Allentown, PA, USA
N152CC
SCHWEIZER 269C-1
The student pilot landed the helicopter after a 40-minute flight and started the engine shut down procedures. She stated that she applied the collective friction and started the timer, running the engine at 2,500 rpm for a cool-down period. She turned off the fuel boost pump and radios and referred to the checklist to ensure she had completed the proper items. She then engaged the collective friction and was tightening the cyclic friction at the cyclic base when the helicopter developed ground resonance. The student pilot could not get the helicopter airborne with the collective friction engaged, so she rolled the throttle to idle. The resonance continued to increase and, "in a matter of seconds, the helicopter shook itself apart." The helicopter had four poppet-type, nitrogen-charged hydraulic units (dampers) in the landing gear assembly to reduce landing shock and help prevent ground resonance. The four dampers were removed from the helicopter and pressure tested; all four failed to meet manufacturer's specifications. Without the airframe protection provided by the dampers, oscillations occurred that resulted in the helicopter rocking on its landing gear and the tailboom twisting toward the right side of the helicopter where it joined the main structure. Maintenance instructions noted that the landing gear dampers would experience a decrease in pressure with helicopter usage and required that they be extension-checked during each 100-hour inspection or every 6 months, whichever occurred first. The helicopter's most recent 100-hour inspection occurred about 2 months before the accident, and it had been operated 58 hours since the inspection. Review of maintenance records for the most recent 100-hour inspection and about 800 flight hours before the accident revealed no mention of any maintenance actions involving the dampers.
On June 17, 2014, about 1205 eastern daylight time, a Schweizer 269C-1, N152CC, operated by Ace Pilot Training, Inc., was substantially damaged while landing at Lehigh Valley International Airport (ABE), Allentown, Pennsylvania. The student pilot sustained minor injuries. The local solo instructional flight was conducted under the provisions of 14 Code of Federal Regulations Part 91.According to the student pilot, she landed the helicopter after a 40-minute flight and started the shut-down procedures. She applied the collective friction and started the timer, running the engine at 2,500 rpm for a cool down period. She turned off the boost pump and radios, and pulled out the checklist to ensure she had completed the proper items. She then engaged the collective friction and was tightening the cyclic friction at the cyclic base when the helicopter developed ground resonance. The student pilot could not get the helicopter airborne with the friction engaged, so she rolled the throttle to idle. The resonance continued to increase and, "in a matter of seconds, the helicopter shook itself apart." The pilot could only hold on until the movement stopped, at which time, she secured the engine, then subsequently climbed out of the helicopter. A photograph showed the tail boom twisted toward the right side of the helicopter where it joined the main structure. The Pilot's Flight Manual (PFM), as part of the preflight inspection, states, "GROUND RESONANCE MAY RESULT IF THE HELICOPTER IS OPERATED WHEN THE LANDING GEAR DAMPERS ARE NOT IN GOOD OPERATING CONDITION. (REFER TO BASIC HMI [Handbook of Maintenance Instructions], SECTION 12 FOR DETAILED INSPECTIONS.)" The PFM further states, in section 7-11, "LANDING GEAR DAMPERS – INSPECTION: Four poppet type nitrogen charged hydraulic units in the landing gear assembly dampen landing shock and help prevent ground resonance. The dampers are mounted between the helicopter center frame section and the landing gear skids (two for each skid, left-and right-hand sides). Ground resonance and possible destruction of the helicopter may result if landing gear dampers are not functioning properly." Postaccident examination of the helicopter included recovery and testing of the four landing gear dampers at the damper manufacturer. A representative of Sikorsky Aircraft Corporation, the type certificate holder for the helicopter was present, along with Federal Aviation Administration (FAA) inspectors to provide government oversight. A report of results was subsequently prepared by the representative of Sikorsky. According to the report, testing was completed 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. All four dampers failed the load stroke test. All dampers exhibited a slight amount of dried hydraulic fluid on the piston and interior of the boot. "The damper identified as left forward, part number 269A3150-19, serial number S0782, was intact with bending and a fracture to the upper bearing lug. The upper mounting cap was removed and a new cap installed to perform the test. No leakage of gas was observed and the upper plug was tight but turned slightly to reach the specified torque. The load stroke test showed that it was within tolerance on the compression stroke at the lower pressure test point, but was low 2500 1bs (should be between 2900 and 3600 1bs). It was out of limits at the ultimate load range. The damper identified as right forward, part number 269A3150-19 serial number S0783 was intact. The load stroke test showed that it was low on the compression stroke lower pressure test point. It was slightly out of limits low 2830 lbs, at the ultimate load range (should be between 2900 and 3600 1bs). The damper identified as left rear part number 269A3150-21 serial number S0742 was intact with minor bending to the upper bearing lug. The load stroke test indicated low on the compression stroke lower pressure test point and out of limits low 2210 lbs, at the ultimate load point (should be between 3200 to 3900 1bs). The damper identified as right rear, part number 269A3150-21, serial number 80729 was intact with a slight bend to the upper lug. The load stroke test indicated an out of limits low 3100 lbs, at the ultimate load point (should be between 3200 to 3900 1bs)." On April 15, 2014, the certificate holder issued 269C Helicopter Alert Service Bulletin (ASB) B-304. The ASB concerned a one-time load stroke inspection of the landing gear dampers to be performed at an overhaul or repair facility. The ASB was effective for all 269C models to ensure the landing gear dampers were serviced correctly. The ASB did not provide any background history as to why the inspection was needed. The ASB stated that compliance was essential and was to be accomplished by January 9, 2015, on all landing gear dampers in service "that have been previously overhauled, serviced, disassembled or otherwise had the charge or fluid level affected." Concurrently, the manufacturer issued a revision to the Handbook of Maintenance Instructions (HMI) for the 269C that added a load stroke inspection procedure to the section of the HMI that pertains to the repair and charging of the landing gear dampers. A review of the 100-Hour inspection criteria revealed that the landing gear dampers were required to be inspected for operation and condition, and for damper extension with a full fuel tank. Section 12 of the HMI indicated that the landing gear dampers would experience a decrease in pressure with subsequent use of the helicopter, and they should be extension-checked during each 100-hour inspection, or every 6 months, whichever occurred first. According to the responding FAA inspector, maintenance was outsourced. The helicopter's latest 100-hour inspection occurred on April 24, 2014, and it had been operated 58 hours since then. The inspector also noted that maintenance records back through 2011, about 800 hours previously, found no mention of any maintenance actions involving the dampers. The student pilot reported that she had accumulated about 40 hours of helicopter flight experience. She also held an airline transport pilot certificate for airplanes and had accumulated about 5,465 hours of flight time in fixed-wing aircraft.
Inadequate inspection and maintenance of the helicopter’s landing gear dampers, which resulted in ground resonance during helicopter shutdown.
Source: NTSB Aviation Accident Database
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