Lightning

[jbrereton]

While working the North Sea and being around when one Super Puma ditched and others limped home after lightning strikes with various degrees of damage, it interested me to see a young lady come along to try to fathom what caused the strikes. Whether it was cloud types, temp, inversion, etc. Has anyone seen the results of this study. One of the things I suggested was that the white strobe lights attract lightning due to the high voltage discharge when the light flashes. Being an ex electrician this seemed obvious to me but they had not thought of it. I used to turn off the white stobe lights when flying near(Ops Manual distance) CB's that were active. Don't know if it helped but never got struck by lightning. Have not seen any other threads discussing the cause only the result. Hopefully someone will have information

[Texdoc]

If lightning is the build up of + charged ions (duh!) and so needs the - ( in respect to the + build up) to finish the circuit. Would turning off lights strobes really be an issue? Don't know the answer just posing the question.

I do know that in ground strikes the + actually "sucks up" a NUMBER (like multiple forks in a tree) of - bolts and picks the best / path of least resitance. So if "struck" by lightning the bolt actually comes from yourself and hence why multiple people can be struck, some suffering more than others. I wonder if this is the same with aircraft.

In any case, if you like golf, carry a 1 Iron. As, according to Seve Ballesteros ( I think it was him who said it ) "Even God can not hit a one iron" :ok:

[chopperman]

In any case, if you like golf, carry a 1 Iron. As, according to Seve Ballesteros ( I think it was him who said it ) "Even God can not hit a one iron"

It was Lee Trevino, after he was struck by lightning.

I was told by a knowledgeable type that the aircraft itself generates much more static than the strobe lights, so turning them off would make no difference. Hmm, think I would turn them of anyway, can't do any harm can it?

Chopperman.

[JimL]

There was a great deal of research effort that was initiated at the time of the G-TIGK accident with several threads:[list=1] Was there a weakness in the structure of the tail rotor of the AS332. Was the certification basis for lightning protection appropriate. Did the certification basis take into account the fact that there were a predominance of positive strikes (or strokes) in the North Sea (and other areas like the Sea of Japan and North West States of the US). Could a method of expeditious forecasting be provided that could enable pilots to avoid such strikes. Was there any (on board) equipment that would allow a pilot to avoid local areas where the potential for strikes was present. In the absence of any solution based upon the above, could operational procedures be improved.[/list=1] Roughly at the same time (circa 1999) there was a symposium at the RAeS on lightning (the proceeding can be obtained from the Society or from any attendee - there were several from the North Sea operators).

From memory, the results were as follows:

There was a change in the maintenance procedures for the 332 and EC offered a modified tail rotor assembly. Although there was a case for amending the certification basis for lightning protection, it was not considered that there was sufficient justification to cater for positive strike protection. (One of the conclusions that resulted from the RAeS symposium was that it would be difficult to establish the appropriate level of protection as the limit of the natural event would be difficult to establish.)

The Met Office were asked to conduct a survey of the conditions present at all of the known strikes to see if there were any common conditions - although there were no conclusive results, it appeared that the predominance of strikes occurred at or around the zero degree isotherm. The met office also produced a mathematical model which divided the North Sea into a series of zones which were coloured to indicate a potential for strikes (based upon the data drawn from previous events); this map was put onto the Met Office web site and made available on a trial basis. This caused a problem as, due to the fact that the potential occurred in air masses, large areas could be set to red for days on end and it was not clear what the operators/pilots were expected to do. (The lady who was referred to was probably from the Met Office on this research project.)

There was also an attempt to produce a methodology that could use the Met Office tools for predicting strikes - based upon real time data (and short term predictive models) - this would have been similar to that used to predict strikes around power stations or other vulnerable locations (France have such a system). Another system (produced by ONERA) required the provision of a number of special masts (measuring specific atmospheric disturbances) located on or around the coast - or offshore. Although this could have been done it would have been experimental with unknown results.

Both of the above systems would have required an end-to-end solution with specific on-board communication systems and involvement of ATC; this was not considered to be a practical proposition.

Concurrent with this, there was a research project to produce an on-board sensor. (This floundered mainly because Bond were sold and the main facilitator of the research - Tony Cox - left the industry to sail round the world.) Although sensors were designed and produced, the build up that they observed took place over seconds, was not directional and would not have provided the pilot with options.

A challenging area!

Jim

[Eurochopper]

Various "experts" tried to fathom this out. The key question was why aircraft in the N Sea were struck relatively often compared to the overall instances of lightning, whereas in other areas with much more lightning (eg Florida, Nigeria) aircraft virtually never got struck.

It seems it was due to the temperature - the voltages needed for lightning are created by charge separation when a mixture of ice and rain separate out in the updraft of a Cb - ice rises because its less dense and rain falls. In order to have this mixture of phases you need to be around zero centigrade - thus this temperature band is where there is the greatest voltage gradient and hence the easiest to trigger the lightning. Of course in the N Sea, operating temperatures are around zero during the winter. The stats show that aircraft on the N Sea are virtually never struck in summer 1/2 of the year, only the winter 1/2.

In the likes of Florida, the zero degree level is way above heli flights, so the only liklihood of getting struck is if you are unlucky enough to be in the way of a cloud to ground strike - very unlikely!

In the N Sea, it seems certain that the aircraft triggers the lighting when it flies through an area of high voltage gradient - the exact mechanism for triggering is not certain but could be either the trail of ionised air resulting from static discharge generated by the rotors, or perhaps an ion trail from the high temperature gases in the exhaust. In either case you are effectively trailing a long conductor behind the aircraft - lightning likes that!

Sorry, the strobe is not an issue as there is no external potential gradient or conductive path. Apart from anything else, if you look at the glass lens of a strobe you will see that there is a wire cage - this is to avoid the emission of radio interference but also eliminates any potential gradient.


The moral is that to avoid getting struck, avoid flying around near zero degrees C - lets say within +- 3 degrees - when lightning is expected. Tricky in winter but it can be done most of the time!

Eurochopper

[212man]

I agree with Eurochopper and JimL; the evidence I have seen/read indicates that the greatest likelyhood of being struck occurs around the zero degree isotherm.

I have operated in a region where the lightning can be pretty horrendous, and the speed of squall build up such that penetration can be the only option. However, to the best of my knowledge, we have never had a lightning strike on a helicopter in 40 years of operating here.

On the other hand, I am told that when we operated turbo-prop fixed wing aircraft, they were often struck and of course they tend to operate at altitudes where the OAT is around zero (about FL150 here).

[delta3]

Very interesting and plausible suggestion.

I have studied (quite some time ago) lightening strikes on the electrical power grid, and all I remember is that it was very difficult to predict where it actually would strike.

d3

[jbrereton]

Thank you all for the replies. Interesting! Did not realise the strobe lens were of such a design. I will look next time I am near an aircraft.
JB

[Mr Toad]

LIGHTNING
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I feel that composite blades may have something to do with this; flying the Chinook with massive wide chord composite blades (especially at night in winter) I remember being frequently singled out for this; sometimes also St Elmo's Fire, quite eerie. Never saw it on 61's or 76's.

Never a big problem, small hole in the blade (easily filled up), bonding leads burnt and a little exit hole in the fuselage, no big deal. But I know Super Pumas had some alarming events; they too employ quite a lot of composites. I agree about the comments on temperature and would add relative humidity.

So anybody got any scientific comment on composites/lightning?

[JimL]

None of these are trivial events; I was struck twice on consecutive days when flying the S61 in the North Sea; on the first occasion the lightning burnt a hole through the rotating swash plate, damaged all blades and required a gearbox change; on the second the strike was to the Spilsbury Tindall aerial which then flew back smashing a cockpit window and damaging all blades.

Nothing quite as frightening at night as feeling a 120kt draft of air whilst both blind and deaf; when we recovered enough of our senses to start to evaluate the problems, the dear old S61 was still flying within a hundred feet of our cruise altitude at 120kts. Only on the second occasion was there any Cb showing on the RADAR but because it was a squall line, it was impossible to fly round.

The S76 that threw the blade was also struck by lightning - it contributed to the subsequent failure.

Extent of damage on composite blades may increase as they age.

Jim