FTW99FA192

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.

History of flight

On July 17, 1999, at 1231 central daylight time, a Messerschmit-Bolkow-Blohm BK-117-B2 helicopter, N110HH, operating as LIFEFLIGHT 1, was destroyed when it impacted terrain following the separation of a main rotor blade during an approach to an unmanned fueling site near Fresno, Texas. The airline transport rated pilot and two medical crewmembers were fatally injured. The helicopter was registered to and operated by Hermann Memorial Hospital Systems, of Houston, Texas. Visual meteorological conditions prevailed for the 14 Code of Federal Regulations Part 91 positioning flight, for which a company VFR flight plan was filed.

According to Hermann Hospital dispatch records, LIFEFLIGHT 1 departed Hermann Memorial Hospital at 1226 and was destined for the Columbia Clearlake Regional Hospital, near Clear Lake, Texas, with an intermediate refueling stop at the Fresno fueling site. At 1227, LIFEFLIGHT 1 was issued a clearance from Houston Hobby air traffic control tower into Hobby Class B airspace. At 1231, LIFEFLIGHT 1 reported that the landing area was in sight. No further communication was received from the helicopter.

According to a witness, who was driving westbound on FM2234, he first noticed the LIFEFLIGHT helicopter flying southbound approximately level with powerlines. He stated that the helicopter turned to the left and hovered for a second, and then he observed "sparks or possible flames shoot out from the top middle of the helicopter," near the main rotor system. He witnessed pieces of the main rotor system separate from the helicopter. Subsequently, the helicopter entered a nose low attitude and passed behind trees out of the witness' view.

Another witness, who was driving northbound on FM521, observed the LIFEFIGHT helicopter traveling southbound, approximately 35 feet agl. He stated that the helicopter made a left turn, as if it was going to land; he then observed an "explosion," and the helicopter impacted the ground.

Personnel information

The pilot was hired by Hermann Hospital on July 23, 1990. On July 27, 1990, he completed initial BK-117 ground and flight training, which was administered by American Eurocopter, Grand Prarie, Texas. On August 2, 1998, the pilot satisfactorily completed his most recent annual recurrent ground and flight training for the BK-117. On August 13, 1998, the FAA issued the pilot an airline transport certificate for helicopters. He also held a commercial certificate with airplane single and multi-engine land ratings and type ratings in the Bell 206 helicopter, Hughes 369 helicopter, and the Boeing Vertol 234. According to Pilot Flight Duty Time Forms that were provided by Hermann Hospital, the pilot had accumulated a total of 1,459.5 flight hours with Hermann Hospital, since his date of hire. There were no restrictions or limitations listed on the pilot's FAA second class medical certificate issued on June 7, 1999. The Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2), which was completed by Hermann Hospital, indicated that the pilot had accumulated a total of 9,233 flight hours, of which 8,763 hours were in rotorcraft.

The two medical crewmembers, a flight nurse and a paramedic, were based at the Hermann Memorial Hospital.

Aircraft information

The 1987, red and black, twin-engine helicopter was equipped with two Honeywell LTS101-750B-1 turboshaft engines, a four bladed main rotor system, and a two bladed tail rotor. The airframe had accumulated a total of 7,815.3 flight hours and 35,971 flights at the time of the accident. The helicopter underwent its most recent inspection, in accordance with its approved inspection program, on July 7, 1999. The left engine had accumulated a total of 6,216.2 hours at the time of the accident and underwent its last 1,200-hour inspection on July 8, 1998. The right engine had accumulated a total of 2,737.4 hours at the time of the accident and underwent its most recent 1,200-hour inspection on October 6, 1998.

The main rotor head and tension torsion straps (T-T straps) had accumulated a total of 7,815.3 hours at the time of the accident. The T-T straps had not been replaced since they were installed on the helicopter in 1987. The main rotor head and T-T straps were last inspected on May 1, 1997, in accordance with the manufacturer's recommended 2,400-hour inspection interval, at a total time of 6,027.1 hours. At the time of the accident, the T-T straps were not time-life-limited.

The T-T straps installed on LIFEFLIGHT 1 were manufactured by Bendix Corporation. A T-T strap contains between 11,900 and 12,000 windings of grade 304 stainless steel wire continously wrapped around two steel bushings that are separated by a distance of approximately 9.6 inches. Each bushing contains an axial tube portion and a radial flange at the end of the tube portion. The area between the straight portions of the bundles of wires, between the bushings, is hollow. A cast polyurethane bushing retention block is installed adjacent to each bushing in the area between the straight portions of the bundle of wires. The wound wires, bushings, and bushing retention blocks are encapsulated in polyurethane, to prevent exposure of the wires to the environment.

Meteorological information

At 1153, the weather observation facility located at the William P. Hobby Airport, Houston, Texas, (located 10 miles northeast of the accident site), reported scattered clouds at 3,000 feet, broken layers at 7,000 and 25,000 feet, wind 160 degrees at 7 knots, visibility 10 statue miles, temperature 88 degrees Fahrenheit, dew point 72 degrees Fahrenheit, and altimeter 30.12 inches of Mercury. The remarks section stated that there were rain showers in the vicinity of the airport.

Reproduced satellite images were obtained through McIDAS and displayed on an NTSB workstation. At 1215 and 1245, the images revealed only one area of cloud coverage, which was located at the flight path's mid-point.

According to cloud to ground lightening strike data, provided by Global Atmospherics, Inc., during the period between 1226 to 1231 there were no cloud to ground lightening strikes within 10 nautical miles of the accident location. Furthermore, no thunderstorms were in the accident site vicinity at the time of the accident.

Wreckage and impact information

A global positioning satellite (GPS) receiver located the accident site at North 29 degrees 34.33 minutes latitude, and West 095 degrees 26.24 minutes longitude. The fuselage of the helicopter came to rest on a measured magnetic heading of 190 degrees, 635 feet southwest of the fueling facility. Ground scars at the accident site revealed that the helicopter initially impacted the ground on the left side of the fuselage in a near inverted position, and then came to rest upright 26 feet southeast of the initial ground impact point. The transmission and main rotor assembly remained secured to each other, but separated from the engines and airframe and came to rest 19.6 feet south of the fuselage. The four main rotor blades were found separated from the main rotor hub (rotor star). One main rotor blade (red) was located 686 feet west of the fuselage, a second blade (blue) was located 275 feet north of the fuselage, a third blade (yellow) was located 329 feet north of the fuselage, and the fourth blade (green) was located 237 feet east of the fuselage.

The yellow main rotor blade, the inner sleeve assembly, the blade mounting fork, and the control lever (pitch horn) remained secured to each other. The T-T strap, serial number 5050, which secures the yellow main rotor blade to the rotor star by way of a quadruple nut and retaining pin in the rotor star and a retaining nut in the blade mounting fork, was separated in two sections. The quadruple nut was found secured within the rotor star and the outer retaining nut was found secured to the blade mounting