Lightning Strikes
 

I have been flying in the tropics for years and have wondered about the effects of a lightning strike on a helicopter. (Usually this comes to mind when it is pissing down with rain in the middle of a storm and you are battling to stay VFR trying to find a field crew you dropped off earlier in the middle of nowhere!!) I know with fixed wing a lightning strike generally does little or no damage if all the components are properly bonded but what about helicopters?
Asking around other pilots and engineers has'nt produced any satisfactory answers. I have heard that lightning can cause static pitting of bearings if all the components are not bonded properly.
Of course the sensible thing is not to be flying around in such weather but in the tropics during the wet season such weather is normal and not flying near storms is generally not always an option.

http://www.aaib.detr.gov.uk/formal/gtigk/gtigk.htm

The usual thorough report by aaib on lightning strike on Super Puma.

While working at Agusta as a RMS consultant I brought the following to the attention of the A-129 Program Manager and other members of the engineering department. I informed them that the rotorblades had inadequate protection from a lightning strike (i.e. embedded aluminum mesh). I also indicated that the bonding of the blades to the rotorhead did not comply with normal design standards. They indicated that the blades, which were of carbon composite structure, would conduct the charge after an attachment. I further indicated that the carbon composition was capable of conducting any static charge off to the rotorhead but if the blade(s) suffered a lightning attachment the conductive capabilities of the carbon composite material would be overwhelmed and the carbon structure would then act as an insulator. As an insulator the temperatures internal of the blade would be so intense as to blow the blade apart. I do not know if they changed the blade design to incorporate the aluminum mesh or, do I know if they improved the bonding.

Normal dissipation of static build up on the rotor through the transmission will cause spark erosion on transmission gears and bearings. Assuming that the bonding is adequate on the rotor system to conduct the amperage of a lightning strike and the full charge passes through the transmission the damage could be much more than just spark erosion.

To: captk

“Asking around other pilots and engineers hasn’t produced any satisfactory answers. I have heard that lightning can cause static pitting of bearings if all the components are not bonded properly”.

There are several things going on inside of a helicopter transmission and they are dependent upon the condition of the oil. It would be assumed that the gears and bearings have solid metal to metal contact when in operation thus providing a bonding path. This is not true. Assuming a properly working lube system there is a very thin oil film at the point of tooth contact on the gears and ball to race contact on the bearings. Oil is an insulator so when the gear teeth are at their maximum point of contact there is no conductivity. As the teeth separate or the balls move in relation to the race the static charge will penetrate or puncture the oil film and the spark will cause pitting. This is called the puncture voltage. The static charge must be high enough to effect the puncture of the oil film. As hours accumulate on the oil wear metals start to build up in the oil. That is why SOAP tests are performed. The wear metals change the conductivity of the oil so the puncture voltage is reduced and the sparking intensifies. When a helicopter suffers a lightning attachment and depending upon how high the amperage is as the charge passes through the transmission the transmission gears and bearings can be severely pitted all the way to being welded together.

[ 08 November 2001: Message edited by: Lu Zuckerman ]

Thanks for your info which has filled a big gap in my knowledge of the subject.
Just another thought but is it possible that a lightning strike could go undetected and perhaps a few hours later the gearbox starts to produce metal as the bearings break down as a result of the lightning strike? This is assuming that all the components were properly bonded and there were no visible marks or debonding on the blades.

Only ever had one lightning strike, but from that experience and what others tell of theirs , you will know about it. The noise alone scares the **** out of you.
Hoping you never have one, (lightning strike that is),

Chopperman.

[ 09 November 2001: Message edited by: chopperman ]

Thanks Chopperman but thats the problem! I have had the **** scared out of me with the flash and loud noise flying near a storm and suspected a strike hence the interest in the subject. There was no damage but wondered about the possibilities. I certainly wouldn't like to go through it again. Unfortunately during the monsoon season thunderstorms are a daily event so lightning strikes are a daily hazzard.

CaptK,
Can tell you that I've been hit by lightning twice. First time in S55T. I got too close to a CB and was hit, Big noise, followed by vibration because a tip cap was blown off the blade end. Not the end of the world, but a few days later, while in a hole picking some water up, I got a momentary flash on the transmission chip light and it caught my eye probably because I was down in the shade of some big trees. Had a look at the chip detector and it was covered in sandy slivers. It turned out the lightning had come in the tailfin, gone along the fuselage and out through blade tip via transmission.
Result: transmission bearings all magnetised and within a few hours were disintegrating rapidly.Transmission had to be replaced.
Second time in a turboprop, lightning came in propellor,through the engine and out through wingtip wicks. Unfortuneately all bearings in engine[PT6] also started to disintegrate withing 10 hours. Our engineers reckoned you had about 15-20 hours before an engine or transmission would fail after lightning passing through it. You'll have an entry and exit point and may be able to determine the route the lightning takes through your airframe.
My advice to you is that if you think any of it passed through your engine or any transmissions, check them all every few hours for at least 30 hours. Don't rely on the chip lights, my S55T light only blinked once even though it was covered in this sand like metal from the bearings. Pull the plugs off and look yourself.

To: captk

ditchy said it all.

article on lightning strikes in latest rotor journal. http://www.actus-multimedia.com/euro...j/eecrj41.html

[ 14 November 2001: Message edited by: widgeon ]

Ditchy, Snap! Also struck twice, once in a Sea King 4 and again in a Jet airliner.

Westland reckoned a lightning strike with the composites would probably be catastophic. I'm pleased to say that they were wrong. There was some damage to the titanium leading edge strip, buthat was all. Taking Lu's point about oil, I seem to remember that the Wessex5 (and possibly the SK4) used O135 in the engines and O127 in the gearboxes. The reason given for the gearbox choice was that it was conductive. Anyone shed light on this?

In the jet it was much more scary, with burning smells, and a very loud bang and flash. Oddly enough, no damage whatsoever.

One thing in common. In both cases the first words were 'What the F*c* was that?' :eek:

Thanks for all the replies and info. My worst fears were well founded but at least I am a lot better informed on the subject.

Lightning strike
 

Anyone ever had this happen to them or have any idea what would happen should a heli get struck by lightning? Instrument damage & structual damage?

lightning
 

I'd suggest the pilot might need a change of pants :O

I have heard of just one incident where a lightning got an Allouette III of REGA. It hit on the rotor head and went out on the tail rotor gear box (or the other way round). It welded the gears together for a short time but they brocke free again. They landed without any problem but during the following inspection, they found the damage. Very expensive.

There was also that incident that took place in the north sea a few years ago, one of the bristow fleet. If the hydraulic pipes had not prevented he tail rotor gearbox seperating from the aircraft, it would have REALLY spoilt there afternoon.

and the case if the S76 where a lightning strike caused local hardening on the MRB spar which led to a catastophic failure years later.
http://www.aaib.dft.gov.uk/sites/aai...ection%204.pdf


and this for a puma.
http://www.aaib.dft.gov.uk/sites/aai...pdf_502987.pdf

http://www.aaib.dft.gov.uk/sites/aai...ma,_G-TIGF.pdf

actually lightning strikes are more destructive on modern composite blades than old fashioned metal ones as the conductive path is normally restricted to the leading edge erosion shield.

Every winter we get the odd lightning strike up in ABZ. The problems are an increase in vibs as as the lightning discharges through the rotor blades (main and tail) and shatters the the blade (kevlar-carbon fibre). There is also an instant "weld" of any moving metal parts ie gears in the gear box.

I was zapped in a Puma on the North sea in SCu, not Cb. Apart from the co-pilot dropping his bacon butty and the VHF1 going to a permanent 'test' there didn't seem to be that much else, as there was nowhere else to go I continued to ABZ.:ouch:

After landing it was discovered that all the bonding straps to the main blades had vapourised, all four blades were written off.:uhoh: Didn't have any problems with the MGB.

The newest helos have to meet lightning strike criteria that older ones did not. The tests zap the parts and show that no damage occurs and that the charge is conducted without the horror stories that some of the observations below describe.
The AB-139 and S-92 are the only types I know that have to meet the new criteria, so one can believe that they are better than the others in this department, along with many other improvements.
The upgrade of safety requirements is a subject that has been bashed about here in the past, but is probably still not fully understood by the industry or pilots. An aircraft first certified in the 60's or 70's is dsigned and built to standards in effect then, to much lower standards, in fact.

Thanks for the feedback - I will have a look on the net to get a better understanding of the damage most likely to be caused. :ok: