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Fly by Wire

July 11, 2007  By James Careless

There are many reasons why helicopter pilots fly into wires.

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.

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.


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.”


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.

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.


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.

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.”

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.


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

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.


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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