History of Flight
On June 2, 2010, at 1400 central daylight time, a Bell 222U helicopter, registration N515MK, operated by CareFlite, collided with the ground following an in-flight breakup near Midlothian, Texas. The flight was conducted as a post-maintenance flight under 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and a company flight plan was filed. The helicopter departed Grand Prairie Municipal Airport (GPM) at 1352, approximately 8 minutes before the accident. Witnesses reported seeing the tail boom, main rotor hub, blades, and other debris separate from the helicopter. One witness heard a loud crack sound. The helicopter then struck the ground and exploded into flames.
Radar data showed the helicopter departed GPM at 1352:03 and proceeded southbound. The last radar target was recorded at 1359:49. The final six radar hits indicated an average altitude of 1,300 feet mean sea level, ground speed of 115 knots, and a ground track of 178 degrees.
Personnel Information
The pilot, age 44, held an airline transport pilot certificate with a rotorcraft-helicopter rating, a commercial pilot certificate with single-engine, multiengine, and instrument airplane ratings, a flight instructor certificate with rotorcraft-helicopter and instrument-helicopter ratings, and a ground instructor certificate. He was issued a second-class medical certificate on January 21, 2010, with no limitations. His total flight experience was estimated at 6,500 hours, with at least 5,300 hours in helicopters and about 346 hours in Bell 222 helicopters. His last Part 135 competency check in the Bell 222U was completed on July 29, 2009. He had flown 19.6 hours in the last 90 days, including 9.8 hours in the Bell 222U.
The mechanic, age 23, held an airframe and powerplant mechanic certificate and a private pilot certificate with a rotorcraft-helicopter rating. He held a first-class medical certificate issued April 13, 2007, with no limitations.
Aircraft Information
The Bell 222U was a five-place twin-engine helicopter, serial number 47515, equipped with high skid landing gear. The pilot’s station was in the right cockpit seat; the co-pilot station lacked flight controls. The main cabin had a medical interior with three passenger seats and a stretcher. Two Honeywell LTS-101-750-1 engines, each rated at 680 shaft horsepower, powered the helicopter. The helicopter had an estimated total time in service of 9,925.1 flight hours. The last recorded inspection was a 600-hour inspection completed on May 27, 2010, at 9,924.0 hours.
Prior to the accident, a swashplate and lever assembly (part number 222-010-400-127, serial number EA136) was removed from N515MK, and another assembly (serial number EA0052) previously installed on N142CF was reinstalled at 9,925.0 total time. The recently installed assembly had about 49.9 hours remaining until overhaul. The outer ring assembly (part number 222-010-403-005, serial number AR-16) had accumulated 4,069.6 hours since new and 2,450.1 hours since overhaul. Drive pins (part number 230-010-402-103, serial numbers A-5 and A-17) were installed during an overhaul on December 23, 1999. Maintenance records showed the drive pins were inspected every 300 hours; the last inspection was at 2,346.4 hours, and they had accumulated about 5.9 hours since then.
Meteorological Information
At 1353, the automated surface observing system at Arlington Municipal Airport, about 11 miles north-northwest of the accident site, reported wind from 200 degrees at 8 knots gusting to 14 knots, visibility 10 statute miles, clear sky, temperature 33°C, dew point 19°C, and an altimeter of 29.77 inches of mercury. At 1350, the system at Grand Prairie Municipal Airport reported wind from 210 degrees at 10 knots, visibility 10 statute miles, few clouds at 5,000 feet, temperature 34°C, dew point 18°C, and an altimeter of 29.79 inches of mercury.
Communications
At 1349, N515MK contacted the FAA contract air traffic control tower at GPM, requested departure southbound to Joe Pool Lake, and was approved with the altimeter setting. No further communications were recorded.
Wreckage and Impact Information
The wreckage was located in a field about 13 miles south of GPM at an estimated elevation of 650 feet msl. The fuselage was oriented on a 044-degree heading, covered in soot, and partially consumed by fire. Other wreckage did not exhibit fire damage. The separated tail boom was found approximately 550 feet northeast of the fuselage, the main rotor about 220 feet northeast, and the fractured main rotor mast section about 80 feet east. Debris included fuselage skin, a patient backboard, tail boom skin with tail rotor blade strikes, and push-out window sections. The most distant debris was about 1,500 feet north-northeast and included main rotor blade skin and paint chips.
The main rotor system was mostly intact. One blade had trailing edge damage consistent with striking a hard object; the other had abrasions and gouges on its bottom surface. A separated tail rotor hanger bearing section matched a gouge on the bottom blade. The A side pitch change link was bent near the center; the B side pitch change link had a fractured bolt at the upper connection. The rotating outer ring swashplate had the A side drive pin fractured and separated; the B side drive pin was intact. The tail boom was found upright, flattened on the bottom, with fractures about 4 inches aft of the fuselage attachment. The left end plate of the horizontal stabilizer exhibited a main rotor blade strike. The tail rotor hub and blades remained attached; both pitch change links were bent.
The main transmission was connected to the fuselage, covered with soot, and had impact fractures. The right aft transmission mount was fractured; the other three were intact. Free rotation was observed through the tail rotor gearbox. The main rotor mast was fractured near the top, consistent with hard contact with the main rotor yoke. Control continuity for the main rotor cyclic, collective, and tail rotor controls could not be verified in the fuselage area due to melted aluminum control tubes. Tail rotor control continuity was confirmed in the tail boom section.
Engine examinations revealed fire damage. The left engine’s power turbine wheel was free to rotate; the axial compressor rotor was intact. The right engine’s power turbine wheel would not rotate, and one turbine blade had a missing piece. Both engines’ accessory and reduction gearbox gears had intact teeth.
Medical and Pathological Information
Autopsies were conducted on June 3, 2010. For the pilot, the cause of death was blunt force and thermal injuries. Postmortem toxicology detected no ethanol in the liver; acetone was present in the liver, and ethanol was detected in the kidney, muscle, and brain. For the mechanic, the cause of death was multiple blunt force injuries. No carbon monoxide or cyanide was detected in the blood; no ethanol was detected in the urine; naproxen was detected in the urine.
Tests and Research
Metallurgical examination revealed that fractures through the mast, B side pitch link bolt, and actuator attachments were consistent with overstress separations. The A side drive pin fracture surface showed predominantly brittle cleavage-like fracture with intergranular separations and small ductile dimples. A comparison new pin subjected to bending overstress showed a 100 percent ductile dimpled fracture. Maintenance records indicated the outer ring assembly was repaired in December 1999, with original constant diameter pins replaced by stepped pins. The drive pins met engineering drawing requirements for material and finish. The material of both pins was confirmed as AerMet 100. Hardness measurements were within expectations.
Atomic hydrogen content was measured: the fractured A side pin contained 8.7 and 9.3 parts per million (ppm), while the B side pin contained 4.6 and 4.5 ppm. Tests were conducted by Bell on new drive pins at the direction of the NTSB Materials Laboratory. After removing cadmium, reheat treating, pickling, and electrolytically plating with heavy cadmium without baking, one test pin (aged at 850°F) fractured through the first engaged thread after being torqued and loaded for 7 days, while the other (aged at 900°F) remained intact. The fractured test pin exhibited a brittle cleavage-like fracture similar to the accident pin. Hydrogen content in the test pins was 2.0 ppm.
The bore of the A-side pin hole had skewed longitudinal scrape marks and circumferential impressions, consistent with gradual extraction of the shank portion of the pin. The spacing indicated relative outward movement with each force reversal.