Incident Overview
On July 16, 2016, at approximately 1315 eastern daylight time, a Schweizer 269C-1 helicopter, registration N6148D, sustained substantial damage after encountering ground resonance while on a taxiway at Tom B David Field Airport (CZL) in Calhoun, Georgia. The commercial pilot in the right seat and a commercial pilot rated passenger in the left seat were not injured. The helicopter was operated as a personal flight under 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and no flight plan was filed for the local flight, which had originated from CZL about 15 minutes earlier.
Sequence of Events
Earlier that day, the right-seat pilot flew with the helicopter's owner for a demonstration flight, during which the owner performed several autorotative landings with either touchdown or power recovery. After that flight, the owner exited, and a commercial-rated pilot friend of the owner boarded the left seat. The helicopter departed, and the left-seat pilot flew briefly while in the airport vicinity. After 10 to 15 minutes, the right-seat pilot landed the helicopter uneventfully on a taxiway and remained there with the engine at idle for 30 seconds to 1 minute. While idling, the pilots discussed differences between helicopters with a correlator versus a governor for rpm maintenance, intending to test them operationally.
The right-seat pilot guarded the collective down with his hand and slowly increased throttle. As rpm rose, he felt a shake, heard a "boom boom" sound, and noticed vibration, which he identified from training as ground resonance. He reduced throttle, but vibration increased. The left-seat pilot advised lifting off and attempted to raise the collective. The throttle was advanced, and with rpm near the green arc, the helicopter shook and did not become airborne despite collective input. The main rotor blades struck the tailboom multiple times. The right-seat pilot searched for the rotor brake and then turned off the ignition switch.
Damage and Maintenance Findings
The helicopter was equipped with four nitrogen-charged poppet-type hydraulic dampers in the landing gear assembly, designed to dampen shock and prevent ground resonance. These dampers were mounted between the center frame and the skids (two per skid). After the incident, all four dampers were removed and tested at the manufacturer's facility in the presence of a Sikorsky representative and an FAA inspector. Each damper was tested twice using a load cell and computer to record stroke versus load. All four dampers failed the load/stroke test:
- Left forward: out of limits at low compression and very low at ultimate load.
- Right forward: within limits at low compression but very low at ultimate load.
- Left rear: high at initial yield and slightly low at ultimate load.
- Right rear: out of limits high during test, did not reach full travel stroke at maximum load.
The dampers were not disassembled after testing, so fluid levels were not determined.
Maintenance records showed that on November 11, 2013, at 2,038.8 hours total time, all four dampers were recharged with nitrogen per the Handbook of Maintenance Instructions (HMI) Appendix C (forward to 350 psi, aft to 725 psi). On April 15, 2014, the type certificate holder issued Alert Service Bulletin (ASB) C1B-041 for a one-time load/stroke inspection of landing gear dampers at an overhaul or repair facility. This ASB was not incorporated into the HMI and was not mandatory for Part 91 operators. Subsequently, on October 15, 2014, Appendix C of the HMI was revised to include complete load/stroke inspection criteria and instructions for fabricating attachment parts. The FAA inspector found no record of compliance with the ASB.
The helicopter's latest Annual/100-Hour inspection was performed on May 12, 2015, with about 61 hours of operation since then. The HMI specifies damper inspection for operation and extension at each 100-hour inspection or every 6 months. A note states that after initial testing and use, charging pressure decreases to a lower operating pressure. There is no specified maintenance procedure for solely charging dampers; the overhaul manual Appendix C covers charging during repair/overhaul. The manual warns that incorrect fluid levels, improper pressure, or inoperable valving can deteriorate damping, potentially causing ground resonance and destruction.
Discussion of Findings
When asked whether the damper deficiencies could have been detected during on-airframe testing per HMI section 12-11, the manufacturer representative indicated that two underserviced dampers would have been detected, the left rear (slightly low) likely would have passed, and the right rear (overserviced) would not have been detected. The representative noted that they considered developing a test for overserviced dampers but could not devise a definable plan. Regarding incorporating the ASB into the HMI, another representative stated their engineering department deemed it unnecessary because the helicopter was out of annual inspection at the time of the accident and because they believed improperly serviced dampers would be caught during the damper extension test at an annual inspection.
The FAA Helicopter Flying Handbook (FAA-H-8083-21A) states that for ground resonance, if main rotor rpm is low, the corrective action is to close the throttle immediately and fully lower the collective to place blades in low pitch. If rpm is in the normal operating range, the helicopter should be flown off the ground to allow blades to rephrase automatically.