Helicopters & High Voltage Lines

[lanichol]

One spring afternoon I heard a helicopter overhead from my office. It sounded as if the chopper was landing on our buildings. In reality, it landed a short distance from the office in a field. The pilot indicated he was low on fuel and concerned he could make the airport. So he landed to refuel.

I noticed the chopper had a generator mounted on a platform attached to the struts. The guys were attaching clips for fiber optics to high voltage power poles

The guy attaching the clips to the power pole was wearing a black wire-mesh jump suite. “Why the jump suite”, I asked? His reply was, “So we can sit on the power lines like birds.” When approaching to work on a high voltage line (30,000 volts I believe) pole, He would take a grounding rod and start about 3 feet out from the line. The electricity would jump the gap and he would then move the rod to the line until the helicopter was the same voltage as the power line. Thus they could then attach the clips.

A few days later I notice a crew stringing a cable through the clips from a spool. Every since that day during my travels I look to the top of high voltage poles for the fiber optic cables that were strung by a two man crew; One crazy pilot and a partner hanging off the platform attaching clips with an electric impact gun.

Here is my question: These type contractors have probably strung fiber cables for thousands of miles on high voltage poles.
Charging and discharging a helicopter has bound to have an effect on the various metallurgy in the chopper

What additional review is required in a helicopter maintenance program that deals with these type voltages?

[Al_Hammer]

Very interesting. I'm not aware of any changes in metallic structure as a result of being charged. However, as the following posts describe, induced currents can flow as a result of being in a changing electromagnetic field and it is important to electrically bond all parts of the helicopter together so that sparking does not occur.

The helicopter doing maintenance on a high voltage line places itself in a
large magnetic field. This field is created by the AC current in the line,
so the field increases and decreases with the current. This will "induce"
or create electricity in metal objects. This effect is good when you're
trying to generate electricity in a powerhouse but not good in an aircraft.
Normal operation of an aircraft results in a "static" buildup of
electricity making the aircraft positive relative to the ground. You may
have felt this effect getting in and out of a car. So aircraft already have
many or most items "bonded". When working around high tension lines more
things will have to be bonded including people.
Bonding means connecting things electrically. For example your car engine
is bonded to the car body via a small cable or strap. This means that the
body and engine will have the same potential or voltage. It doesn't mean
that there isn't current flowing between the engine and the car body but as
long as the engine is connected to the body you will not get a spark by
touching the engine and body. For electronic devices, they are normally in
a metal case (like your computer). If you wanted to bond the case to the
chassis you would connect a wire from the case to the chassis.
An unbonded or poorly bonded fuel sensor in a fuel tank could cause a
spark, which would not be much of a problem if the tank was full but an
empty tank would explode (unless it has been purged with an inert gas).
************************************************** ***

>>I'm sure many of you have seen maintenance work being done to HV (and
>>EHV?) aerial lines by men in helicopters. I noticed that as the helicopter
>>settles into position prior to any work commencing, a chain is thrown over
>>the line, bringing the helicopter into electrical contact with the line.
>>This may seem like an odd question, but why is this done? Since the
>>helicopter is not in contact with any ground, I don't understand how the
>>workers would be harmed by coming into contact with the line without first
>>connecting to it.
>>
>>Thanx,
>>Jim
>
>
>Hi,
> This is done because of the charge current required to bring the
>helicopter to the same potential as the line. The line has an excess
>or deficit of electrons depending on the part of the cycle you are
>looking at. The helicopter has no excess or deficit, it is at zero.
>When tie line, (I have seen standard grounding lines used but not
>chains), is attached, if the line has an excess of electrons, they
>rush into the helicopter until the helicopter is at the same potential
>as the line. This much currant running through a mans arm would most
>likely kill him. And as the line swaps potential 120 times a second,
>the connection needs to be solid and left on until last.
********************************************
Due to the high levels of induction (caused by being in a thick magnetic
field) the chopper and everything in it must be bonded (connected
electrically) to reduce circulating currents and voltage differentials.
This means that everybody will be wearing semi-conductive clothes and be
bonded to the airframe themselves. If something was not bonded it could
develop a potential difference high enough to cause an arc. Not good for
electronics (or aircraft fuel tanks) or people. Hope this answers your
question.
***********************************************
It is noticable that birds do not sit on high voltage transmission
lines, although you may see them on the topmost grounding cable. The
voltage potential is high enough that the flow of electrons to achieve
a coulombic balance is enough to kill. Line maintenance is often done
by helicopter instead of tower truck. The visual effect of raising the
chopper and crew to the potential of the transmission line is
spectacular.

[PW_Plack]

This is not just an AC concern. If the power line was carrying DC, any large, metal, moving parts of the helicopter would generate very high voltages when they cut the magnetic lines of force around the line. For that matter, flying near a large magnet would do the same thing.

[Al_Hammer]

Valid point, although you don't see many DC power lines these days.
Even if the wires were made of spaghetti, and there was no current at all, it would still be a problem. Not from induced current, but from static charges. The helicopter builds up a charge in flight which must be bled off.

static electricity

Recently whilst in New Zealand i had a coversation with a crewman from the venison recovery deer culling days of the 70's. He remembers a number of time being badly shocked with static electricity whilst hooking loads on H500's, some had static straps fitted to stop this. No word of it when I did my hook/sling endorsement a couple of years ago. Any comment??
helibiz



Helibiz,
Whoever did your sling endorsment should have covered this. It's a common occurance in sling / winch op's to get static buildup on the line. This static charge will "go to ground" at the first opportunity (ie: through the loadmaster / rescue crewman) if the hook is not grounded before they touch it. Normally, allowing the hook to briefly touch the ground first will cause sufficient discharge, also a well insulated person is less likely to recieve a shock. Dry & dusty environmental conditions will normally make static buildup more likely too.

[darrellwittke]

While working on a forest fire rehabilitation project, we would do sling loads of material almost all day. If the pilot didn't place the long lead hook and swivel on the ground and a eager young smokejumper rushed to hook it up, he would receive a healthy shock (felt like appx. twice of what an electric fence delivers.) A not to quick learning guy may have done it twice and found out the shock varies from load to load.

Another thing of interest to heli pilots may be that we were working in proximity to high tension cross-country electrical transmission lines. I remember the pilots wouldn't get closer than 150-200 ft to them for fear of arcing from wires to helicopter.

[KenSandyEggo]

I was also concerned about the hi-voltage lines around here. I spoke to the Captain of my air-unit while in Karjakistan during the last flare-up, and he gave me a little safety tip. They only have one wire there, but it has a lot of current, he said. He told me to get a light socket and bulb from Home Depot and afix it to one of my control-sticks, so that the bottom of the socket is touching the stick and to ground the screw part to my left ankle with a large conduit clamp. Now if I approach some hi-voltage lines inadvertantly, the light bulb in the socket lights up and I know to change course.......Hey! It could happen!