Schweizer 269C N902CP accident near Temple, Texas

Probable cause

Ground resonance due to maintenance personnel’s improper inspection of the landing gear dampers, which resulted in the dampers operating outside of allowable limits.

NTSB analysis

The pilot reported that, during initial taxi, he noticed that the intercom voice operated relay (VOX) volume was set too high, so he landed the helicopter on the asphalt taxiway to adjust the VOX. With the helicopter at flat-pitch operating rpm and just as the pilot reached to adjust the VOX, the helicopter began oscillating due to ground resonance. Within 5 seconds of touchdown, the oscillating became severe. The pilot ensured that the collective was at flat pitch, and he rolled the throttle to flight idle. Within 15 seconds, the main rotor blades impacted the ground, tail boom, and other helicopter components.

During postaccident examination, all four main landing gear dampers failed load stroke tests. The helicopter's maintenance records indicated that the main landing gear dampers were serviced by charging them with nitrogen in accordance with the helicopter's maintenance manual about 1 month before the accident. However, about 14 days before the dampers were serviced, the helicopter manufacturer had issued an alert service bulletin (ASB) that recommended a one-time load stroke inspection of the landing gear dampers to be performed at an overhaul or repair facility to ensure that the dampers were serviced correctly. Concurrently, the helicopter manufacturer had issued a revision to the maintenance manual that added a load stroke inspection procedure to the section of the manual pertaining to repair and charging of the dampers. The company's maintenance manager reported that maintenance personnel were unaware that the ASB and maintenance manual revision had been issued until after the accident. If the load stroke inspection of the dampers called for by the ASB or the maintenance manual revision had been performed, it would have identified the out-of-specification condition of the dampers before reinstallation. Because the dampers were out-of-specification, ground resonance occurred after the pilot landed the helicopter.

Text from the NTSB. Open the NTSB docket for CEN14LA269 for the full record.