight. Visual meteorological conditions prevailed, and no flight plan was filed. The helicopter had departed TPL at 1010 on a local flight.
Pilot's Account
The pilot reported that during initial taxi, the intercom voice operated relay (VOX) volume was set too high. He landed the helicopter on the asphalt taxiway into the wind to adjust the VOX. With the helicopter at flat pitch and operating rpm, as he reached to adjust the VOX, the helicopter entered ground resonance. Within five seconds of touchdown, the oscillations became severe. The pilot ensured the collective was at flat pitch and rolled the throttle to flight idle. Within 15 seconds, the main rotor blades impacted the ground, tail boom, and other components. The pilot shut down the engine using the magneto switch and evacuated the helicopter with the passenger.
Examination of Landing Gear Dampers
All four main landing gear dampers were removed and sent for a load stroke examination. A load cell applied pressure, and a computer recorded stroke versus load. Visual examination revealed varying amounts of dried hydraulic fluid on the pistons, dust seals, and inside the boots. All four dampers failed the load stroke test.
- The left forward damper had minor bending to the upper bearing lug. It was within tolerance on the compression stroke at the lower test point but slightly high at 3,620 lbs (limit 2,900–3,600 lbs) and out of limits at the ultimate load range.
- The right forward damper was intact; it was within tolerance at the lower test point but out of limits low at 1,787 lbs at the ultimate load range (limit 2,900–3,600 lbs).
- The left rear damper was intact but out of limits high, exceeding 3,900 lbs at 3.12 inches of travel (nominal travel ~3.5/3.6 inches).
- The right rear damper was intact but slightly high at 3,910 lbs at the ultimate load point (limit 3,200–3,900 lbs).
Maintenance History and Service Bulletin
On April 15, 2014, Sikorsky issued Alert Service Bulletin (ASB) B-304 for the 269C helicopter, requiring a one-time load stroke inspection of landing gear dampers at an overhaul or repair facility. Compliance was essential by January 9, 2015, for dampers that had been overhauled, serviced, disassembled, or had their charge or fluid level affected. Concurrently, Sikorsky revised the Handbook of Maintenance Instructions (HMI) to include a load stroke inspection procedure in Appendix C, Part VI.
Maintenance records showed the last annual inspection occurred on April 21, 2014, at 5,204.5 total aircraft hours. On April 29, 2014, the main landing gear dampers were serviced by charging with nitrogen per HMI paragraph 5-4 of Appendix C Part VI. The maintenance facility did not comply with ASB B-304 or the concurrent HMI revision. The company's maintenance manager reported they were unaware of ASB B-304 and the HMI revision until after the accident. Subsequently, they inspected dampers on two other Schweizer 269C helicopters and found 5 of 8 dampers failed the load test and required overhaul.