CHI95TA243

The pilot reported that he made a normal, smooth landing. He said the helicopter had been on the ground about 5 to 6 seconds and that he had reduced the engine rpm back between 2700 and 2800 RPM's, when he noticed a vibration in the helicopter. The pilot said he rolled in the throttle to 3100 rpm and attempted to get the helicopter into a hover. He reported that the vibrations were severe, and about one second later, the left skid collapsed and the main rotor contacted the tailboom. He secured and evacuated the helicopter after all motion had stopped. Tests revealed that the charge pressure for all four landing gear dampers was lower than that specified by the maintenance instructions. The maintenance instructions warned that improper shock strut pressure levels may result in ground resonance and destruction of the helicopter. Examination of the main rotor head dampers revealed no evidence of preexisting cracks or fatigue.

On July 27, 1995, at 2350, central daylight time, a Schweizer, 300C, sustained substantial damage when the main rotor blades contacted the tail boom while at the Kansas City Downtown Airport, Kansas City, Missouri. The commercial pilot received minor injuries. The helicopter is operated by the Kansas City Police Department. The 14 CFR Part 91 flight departed from the Kansas City heliport and had been on police patrol before it had landed at the Kansas City Downtown Airport. Visual meteorological conditions prevailed and a company flight plan was filed.

The pilot reported that he had air taxied and landed near the front of Executive Beechcraft, a local fixed based operator. He repositioned the helicopter a short time latter in order to keep clear of an aircraft that was getting ready to load passengers. He reported that he had hovered the helicopter to a clear area and made a normal, smooth landing. He flipped the governor switch off (on top of the cyclic). The pilot reported that he had been on the ground about 5 to 6 seconds and had reduced the engine RPM's back between 2700 and 2800 RPM's.

The pilot reported that at that point he noticed a vibration in the helicopter. He immediately thought he might be in ground resonance. He flipped the governor switch on, and rolled in the throttle to 3100 RPM's and attempted to pick the helicopter into a hover. The helicopter yawed to the left and banked downward to the left. The pilot reported that the vibrations were severe, and that it seemed to be only a matter of a second before the helicopter began collapsing around him. The pilot evacuated the helicopter after all motion had stopped and he had secured the aircraft.

A helicopter mechanic with fifteen years of experience witnessed the accident. The witness reported that he saw the pilot reposition the helicopter and that everything looked normal as the helicopter landed from the hover. He witnessed the pilot lower the collective and reduce RPM's. He reported that at that time the aircraft entered ground resonance. He heard the pilot apply power and collective. The aircraft swung about 20 degrees to the left with the nose a little above level. The helicopter was less than a foot off the ground when it sounded like the main rotor blades struck the tailboom. Within seconds the aircraft impacted the ground on the left side and the left skid collapsed.

The witness reported that he believed the helicopter experienced ground resonance, but had not observed anything which would have caused it to do so. He reported that the pilot had not made any abrupt control movements before the vibrations started. He also reported that the rotor disc appeared to be level prior to the vibrations and the blades were in a flat pitch position. He stated the pilot had just started reducing power when the resonance began. It began to slightly oscillate or bounce from each corner of the landing skids at first. It progressively worsened within 2 to 3 seconds. The witness reported that the pilot had attempted to increase power, but by the time the pilot tried lifting the helicopter off the ground, it was well into ground resonance.

The four landing gear (skid) dampers were removed from the aircraft and tested at Schweizer Aircraft Corporation. The load stroke tests revealed that all four landing gear dampers exhibited a charge pressure lower than that specified by the Schweizer Handbook of Maintenance Instructions.

The Handbook of Maintenance Instructions (HMI) contains a warning that states that, "Incorrect fluid levels, improper pressure or inoperable valving will deteriorate the damping capabilities of the landing gear dampers. These conditions may result in ground resonance and destruction of the helicopter. Follow all instructions in the HMI carefully, to ensure safe helicopter operation."

The manufacturer's report states that the damage is consistent with ground resonance, and that the low pressure of the landing gear dampers was not sufficient to attenuate the rocking of the fuselage. (See attachments)

Additionally, the manufacturer's engineering technical manager reported in separate correspondence that: "In conclusion the accident was the result of ground resonance. All ground resonance accidents investigated at Schweizer have had landing gear dampers that do not meet the specification for charge and pressure. If performed properly the 100 hour inspections are adequate to determine the airworthiness of landing gear dampers. The initiator for the main rotor oscillation is not known. The dampers from this accident when tested at Schweizer were below limits and therefore not capable of attenuating the main rotor oscillations." (See attachments)

The three main rotor dampers were sent to the Lord Corporation for examination. In their conclusions, they reported that, "The lack of damage to the elastomer elements of S/N 5125 (yellow damper) and the fracture of its rotor head attachment bracket indicates that this damper assembly probably initiated the damage sequence. Excessive damage to the damper side fracture prevented detailed analysis that could have aided in understanding the root cause of this mishap." (See attachments)

The main rotor head dampers were sent to the Materials Laboratory of the National Transportation Safety Board for examination. The examination revealed that no evidence of preexisting cracking (such as fatigue) was found.