Fly by Wire

Wire strikes and technology to prevent them
James Careless
July 11, 2007
By James Careless
155Wire strikes are one of the deadliest threats faced by helicopter pilots. Two-thirds of such strikes destroy the aircraft. At least half of them kill one or more of the aircraft occupants.

There are many reasons why helicopter pilots fly into wires. The main one is ‘virtual invisibility’: “60% of the time, the pilot never saw the wire he hit,” said Robert Feerst, president of Utilities/Aviation Specialists Inc., a Crown Point, Indiana, consulting firm that specializes in wire-strike avoidance research and solutions for clients such as Transport Canada and the Federal Aviation Administration (FAA). “It takes a very minor change in the outdoor light intensity for a wire to go from visible to invisible,” he explained. “Many experienced pilots have seen the wire one minute, only to lose visual contact and then hit it the next.”

Other factors also contribute to wire strikes, such as the pilot flying low over unknown terrain, or simply not paying enough attention to the fact that wires can be anywhere. “You have to understand the dynamics of the flying environment,” said Feerst. “There are 4.6 million miles of suspended wire in the US alone, and Canada is similarly wire-infested.”

One thing is certain: wire strikes are not a rookie mistake. Information drawn from Civil Wire Strike Assessments by the National Aeronautics and Space Administration (NASA) indicate that the average wire strike pilot is 40 years old with 2,300 hours of flight experience. 86% of wire strikes happen in clear weather with good visibility, and 40% of the pilots who hit wires knew before the accident that they were there.

Just how deadly a wire strike can be to the average helicopter is graphically shown in a video Feerst uses in his “Flying in the Wire & Obstruction Environments” seminar, which he’s been presenting at Heli-Expo annually since 1992. Produced by the US Army, the video shows a small Hughes OH-6A helicopter suspended from a crane at NASA’s Langley Flight Test Center. The unmanned OH- 6A is swung front-first at 40 knots into a 3/8" seven-strand messenger wire. Hitting the OH-6A squarely across the windshield, the messenger wire slices horizontally through the airframe like a knife slicing through bread. The main rotor and upper third of the OH-6A end up hanging from the wire’s suspension cable, while the rest – having exploded into pieces during the impact – falls to the ground.

Aviation authorities and manufacturers have been working on wire-strike prevention technology for years. A number of aircraftbased products are on the market today, plus a new ground radar-based device whose deployment Feerst is spearheading in North America. However, despite the availability of wire-strike prevention technology, Feerst is quick to focus on pilots when asked which prevention approach works best. “Despite all the technology that’s coming out, the best way to prevent wire strikes is through pilot situational awareness, he told HELICOPTERS. “The person at the controls needs to know how to read the wired environment and how to make his way safely through it. Unfortunately, these skills are not taught at conventional flight schools.”

ACQUIRING SITUATIONAL AWARENESS
In its simplest sense, situational awareness means knowing what’s going on around you. Helicopter pilots must do more than just look ahead. They need to be constantly aware of what’s coming up below, above, and to the sides as well as to the front. But this is just the beginning, because situational awareness includes reconnoitering the route before flying it, whether on paper, computer, or in person at safe high altitude. In particular, such recons need to include not just spotting where wire and obstacle hazards may exist, but what landmark indicators exist to warn of their imminent closeness. In short, before pilots take to the air in the wired environment, they need to have figured out where the wires are, as surely as they would scout out the locations of mountains, tall buildings, and other obvious flight hazards.

Further, it is important for helicopter pilots to understand how power line grids are laid out and what kinds of wires are deployed, in order to accurately read the wired landscape. It is even possible to determine which way the wires are ‘travelling’ – if they are invisible – by looking at the larger insulating connections that can be seen from the air and observing how they are arranged.

Add constant awareness of the aircraft’s location and altitude, and helicopter pilots can substantially improve their chances of avoiding wire strikes.

DON’T RAISE THE BRIDGE; LOWER THE RIVER
Of course, there are times when even the most situationally aware pilot can’t detect upcoming wires; especially if it is approaching dusk, and the aircraft isn’t equipped with a wire detection system. In such situations, the old adage “Don’t raise the bridge; lower the river” comes to bear: If the pilot can’t detect the wires, then perhaps the wires should detect him.

This is the concept behind the Obstacle Collision Avoidance System (OCAS). Developed by Morten Mork and Rolf Bakken – formerly pilots with the Royal Norwegian Air Force, whose pilots fly in wire-crossed fjords – OCAS is a low-power radar system mounted on utility towers. Operating in the L-Band, OCAS is what’s known as a ‘sleeper’ system; its solar-powered brain doesn’t turn on the tower’s strobe lights unless it detects an aircraft approaching on a collision course. If this sudden light show doesn’t stop it, the OCAS unit broadcasts a verbal warning over all aeronautical VHF frequencies.

“The OCAS system is being tested in five locations – one site in Canada, four in the US – with great results,” said Feerst, who is managing the North American trials for both Transport Canada and the FAA. “One particular benefit is that the solarpowered OCAS is priced to be competitive with conventional tower lighting systems. This makes it affordable to utility companies and others with wired towers.”

In Canada, the OCAS system is being tested by BC Hydro at the Ruskin River crossing, nine nautical miles east of Pitt Meadows Airport near Vancouver. In this trial, the OCAS radar triggers a third strobe light (there are already two strobes operating, one on each side of the river) whenever an aircraft is 15 seconds away from being within 165 feet above the obstacle or closer. At 5.5 seconds, the OCAS transmitter will broadcast “wires, wires” repeatedly on local training, control tower, and VFR enroute radio channels.

Such is the success of the OCAS trials that Transport Canada plans to certify the Ruskin River site as operational in mid-May. “The week after that, we expect the FAA to certify the three OCAS sites in Tennessee and the one in Kentucky,” Feerst said.

ONBOARD WARNING SYSTEMS
For pilots with a budget, one of the best ways to avoid wire strikes is to install wire-strike warning systems on their aircraft. There are a number of these products on the market, each with its own approach to the wire detection problem.

EADS’ Helicopter Laser Radar (Hellas; www. e a d s - n v.com) system searches for obstacles using eye-safe laser light. With this data, Hellas warns the pilot of approaching obstacles by both visual and audio cues. The unit can also input this data into digital maps, which can be viewed by the pilot on a colour cockpit display.

Honeywell’s Enhanced Ground Proximity Warning System (EGPWS) uses the company’s Terrain Awareness & Warning Systems (TAWS) technology to alert helicopter pilots to all kinds of obstacles, including wires and towers. EGPWS works by comparing the system’s onboard mapping database with real-time GPS readings and the aircraft’s instrument data. Wire hazards are included in the EGPWS database and warning routines.

Safe Flight’s Power Detection System (PDS) watches for wires by detecting the electromagnetic currents they generate. It does this using an off-the-shelf aircraft antenna mounted externally on the helicopter’s airframe, connected to a small panelmounted device “about one inch square and 6-7” deep,” said Peter Fleiss, the company’s director of corporate communications. “When an electromagnetic field is detected, a clicking begins in your headsets, similar to what you might hear from a Geiger counter. The stronger the field grows, the faster the clicking, letting you know that you are closing in on wires even if you can’t see them.” Safe Flight’s PDS can detect larger power lines up to 3,000 feet away, and smaller lines – “like those that run up to your house,” said Fleiss – at up to 2,000 feet away.

WHAT SHOULD A WIRE-SAVVY PILOT DO?
Clearly, there any many options for preventing wirestrike accidents. The place to start is with situational awareness training, through courses such as those offered by Feerst at Heli-Expo and other locations. The next step is to equip your aircraft with some form of wire detection system, just in case. Granted, the time may come when major towers are protected by OCAS systems; however, a wise pilot doesn’t depend on others to keep him safe.

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