Calhoun, GA, USA
N6148D
SCHWEIZER 269C
The commercial pilot flew with the helicopter's owner earlier that day for a demonstration flight, during which the owner performed several autorotative landings to either touchdown or power recovery. After that flight, the owner exited the helicopter, and a commercial pilot-rated passenger boarded and sat in the left seat of the helicopter. They departed, and, after about 10 to 15 minutes, the pilot landed the helicopter uneventfully on a taxiway. With the engine operating, they discussed a helicopter system. The pilot intended to demonstrate the system that they were discussing, and, while guarding the collective down, he slowly advanced the throttle. As the rpm increased, he reported feeling a shake, then a "boom boom" sound, and noticed a vibration, which he identified from training as ground resonance. He reduced throttle, which was the correct procedure to perform for ground resonance, but the vibration increased. The pilot-rated passenger then advised him to pick the helicopter off the ground and attempted to assist by raising the collective. The throttle was advanced, and, with the main rotor rpm near the green arc, the helicopter began shaking and did not become airborne, although the collective was raised. The main rotor blades contacted the tailboom several times. The helicopter's latest annual/100-hour inspection was performed about 1 year 2 months before the accident, and the helicopter had been operated about 61 hours since then. The handbook of maintenance instructions for the helicopter specified that the dampers are to be inspected for operation and condition, and for damper extension on airframe, at each 100-hour inspection or every 6 months, whichever occurred first. Postaccident operational testing of the dampers from the unairworthy helicopter revealed that all failed a load stroke test. Two of the four dampers, which were underserviced, likely would not have passed an on-airframe extension test during the annual/100-hour inspection that was overdue, while one damper, which was slightly underserviced, would likely have passed. The remaining damper was overserviced, but that condition would not have been detected during the on-airframe extension test because there was no test procedure in place to identify that condition. Without the airframe protection provided by the dampers, ground resonance occurred. Further, the Federal Aviation Administration's Helicopter Flying Handbook states that with respect to ground resonance, if the main rotor rpm is low, the only corrective action to stop ground resonance is to close the throttle immediately and fully lower the collective to place the main rotor blades in low pitch. Thus, raising the collective and advancing the throttle likely contributed to the helicopter's damage.
On July 16, 2016, about 1315 eastern daylight time, a Schweizer 269C-1, N6148D, was substantially damaged after it experienced ground resonance while on a taxiway at the Tom B David Field Airport (CZL), Calhoun, Georgia. The commercial pilot and a commercial pilot rated passenger were not injured. The helicopter was being operated under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed and no flight plan was filed for the local flight, which originated about 15 minutes earlier from CZL.The commercial pilot, who was seated in the right seat flew with the helicopter's owner earlier that day for a demonstration flight which included the owner performing several autorotative landings to either touchdown or power recovery. At the conclusion of that flight, the owner exited the helicopter and a commercial-rated pilot who was a friend of the owner boarded the left seat of the helicopter. The helicopter subsequently departed and while in the vicinity of the airport, the left seat pilot flew the helicopter briefly. After about 10 to 15 minutes, the right seat pilot landed the helicopter uneventfully on a taxiway and remained there between 30 seconds and 1 minute with the engine at idle. While there, they discussed the differences between helicopters equipped with a correlator and a governor to maintain rpm, which they intended to operationally test. The right seat pilot guarded the collective in the down position with his hand and slowly applied throttle. As the rpm increased he reported feeling a shake, then a "boom boom" sound and noticed a vibration, which he identified from training as ground resonance. He reduced throttle but the vibration increased. At that time, the left seat pilot advised him to pick the helicopter off the ground, and attempted to assist by raising the collective. The throttle was advanced and with the rpm near the green arc the helicopter began shaking and did not become airborne, although the collective was raised. The main rotor blades contacted the tailboom several times and the right seat pilot attempted to locate the rotor brake, and then turned off the ignition switch. The helicopter was equipped with four poppet type nitrogen charged hydraulic units in the landing gear assembly designed to dampen landing shock and help prevent ground resonance. The dampers were mounted between the helicopter center frame section and the landing gear skids (two for each skid, left-and right-hand sides). All skid dampers were marked for location, removed from the helicopter and tested at the damper manufacturer's facility. A representative of the type certificate holder for the helicopter, Sikorsky Aircraft Corporation (Sikorsky), was present, along with a Federal Aviation Administration (FAA) inspector. A report of results was subsequently prepared by the representative of Sikorsky. Testing of each damper was performed twice using a load cell to apply pressure and a computer to record the stroke versus the load required for compression. The computer program was the same as used to verify proper assembly and operation of new production dampers. According to the report, all dampers failed the load stroke test. The left forward damper was out of limits at the low range compression test point and very low at the ultimate load range. The right forward damper was within limits at the low range compression test point and very low at the ultimate load range. The left rear damper was high at the initial yield point and slightly low at the ultimate load range. The right rear damper was out of limits high during the load stroke test, and did not reach full travel stroke with the load reading at the maximum range limit. The dampers were not disassembled following testing; therefore, the fluid levels were not determined. A review of the maintenance records revealed an entry dated November 11, 2013, at helicopter total time 2,038.8 hours, indicating, "recharged all 4 landing gear dampers with nitrogen utilizing [Sikorsky formerly] Schweizer special tools. Work accomplished IAW Model 269C-1 [Handbook of Maintenance Instructions] HMI [Appendix] C. [Forward] dampers charged to 350 psi and aft dampers charged to 725 psi." On April 15, 2014, the certificate holder issued 269C-1 Helicopter Alert Service Bulletin (ASB) C1B-041 specifying a one-time load/stroke inspection of landing gear dampers to be performed at an overhaul or repair facility. The ASB was not incorporated into the HMI, and was not considered mandatory for Part 91 operators. Appendix C of the HMI was revised by Sikorsky on October 15, 2014 to include the complete load/stroke inspection criteria as well as directions for obtaining the load/stroke inspection apparatus. Additionally, instructions were included for fabricating parts that are used to attach the landing gear damper to the load/strike inspection apparatus. According to the FAA inspector, there was no record that the ASB had been complied with. The helicopter's latest Annual/100-Hour inspection was performed on May 12, 2015, and the helicopter had been operated about 61 hours since then. Section 12-14 of the HMI specifies the dampers are to be inspected for operation and condition, and for damper extension on airframe at each 100-Hour inspection or every 6 months, whichever occurred first. A note in section 12-11 of the HMI pertaining to the dampers states that after initial testing at Sikorsky and subsequent use on the helicopter, original charging pressure will decrease to a slightly lower operating pressure. There is no specified maintenance procedure for only charging the dampers, but the damper overhaul manual Appendix C to HMI, does specify a charging procedure associated with damper repair or overhaul. The overhaul manual also specifies that incorrect fluid levels, improper pressure or inoperable valving will deteriorate the damping capabilities of the landing gear dampers, which may result in ground resonance and destruction of the helicopter. The airframe manufacturer representative was asked whether the findings of the accident dampers could have been detected during on-airframe testing in accordance with section 12-11 of the HMI. The representative indicated that 2 of the 4 dampers that were underserviced would have been detected in the on-airframe test, the left rear damper that was slightly low would likely have passed the on-airframe testing, and the right rear damper that was overserviced would not have been detected in the on-airframe test. The representative also indicated that they thought about developing a test to identify an overserviced damper, but could not come up with a plan/test that was definable. The representative was also asked if they would consider adding the ASB into their HMI, but a different representative indicated their engineering department determined that action was not necessary because the helicopter was out of annual inspection at the time of the accident, and also because they believed the improperly serviced dampers would have been detected during the damper extension test at an annual inspection. Helicopter Flying Handbook publication FAA-H-8083-21A states that with respect to ground resonance, if the main rotor rpm is low, the only corrective action to stop ground resonance is to close the throttle immediately and fully lower the collective to place the main rotor blades in low pitch. If the rpm is in the normal operating range, fly the helicopter off the ground and allow the blades to rephrase themselves automatically.
The helicopter owner’s failure to ensure the helicopter’s airworthiness and the inadequate inspection and maintenance of the helicopter's landing gear dampers, which resulted in ground resonance during throttle application. Contributing to the damage of the helicopter were the improper actions of the pilots after encountering ground resonance.
Source: NTSB Aviation Accident Database
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