TOULON, IL, USA
N9375S
HUGHES 269B
THE HELICOPTER HAD BEEN REASSEMBLED AFTER AN ANNUAL INSPECTION, WITH INSTALLATION OF 3 OVERHAULED MAIN ROTOR BLADES. THE MAIN ROTOR BLADE DAMPERS HAD BEEN DISASSEMBLED, INSPECTED AND REASSEMBLED. DURING REASSEMBLY, THE INTERNAL SEQUENCE OF FRICTION DISCS WAS INCORRECT, RESULTING IN IMPROPER AND NON- UNIFORM DAMPING ACTION BETWEEN THE BLADES. THE SHOCK STRUT OLEOS WERE NOT SERVICED. POST ACCIDENT INSPECTION REVEALED LOW PRESSURE IN THE 3 REMAINING INTACT STRUTS. DURING THE FIRST TAKEOFF AFTER REASSEMBLY TO TRACK THE MAIN ROTOR BLADES IN A HOVER, THE HELICOPTER ENCOUNTERED GROUND RESONANCE AND WAS DESTROYED.
On May 25, 1994, at 1830 central daylight time, a Hughes 269B helicopter, N9375F, owned and piloted by Dwain Webster of Toulon, Illinois, was destroyed while performing main rotor blade tracking. The private pilot in command and mechanic passenger received minor injuries. Visual meteorological conditions existed at the time of the accident and winds were reported as 10 knots. The flight was operated under 14 CFR Part 91 as a maintenance flight, no flight plan was filed and there was no intent to depart from a hover. The pilot reported that upon completing the fourth rotor engagement to track the newly installed main rotor blades, he began to increase collective pitch to check blade track in a hover. With approximately 2 inches of up collective, the helicopter began violent vertical vibrations. The pilot reduced collective and secured the throttle. The tail boom separated from the helicopter, the cockpit and skids were destroyed, and all three main rotor blade dampers separated from the main rotor blade trailing edges. A post-crash inspection of the helicopter revealed that all the landing gear oleo attaching lugs separated from their attaching points. Flight control continuity was established to the rotor head, and all controls still moved freely. All three main rotor blade spindles rotated smoothly. The red and white main rotor blade flap restrainers were intact. The blue main rotor blade flap restrainer was broken in an upward direction. The main transmission rotated freely. All 3 main rotor blade dampers were able to be moved through their ranges, exhibiting some resistance, which was not measured due to a lack of a calibrated torque wrench. The main rotor blade dampers and the four shock strut oleos were removed for testing at Schweizer Helicopters. The test and disassembly was attended by FAA representatives from the Rochester, NY FSDO. These tests revealed that all the dampers were set 70 to 100 inch-pounds below the value specified in the maintenance manual. The red damper internal discs were assembled in an incorrect order. Blade damper torque values were measured as: 1st stage lead 1st stage lag 2nd stage BLUE 140 in-lb 150 in-lb 220 in-lb WHITE 150 in-lb 150 in-lb 250 in-lb RED 130 in-lb 120 in-lb 200 in-lb normal 200-230 in-lb 200-230 in-lb --------- The 3 intact oleos were compression tested as follows: initial maximum LFWD 70 lbs 600 lbs RFWD 75 lbs 500 lbs RREAR 50 lbs 250 lbs normal 150 lbs 3300 lbs The Airframe and Powerplant mechanic who was performing the annual inspection stated he did not remember servicing the oleo struts. He stated the struts looked ok, and were hand checked by lifting up the tail to check for reaction. They were not checked with any pressure gauge. The same mechanic stated that every other year during the annual inspection the dampers were disassembled, the plates cleaned using emery cloth, and reassembled with new fluid. The same parts that were removed were reinstalled into each damper, the torque was set using a dial calibration type torque wrench calibrated in inch-pounds, and the tension was checked using a scale and pulley. Lead to lag should be set from 6.5 to 7.0 inch pounds according to the maintenance manual. The Pilot Operating Handbook for the Hughes 300 Model 269B contains an OPERATIONAL CHECK - OLEO DAMPERS section, which includes the following: "CAUTION" "Ground resonance can result if aircraft is operated when oleo damper extension, fluid type, and/or fluid-to-air proportions are incorrect." The 269 Series - HMI states: "5.3.f(7) Cycle overhauled dampers and readjust locknut as many times as necessary to stabilize torque reading at 230 inch-pounds." "NOTE: ...if torque readings from lead-to-lag and lag- to-lead positions for first stages of all dampers are not equal, back off cover assembly on individual dampers as necessary...Damper torque may vary from 200 to 230 inch-pounds in actual service." "WARNING" "INCORRECT PHASING AND/OR TORQUE ADJUSTMENTS CAN LEAD TO CONDITIONS THAT MAY RESULT IN GROUND RESONANCE AND DESTRUCTION OF THE HELICOPTER..." Chapter 15 of Helicopter Aerodynamics, by Raymond Prouty, an aeronautical engineer specializing in rotorcraft, discusses the phenomena of ground resonance. The phenomena is associated with fully articulated rotor heads, and results when a resonant frequency is obtained between the ground, a shock strut system, and the rotor system. This resonance is initiated when the blades move on their respective lead-lag hinges, placing their combined center of gravity outside the center of rotation of the rotor disc. Damping to prevent this resonance is required in both the landing gear system and the lead-lag plane of a fully articulated rotor head.
the improper reassembly of the main rotor blade dampers and the underservicing of the landing gear skid hydraulic struts which resulted in ground resonance of the helicopter when the first takeoff after reassembly was attempted.
Source: NTSB Aviation Accident Database
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