Wire strike protection system
Passive mechanical cutter reducing helicopter wire strike risks.
A wire strike protection system (WSPS) uses guides and a passive mechanical cutter to reduce the danger of helicopters hitting wires while flying low. Wire strikes were a major hazard: from 1974 to 1979, they caused 8% of U.S. Army helicopter damage, 6% of injuries, and 16% of fatalities. In the decade starting 1970, 208 civil helicopters were involved in wire strikes; 88 were destroyed (42%), and 37 of 331 people aboard (11%) died. Between 1975 and 1977, hitting wires and poles was the most common cause of civil helicopter accidents.
Bristol Aerospace developed a WSPS for the Bell OH-58 Kiowa under a Canadian Armed Forces contract in May 1979. Nelson Chan invented the system and received patents in 1980. Bristol ran 52 tests by mounting the WSPS on a wrecked Kiowa fuselage, loading it onto a flatbed truck, and driving into wires at 15–60 mph (13–52 knots; 24–97 km/h), with yaw angles of 0–45°, using typical overhead power and telecom cables—including a 3/8-inch (9.5 mm) seven-strand steel cable with a tensile strength over 10,000 lb (4,500 kg).
Because Bristol’s tests didn’t check the lower fuselage cutter’s effectiveness or how a wire strike would affect flight attitude, the U.S. Army Research Laboratory ran pendulum swing tests at Langley Research Center in October 1979 using a Kiowa. An extra deflector for the skids, landing gear, and tail boom (including rotor and vertical stabilizer) was tested but proved ineffective. In the Langley tests, an OH-58 was attached to a 196-foot (60 m) cable, pulled back, and released to swing through wires mounted horizontally about 22 feet (6.7 m) high. Similar pendulum tests were done for Bell UH-1H and AH-1S helicopters in 1981 and 1982.
All small to medium U.S. Army helicopters received WSPS in a retrofit program completed by 1992; from 1996 to 2002, the Army had no fatal wire strike accidents. In civil operations, wire cutters were considered most useful for agricultural flights. Of the 208 wire strikes in the 1970s, nearly half could have been prevented with cutters and other recommended mechanical upgrades.
The system is typically mounted on the front of U.S. military helicopters, rescue helicopters, and civilian agricultural helicopters. It works when the helicopter hits wires at an angle less than 90 degrees and at speeds above 30 knots (35 mph; 56 km/h).
- Inventor
- Nelson Chan
- Patent year
- 1980
- Developer
- Bristol Aerospace
- First qualified aircraft
- Bell OH-58 Kiowa
- Qualification date
- May 1979
- Weight
- 16.3 lb (7.4 kg)
- Installation time
- 40 man-hours
Lore & Background
Between 1974 and 1979, wire strike accidents accounted for 8% of United States Army Aviation aircraft damage, 6% of aircraft-related injuries, and 16% of aircraft-related fatalities. For the ten years from 1970, 208 civil helicopters were involved in wire strike accidents; 88 were destroyed (42%) and 37 (11%) of the 331 people involved were killed. The commonest cause of civil helicopter aerial accidents between 1975 and 1977 was striking wires and poles.
Bristol Aerospace developed a WSPS qualified for the Bell OH-58 Kiowa under contract to the Canadian Armed Forces in May 1979. Nelson Chan invented WSPS, and was granted patents in 1980. Bristol staged a series of 52 tests by mounting the system on the fuselage of a wrecked Kiowa loaded onto a flatbed truck and driven into wires at speeds from 15 to 60 mph, yaw angles from 0 to 45°, and various cables including a 3/8-inch 7-strand steel cable with tensile strength exceeding 10,000 lb. Because the Bristol testing did not determine effectiveness of the lower fuselage-mounted cutter and was ground-based, the United States Army Research Laboratory conducted supplemental pendulum swing tests at Langley Research Center with a Kiowa in October 1979. An additional deflector for skids, landing gear, and tail boom was tested but found ineffective.
The system is typically mounted around the front of American military helicopters, rescue helicopters, and civilian helicopters involved in agricultural work. It is effective when the helicopter strikes wires at an angle of less than 90 degrees and at speeds more than 30 knots. The system is designed to cut a 3/8-inch steel cable with a breaking strength of 12,000 lb. The Bristol WSPS consists of a roof-mounted cutter, a lower cutter fitted to the fuselage, and a deflector fitted to the middle of the windshield. Sometimes a windshield wiper protector frame is used. As installed, the OH-58 WSPS weighs 16.3 lb and requires 40 man-hours to install.
Reader's Guide
The wire strike protection system's significance is underscored by accident statistics: between 1974 and 1979, wire strikes caused 16% of US Army aviation fatalities, and in the 1970s, 208 civil helicopters were involved in wire strike accidents, with 42% destroyed. The system was developed by Bristol Aerospace under contract to the Canadian Armed Forces, qualified for the Bell OH-58 Kiowa in May 1979, and invented by Nelson Chan, who received patents in 1980. Testing included 52 flatbed truck tests and pendulum swing tests at Langley Research Center in October 1979. All small to medium US Army helicopters were retrofitted with WSPS in a programme completed by 1992; between 1996 and 2002, the US Army had no fatal wire strike accidents. In civil operations, wire cutters were thought most effective for agricultural flights; of the 208 wire strike accidents in the 1970s, almost half could have been avoided with wire cutters and other recommended mechanical upgrades. The system is a registered trademark of Magellan Aerospace, and Dart Aerospace markets an equivalent under the name Cable Cutter System. Other manufacturers have developed similar systems, including MD Helicopters (1981), Custom Air (1987), Airbus Helicopters (2008 and 2011), and Bell Helicopter (2014).
Did You Know?
- Nelson Chan invented the WSPS and was granted patents in 1980.
- Bristol Aerospace tested the WSPS by driving a wrecked Kiowa fuselage on a flatbed truck into wires at speeds up to 60 mph.
- Between 1996 and 2002, the US Army had no fatal wire strike accidents after retrofitting all small to medium helicopters with WSPS.
More in Aircraft Components, Part 2 1-24
Spotted an error? Know more?
Reader corrections go straight into our review queue. Suggest an edit · How this site is sourced