A Bond Super Puma L2 got struck by lightning this morning just off the North Coast of Scotland, and made a safe return to Aberdeen.
I know over the last 20 years we've had a fair few lightning strikes on helicopters, often in this mystery -2'C to +2'C band and they've caused a wide variety of damage from lost tail rotors, broken blades, and then some aircraft with no visible damage at all.
There were various rumours floating about that all aircraft have to be stripped down following a strike and de-magnatized and/or checked for damage to even the smallest of components as the path of the electricity can be so unpredictable.
Is this the case or can an inspection be carried out in a few days off line just to check no major damage ?? How did these two aircraft fare ??
Lightning strikes can cause problems years later, (S76 crash in the 90's caused by blade strike) but checks are carried out before the a/c goes back on the line. depends on the intensity I guess.
I think there's something like 18+ strikes a year at least for offshore heli's but not sure if thats worldwide.
Since Bristow lost the 76 as a direct result of the lightning strike....folks are taking these things much more seriously. The accident report for that crash makes for some sobering thoughts.
Last winter a CHC L2 was hit twice, I think the second was only a day or two after it returned to service from the first. Hydraulic fluid was lost as a result of arcing damage done to the servo. Part of the damage was probably from the first strike but was undetected following the standard checks. Following the AAIB report EC have recommended more in depth checks. So in answer to the previous question, yes very in depth inspections and usually significant component changes follow a strike. I gather the results are not cheap!
The question is how can we stop being hit? The advice of using the weather radar to avoid build ups seems not be the whole answer, any new suggestions?
How to stop beeing hit ? it's an old topic....It's seems that lightning strikes on helicopters are more frequent at relatively low OAT(around 0)....apparently few are reported under the tropics (except this L2 in Malaysia, having experienced a servo leak after a lightnng strike....I also have witnessed a strike on a parked AS 365 in Africa)....so...I personnally try to avoid vicinity of clouds with a marked electrical activity. Without a stormoscope, I try to assess the risks with the cloud vertical development and also by listening the static discharges in HF and NdB frequencies. In doubt, I avoid.
Unfortunately none of Stormscope, monitoring cloud vertical development or static on HF/ADF will guarantee not being struck in the N Sea. In the tropics etc, they will but the type of lightning is different there - mostly cloud to ground and the only time the aircraft gets struck is if it happens to be in the wrong place at the wrong time and a strike passes through it - because the strike originates (ie the zero C level is) a long way above normal heli operating altitudes.
In the N Sea since we have to operate around zero C in winter, the helicopter can act as the trigger for the first strike. So the Stormscope etc only tells you that you have just been struck.
The mechanism is that mixed phase hydrometeors are present around zero C (well I had to get that in somewhere!) ie its where the lighter ice and heavier liquid water are present. The lighter ice goes up in the updraft, the heavier water goes down and the friction between the two generates the static which builds up to form a strong electric field which can ultimately break down to give a strike. The maximum electric field strengh is at the zero C level. If a heli passes through that generating ionised air (from the rotor blades and turbine exhaust) the ionised air acts like the heli is trailing a conductor and triggers the discharge.
All this is confirmed by the fact that virtually all strikes on N Sea helis have been during the winter half of the year, despite the fact that lighting is more prevelant in the summer.
Each strike is potentially disasterous as even though the helis are certified to withstand a certain strike energy level, Nature does not live in JAR/FAR29-land or if it does, does not bother read the specs and may decide to exceed the certification energy levels.
Though most are not disasterous, they are very expensive with transmission trains, heads, blades etc normally needing replacement. If there was a way to avoid getting struck, we would be using it!
HC
(who once got struck whilst flying though thin stratus, with no vertical development, only speckled green on the radar, no turbulence, lighting not forecast or present anywhere else in N Sea that day etc etc (but it was around zero!))
HC, I agree with what you say, but I believe the odds to be striken must be higher if certain conditions are met..... Where I use to fly, OAT is usually around Std+15°c and never less than 18°C at our levels. It just makes sense to me to avoid places where lightning activity seems to be the strongest. I also know it is not an absolute vaccine but it should look a little provocating to fly right trough it....Some of those ancient Gods barely allow us to fly, no need to challenge them ! As for possible damages,I have read in an old lightning strike thread, that a German BO105 had its blades destroyed in flight by a lightning.
No science behind this but if I find myself in potential lightning strike conditions, I tend to go low. As far as I am aware, virtually all the North Sea lightning strikes have been in the 2000' -3000' altitude band (because these are the most common flight altitudes over the NS).
In a previous life, also on the NS, after we had had a couple of strikes within a few days, our technical director contacted NASA as they have some of the best knowledge about lightning strikes. In those days, the space shuttle probably would not survive a lightning strike so NASA did a lot of research on this subject.
One of their experts came over from Florida and gave us a presentation on the subject. One thing I do remember from the presentation was that he confirmed that the 2000'-8000' band was the highest risk altitude band for aircraft lightning strikes.
I do get some raised eyebrows from some of my younger copilots when I say "lets go down to 500 feet until we are clear of this" as low level flight is virtually unheard of nowadays on the North Sea.
I agree about the winter-half of the year being most risky in the North Sea from about 55N northwards from what I have seen on countless plotted hourly UK weather and sferic charts over many decades, as well as climatological stats.
A lot of winter thunderstorms with hail/sleet/snow/granular snow/graupel seem to occur at places like Shetland and Orkney (and the Hebrides) due to the relatively squat icy CBs which sweep the area in unstable polar or arctic airstreams.
Winter thunderstorms in the Western Isles and West Highlands have always seemed to me to give a disproportionately high amount of trouble to the electricity suppliers, too.
The co-existence of ice and water does seem to be important, too, as described in a previous excellent post by HeliComparator. Rainfall radar displays always show these - often surprisingly small - but lively CBs and electrical storms as having red or white centres. I believe that precipitation passing thro’ the ice to water phase (or vice versa) is responsible for these particularly vivid echoes and you can see them on several days in every winter.
Climatology for Lerwick for 1961-90 gives average monthly "days of thunder heard" as follows:
Jan - Feb - Mar - Apr - May - Jun - Jul - Aug - Sep - Oct - Nov - Dec - Year 0.9 - 0.8 - 0.5 - 0.0 - 0.1 - 0.3 - 0.4 - 0.3 - 0.1 - 0.3 - 0.9 - 0.5 - 5.2
The highest monthly values are indicated in bold.
IMO these will be storms that have developed in cold airstreams being heated by a relatively warm sea. We could regard them as “maritime”.
Leuchars in Fife gets around 5.2 "days of thunder heard" per annum, too, (1957-95) but the seasonal distribution is quite different, the highest frequency of thunder being in the “summer” period May-September where storms have developed over land due to summer heating.
Thanks for that, enlightning (excuse punn). A question, the movement of heavier water against light ice -does this ever occur with no indicative cloud formation? Also, regarding your last comment-flying in stratus with no vertical development and still getting struck..at 0 degC... what are your theories on that? eg: Are there any risks inland of this phenomena (for want of a better wrd) of 0deg lightning?
Cheers K48
Last edited by K48; 15th Mar 2008 at 07:27.
Reason: more to add
Its hard to know what caused my strike when there was no updraft etc. I can only assume it was left over from earlier cumuliform activity - the cloud had evaporated but the electric charge remained lurking until I was stupid enough to fly by! It was not a particularly bad strike, though 1 rotor blade had to be replaced.
In the hangar, at first it did not look as though the blades were damaged but standing on a ladder and sniffing the end of each blade, the damaged one had a very marked smell of burning plastic and sure enough the leading edge/bonding strip had been pushed up a bit by boiling plastic inside, at the junction between 2 strips where I guess there was relatively high resistance.
Whether this could occur onshore I don't know. The mechanisms tend to be slightly different onshore - as PKPF says the trigger offshore in winter is the warm sea (I use that term relatively!) whereas onshore it tends to be sun warming ground. In other words onshore lightning occurs in summer when 0C is well above your cruising altitude, whereas in winter offshore its at normal heli cruising altitudes.
HughM's policy of going low when there is a risk of lightning is one I also subscribe to (since I was struck!) and I think the primary reason why it is sucessful is because you are then well below the 0C altitude (low level air being "warmed" by the sea)
Would fitting diffusers to the exhausts like we did on the Gazelles and Lynx's not help avoid the winter strikes? im guessing - but surely the temperature difference at the zero layer would be as sensitive as a heat seeking missile - therefore the lower the signature you can paint the less likely hood of creating the static source and flashpoint??
Clearly low level is the better option but normally at a comms cost and no doubt frowned upon for some ludicrous reason by those assuming your a loose cannon and war hero or poser...!!
Can you north sea drivers decide your own level subject to conditions or is your flying and levels as restrictive as airways for plank drivers?
I have to say it is a very interesting topic especially the difference between over water and land ops vis-a-vis weather....
Rather, slack pressure fields with the development of one or two lows in the cold air is likely to occur. Freezing level will be relatively low.
Clearly then, there will be plenty of cold/icy CBs about. Being April, though, and with more powerful sunshine, development over land as well as sea, of towering CU and CB can be expected, often with hail. Archetypal British April wx, really.
Not all bad, though, this scenario usually gives astonishing vis and clarity of air, with lots of blue between the towering clouds containing the hard bits!
Pleasant and safe flying, all.
Last edited by PKPF68-77; 4th Apr 2008 at 11:18.
Reason: add link
Here is one of the arctic air's CBs that found its way as far south as the Forth valley. Mature cloud with fibrous anvil and a column of hail reaching the ground. We had three or four hail showers in the afternoon, not even amounting to 0.2mm of ppn, though.
Regions of Scotland further north will have been receiving numerous showers and whiteout conditions at times. Freezing level not far above 500ft up there, with minus 5 recorded (and forecast yesterday) on Cairngorm.
Nice picture PKPF...bettrer keping away from such column of hail, by the way, does a hail shower shows the same picture as as rain shower on a weather radar. Is there any hint to tell the difference?
