DFW06LA057

The 6,500-hour commercial helicopter rated pilot inadvertently encountered ground resonance while ground taxiing. The pilot performed a normal startup and prepared for ground taxi. He moved the helicopter forward and then started the helicopter in a 90-degree ground taxi turn at 30-degree intervals. He pushed on the toe brakes to stop the helicopter to give right of way to an aircraft taxiing down the taxiway. At this time the helicopter started to shake and bounce. Before the pilot could get the aircraft shutdown, the aircraft executed an uncommanded 90-degree turn to the left. The helicopter's main rotors and transmission assembly separated from the airframe. The wind at the time of the accident was reported to be from 170 degrees at 17 knots, gusting to 23 knots.

On January 19, 2005, approximately 1515 central standard time, a twin-engine Augusta A109E turbo-shaft helicopter, N142CF, registered to North Central Texas Services of Grand Prairie, Texas, and operated by CareFlight of Grand Prairie, Texas, was substantially damaged while taxiing at the Fort Worth Meacham International Airport (FTW) near Fort Worth, Texas. The commercial pilot, sole occupant of the aircraft, was not injured. Visual meteorological conditions prevailed, and a company visual flight rules flight plan was filed for the repositioning flight conducted under 14 Code of Federal Regulations Part 91.

The 6,500-hour pilot reported in the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1) that he performed a "normal startup and prepared for ground taxi. I started the aircraft forward then started the aircraft in a 90-degree ground taxi turn in 30-degree intervals. I pushed on the toe brakes to stop the aircraft to give right away for aircraft taxiing down the taxiway. At this time the aircraft started to shake. Before I could get the aircraft shutdown, the aircraft did a 90 degree turn to the left."

According to several witnesses, the helicopter was taxiing to the south and then stopped just as a fixed wing airplane was taxiing to the east into the ramp area for a fixed base operator. The helicopter began to shake and bounce very rapidly from side to side as if it were trying to roll about its longitudinal axis. The helicopter then turned 90-degrees to its left followed by the separation of the rotor and mast components and part of the upper fueslage, from the airframe. One of the witnesses stated that "the whole event probably took no more than 5 to 10 seconds."

Examination of the helicopter by a Federal Aviation Administration (FAA) inspector, who responded to the accident site, revealed that the helicopters main rotors and transmission assembly separated from the airframe. The main rotor actuator attachment fitting had separated into three pieces ( a left leg, right leg and fitting). All three pieces were retained for further examination.

On March 22, 2006, at the NTSB's Material Laboratory in Washington, D.C., an examination of the main rotor actuator attachment fitting was conducted by an NTSB metallurgist and representatives from the FAA, Agusta, and CareFlight.

A bench binocular microscope examination of the fracture face from the left leg revealed that the fracture was largely on a flat plane and had two distinct fracture regions. One of the regions had a woody, matte appearance typical of an overstress fracture region. The remainder of the fracture was heavily damaged from contact with the mating fracture surface and contained fracture lines oriented primarily in the forward/aft direction.

A scanning electron microscope (SEM) examination of the fracture face on the left leg revealed the "damaged region" contained mechanical damage that resulted from shear stress. The damaged portion of the fracture at one isolated area showed a feature that appeared similar to striations, but this striation-like feature was not found in other areas of the fracture or on the fracture face of the other separated leg. The striation-like feature also resembled the highly textured fracture features in the overstress region. The fracture face did not contain any other features that are typical of a fatigue crack (such as radial marks that point to a fatigue origin, crack arrest bands, thumbnail shaped crack regions, or a terminus of a fatigue region). During the SEM examination, areas of undamaged features were found near the top of the fracture in the left leg. These areas contained a highly textured surface with features that were oriented parallel to the length of the leg and ductile dimple features typical of overstress separation.

The right leg portion showed fracture features that were similar to those found on the left leg.

At 1453, the automated surface observing system (ASOS) at FTW reported the wind from 170 degrees at 17 knots, gusting to 23 knots, visibility 10 statute miles, clear sky, temperature 73 degrees Fahrenheit, dew point 44 degrees Fahrenheit and a barometric pressure of 29.87 inches of Mercury.