CHI07LA309

The pilot reported that he felt vibrations in the main rotor while he was approaching the landing zone. The vibrations grew worse and when the helicopter was about 10 - 15 feet above ground level (agl), the vibrations were severe enough that he decided to lower the collective and land. The tail rotor impacted the tall corn and the tail rotor gearbox separated from the tail rotor boom. The helicopter rotated about 180 degrees during ground impact. The main rotor blades impacted the ground, and the landing gear separated from the fuselage. The on-site inspection of the helicopter revealed no preexisting anomalies. Components of the helicopter were shipped to the Robinson Helicopter Company (RHC) for inspection under the oversight of a National Transportation Safety Board aviation safety investigator. The inspection of the helicopter components revealed no preexisting anomalies that would account for the vibrations reported by the pilot.

On September 30, 2007, at 1630 eastern daylight time, a Robinson R44 helicopter, N8353K, sustained substantial damage when the tail rotor hit tall corn and impacted the terrain during an approach to a private landing zone near Jackson Center, Ohio. The pilot, the sole occupant, was not injured. The 14 Code of Federal Regulations personal flight departed from the pilot's private landing zone on a local flight. Visual meteorological conditions prevailed at the time of the accident, and no flight plan was filed.

The pilot reported that he felt vibrations in the main rotor while he was approaching the landing zone. The vibrations grew worse and when the aircraft was about 10 - 15 feet above ground level (agl), the vibrations were severe enough that he decided to lower the collective and land. The tail rotor impacted the tall corn and the tail rotor gearbox separated from the tail rotor boom. The helicopter rotated about 180 degrees during ground impact. The main rotor blades impacted the ground, and the landing gear separated from the fuselage. The helicopter rolled onto its right side, and the pilot exited the helicopter.

Inspectors from the Federal Aviation Administration examined the helicopter at the accident site. The inspection revealed that there was continuity of the collective and cyclic system, although the range of motion was restricted due to control rod impact damage. The main rotor hub was intact. One blade remained attached to the hub and it was entirely accounted for and it remained intact. Several points along the bonding seam were separated, but were consistent with impact damage. The other blade was separated from the hub but there was no indication of delamination. All damage to the blade was consistent with impact damage. The tail rotor system inspection revealed that the tail rotor shaft spun freely and the pitch control functioned normally. One tail rotor blade separated from the hub and it exhibited leading edge crush. The crush of the tail rotor blade "fit" the contour of an ear of corn. Both tail rotor blades and associated hardware was accounted for. The inspection of the transmission, transmission mounts, and engine mounts revealed no preexisting anomalies.

Components of the helicopter were shipped to the Robinson Helicopter Company (RHC) for inspection under the oversight of a National Transportation Safety Board aviation safety investigator. The components shipped to RHC included:

1. Main rotor hub with blade root sections attached. The blade sections bore serial numbers 1830C and 1827C. The spindles carried serial numbers 0825 and 0802.

2. One blade skin section that mated with the section of blade 1830C that was attached to spindle 0825.

3. One 4-foot section (from tip inboard) of unidentified main rotor blade, along with 2 smaller pieces that mated to missing areas of the blade section.

4. Tail rotor hub (serial number 0365, with blades 0824A and 0817A.

5. Low rotor speed warning horn unit

6. Governor controller, serial number 0336

7. Low rotor speed warning logic unit, identified as number 4 of lot 20 in the manufacturing sequence.

The blade root sections still attached to the main rotor hub were removed. All bearings, bolts, bushings, and bores were examined. No abnormal operating signatures were present on any of these items.

Spindle 0802 rotated easily and smoothly on its bearings.

Spindle 0825 was rotated, with a rough spot felt over a 10-degree arc of motion. The spindle was disassembled and the bearing stack was removed from the spindle shaft. No unusual markings or operating signatures were observed on the spindle shaft. All of the bearings rotated easily in their respective races.

The main rotor blade sections were examined. The skin panel separations from the underlying metal honey comb matrix were observed to be fractures of the honeycomb matrix rather than the honeycomb matrix separating from the adhesive. All skin panel separations were in areas of heavy bending and impact damage. No unusual adhesive separation signatures were observed.

The tail rotor assembly was examined. The blades moved smoothly on the hub bearings. Blade 0817A was torn off near the blade root, but was other wise undamaged; some leading edge polishing with dirt deposits were noted at the tip end. Blade 0824A was bent about mid span and had a 6 to 7-inch tip end section torn off. From just inboard of the tip doubler to the blade piece separation point, the leading edge was crushed and bent backward with large sections of paint missing exposing the zinc chromate primer underneath. Within the bent and crushed area, an impression of an oval like object was observed.

The low rotor warning horn was connected to a 13.4-volt power source. The horn functioned.

The governor controller, part number D278-1, serial number 0336, was noted to be a revision 'C' unit. It was installed on a calibrated test bench and subjected to the acceptance test protocol in Robinson Process Specification RPS-69. The unit met all acceptance criteria for a new or factory overhauled unit.

The low rotor speed logic unit does not have serial numbers assigned by Robinson but was identified by stamps on the external case as unit number 4 of production lot 20. It was a revision 'S' unit. The unit was installed on a calibrated test bench and tested to acceptance test protocol in Robinson Process Specification RPS-67. The unit was found to send a signal to the warning horn at a rotor speed of 94.738%, and turn off the warning at a rotor speed of 95.708%. The process specification calls for the unit to signal a warning at main rotor speed 96% and turn off the warning at main rotor speed 98%.