19990717009089A

Air taxi, lifeflight crashed with three fatalities (other circumstances are unknown) near fresno, TX. (.19)on July 17, 1999, at 1231 central daylight time, a messerschmitt-bolkow-blohm BK-117-B2 helicopter, N110HH, operated as Life flight one, was destroyed when it collided with terrain near fresno, Texas. The helicopter was registered to and operated by hermann memorial hospital system, Houston, Texas. The airline transport pilot and two crewmembers received fatal injuries. Visual meteorological conditions prevailed for the 14 CFR part 91 repositioning flight. The local flight originated from the hermann memorial hospital, Houston, Texas, at 1226. According to witnesses, the helicopter was traveling southbound, parallel to and 100 yards west of fm 521. The helicopter appeared to initiate a left turn, and pieces "shot out" from the top of the helicopter. The helicopter descended in a nose low attitude behind trees and impacted terrain. The main wreckage was located at latitude 29 degrees 34.220 minutes north and longitude 95 degrees 26.272 minutes west. The fuselage came to rest upright on a measured magnetic heading of 180 degrees. The main transmission and main rotor hub remained attached to each other, but separated from the airframe. The transmission and main rotor hub came to rest approximately 20 feet from the airframe. All four main rotor blades separated from the main rotor hub. One main rotor blade (red) was located .13 miles west of the main wreckage, and a second blade (green) was .13 miles east of the main wreckage. The two other main rotor blades (blue and yellow) were located north of the main wreckage at .20 and .27 miles, respectively. (-23) according to several eyewitnesses, the helicopter appeared to initiate a left turn when pieces "shot out" from the top of the helicopter and the helicopter then descended and impacted terrain. Evidence at the scene showed the tt strap (p/n 2604067) failed in flight and all four main rotor blades separated from the rotor hub before terrain impact. The red rotor blade was located 686 feet west (260 degrees) of the main wreckage; the green rotor blade was 293 feet east (057 degrees); the blue rotor blade was 329 feet north (330 degrees); and the yellow rotor blades was 525 feet north (340 degrees). The blade mounting fork, pitch horn, and damaged tt strap were found still attached to the yellow blade. The main gear box (transmission) and rotor hub were found 20 feet from the fuselage and were not attached to the fuselage when the fuselage impacted terrain in a partially inverted nose low attitude. (.4) the helicopter was making a VFR approach to a fueling site when witnesses observed components of the main rotor system separate from the helicopter. Subsequently, the helicopter entered an uncontrollable descent and impacted the ground. Examination of the helicopter revealed that one of the four main rotor blades separated due to the failure of the tension-torsion strap (t-t strap), which secured the blade to the main rotor head. Examination of the failed t-t strap revealed that the polyurethane protective coating had debonded from a portion of the strap allowing corrosive agents, including carbon, sulphur, and chlorine into the strap. The strap separated as a result of fatigue cracking of the wires making up the strap. The fatigue cracking emanated from corrosion pits and deposits that were introduced to the strap when the polyurethane protective coating debonded. The t-t strap had accumulated a total of 7,815.3 flight hours and 35,971 flights at the time that it failed. It had been in service since the helicopter was manufactured in 1987. Following the accident, a Life limit of 120 months or 25,000 flights (whichever comes first) was established for the t-t straps.