Ballistic cable cutters

[Scattercat]

Several weeks ago I was involved in a winch rescue that required operating quite close to a set of high voltage powerlines. Of concern at the time was whether EMT from the lines would or could have any effect on the ballistic cable cutter system. We routinely have boats switch radars / HF radios off, partly for this reason. Conditions at the time were very dry and dusty & nessitated a hover height of aprox' 100ft to remain above the dust. The paramedics reported quite severe static discharge problems which got me thinking as to whether the static was being caused by the atmospherics, or EMT from the powerlines. Has anyone any experience or knowlege on this?
This was in a B412 with an external hoist.
Cheers.

[trimpot]

Scatter,
my experience is that static caused by dust, salt water spray etc will not cause the cable cutter to fire but EMT from power lines most certainly will. From memory this is the reason Careflight Sydney dropped one of their doctors a few years back.

[Shawn Coyle]

Two points on this-
First of all, if it's a civil helicopter, the stuff on the hoist should have been tested for some pretty stringent EMC / EMI fields.
Secondly, the presence of something like this on a hoist in a civil helicopter is one of the reasons why you'll see phrases in the Flight Manual Supplements for hoists like 'Not approved for human cargo'. Strange isn't it?
But this is the only way engineers in the certifying authority can put their hands on their hearts and say that they've done their job. How you as an operator are supposed to use the equipment is not their problem
Hint- you can invoke the 'emergency' clause in most operating rules to override anything in the flight manual to save human lives.
And I'm sure that will help when the lawyers come around after the cable cutter blows when operating near a power line.

[Lu Zuckerman]

Any moons ago when I was a Tech Rep on the Atlas ICBM I had the misfortune of standing beneath two 1000-Pound thrust solid rockets. They were running a test of the flight control RADAR and were painting the launch complex. Although the squibs in the rockets were properly grounded the RFI energy level was sufficient to fire the squibs. It was like a shotgun going off next to my ears.

In a similar case in Vietnam a large cargo aircraft had landed and had not shut down its’ RADAR. The beam painted a Cobra that was going out on a mission. The RFI fired the squibs on the FF rockets and fired the ammunition for the gun. The ammunition was fired electrically and it all went off at the same time. The missiles fired and went down to the end of the airfield and exploded and the helicopter was totally destroyed by the exploding ammunition.

The wires leading to the squibs on the hoist should be a twisted pair and properly shielded to minimize transformer coupling from the electrical field surrounding high-tension wires. I believe that hoists on military aircraft have a more stringent design spec than those developed for civil aircraft unless the hoist is specified for use on a specific helicopter and then the airframe manufacturer applies their specification requirements.


:E :E

[SawThe Light]

Generally there will be no problems with the squib circuit in civil helicopters provided the system receives its required maintenance. Analysis of the few that I am aware of revealed system defects in each case (except for one where there was a pilot defect) that should have been detected with correct maintenance. None were as result of an external "trigger".

The most common problems we have observed with un-commanded firing (and thank God there are not many) is simply the result of a lack of necessary maintenance, a lack of a good bonding throughout the system and/or very occasionally, poor electrical system design. The problems magnify where the hoist is classified as role equipment and is being installed and removed by the flight crew. (Don't get me wrong any of you flight crew that do carry out the on/off tasks. Some of you are possibly properly trained in the use of the correct test equipment to ensure the hoist system is correctly earthed/bonded after installation)

For the record it was simply poor earthing/bonding that caused the Sydney accident. Over a period of time deterioration of the earthing/bonding went un-noticed and eventually the system finally found a ground through the squib circuit. Note that that particular installation was a local design installation, not a factory setup.

[trimpot]

Saw The Light,
I was under the impression that at the time the Careflight crew were winching near high voltage lines and this is what the accident was attributed to. However, your explanation does sound more plausible and is no doubt closer to the truth. This being said, I was always advised (SOP style) not to arm the cable cutter if in the vicinity of high voltage lines.

As a side note my wife worked with the Doctor that was dropped (he made a full recovery by the way). It was interesting to compare notes with him, as, I have been cut off the wire myself (Thanks Geoffro, you saved my life!) and it's not fun, it's bloody scary! :oh: