voiceofgoa
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Lightning Strike

Fri Aug 31, 2012 1:19 pm

I was on LH757 in a B744 from BOM-FRA yesterday, sitting in the nose of the aircraft (in B class). Barely two minutes into the climb after take-off there was a very loud bang and two flashes of light outside. People were startled but I suspected it was a lightning strike. Soon thereafter the captain announced that yes, that was a lightning strike "but the aircraft is perfectly fine as always."

Under what conditions would that strike be dangerous? Do the engines have special protection? And the loud bang - was that a physical blow to the airframe?
 
flyingturtle
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RE: Lightning Strike

Fri Aug 31, 2012 1:51 pm

Quoting voiceofgoa (Thread starter):
Under what conditions would that strike be dangerous?

Aircraft are hit all the time by lightning, and accidents caused by lightning are extremely rare.

In commercial aviation, only these two accidents seem to have ever happened due to lightning:

http://en.wikipedia.org/wiki/Pan_Am_Flight_214
http://en.wikipedia.org/wiki/LANSA_Flight_508


David
Keeping calm is terrorism against those who want to live in fear.
 
CosmicCruiser
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RE: Lightning Strike

Fri Aug 31, 2012 4:59 pm

The loud bang is called thunder.
 
fr8mech
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RE: Lightning Strike

Fri Aug 31, 2012 9:21 pm

Quoting voiceofgoa (Thread starter):
Under what conditions would that strike be dangerous?

The probability of a dangerous problem approaches zero.

Quoting voiceofgoa (Thread starter):
Do the engines have special protection?

Modern aircraft are well protected against lightning strike. All large surfaces are bonded. Control wiring is shielded (not just for lightning protection). The engines and the electronic controls are shielded.

Do problems occur? Yes. Are they serious enough to cause a dangerous condition? No.

The airframe on the other hand, is a different story. Usually, there will be burn marks. Markedly, at the entry point (usually small) and at the exit point (usually big and ugly). Quite frequently there will be strikes all along the airframe between these 2 points. Each burn must be addressed in some fashion. I've seen aircraft out-of-service for a couple of hours and I've seen them out-of-service for several days.

Quoting voiceofgoa (Thread starter):
And the loud bang - was that a physical blow to the airframe?

That noise was thunder.
When seconds count...the police are minutes away.
 
CosmicCruiser
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RE: Lightning Strike

Fri Aug 31, 2012 10:01 pm

I had a strike in a DC-10 that resulted in 7 burnt spots around door 1R and a piece of elevator about the size of your palm missing. We didn't experience any difficulties and continued to destination. It was then that we saw the damage.
 
tdscanuck
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RE: Lightning Strike

Sat Sep 01, 2012 2:00 am

Quoting voiceofgoa (Thread starter):
Under what conditions would that strike be dangerous?

The only likely conditions to be dangerous are a strike so large that it exceeds the conductive capability of the aircraft (physically possible but only in weather that aircraft have no business flying in) or a fault in the bonding/grounding system that allows lightning current to go where it shouldn't (inside the cabin or through a fuel tank).

Quoting voiceofgoa (Thread starter):
Do the engines have special protection?

They're protected, but not in any particular way that the rest of the airplane isn't (i.e. it's not "special"). Nacelles are struck fairly often but the engines themselves aren't very good lightning paths compared to the airframe.

Tom.
 
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Starlionblue
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RE: Lightning Strike

Sat Sep 01, 2012 4:34 am

I've heard the figure in docus that every airliner is struck by lightning once a year on average.


On a related note, how do the static wicks help? Do they sacrifice themselves and burn off to discharge powerful strikes?
"There are no stupid questions, but there are a lot of inquisitive idiots."
 
David L
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RE: Lightning Strike

Sat Sep 01, 2012 10:23 am

Quoting Starlionblue (Reply 6):
On a related note, how do the static wicks help? Do they sacrifice themselves and burn off to discharge powerful strikes?

I think they're there to dissipate a build-up of static charge from friction on the airframe. I assume they're vulnerable to damage by lightning strikes rather than being designed to sacrifice themselves - more a case of "accidental death" than "suicide"?  
 
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Aaron747
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RE: Lightning Strike

Sat Sep 01, 2012 10:42 am

Quoting tdscanuck (Reply 5):
The only likely conditions to be dangerous are a strike so large that it exceeds the conductive capability of the aircraft (physically possible but only in weather that aircraft have no business flying in) or a fault in the bonding/grounding system that allows lightning current to go where it shouldn't (inside the cabin or through a fuel tank)

This I have always been a bit unclear about. The vast majority of lightning is negatively-charged, but rare positive strikes are incredibly powerful by all accounts and usually eminate from the anvil of a CB. These strikes have been known to travel great distances from the generating cloud, so in theory, it should be possible for them to strike aircraft that are well clear of the weather itself. If I remember correctly, there is a lot of speculation that the above-referenced Pan Am flight was downed by a positive strike.
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flyingturtle
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RE: Lightning Strike

Sat Sep 01, 2012 1:41 pm

http://www.damninteresting.com/the-power-of-positive-lightning/ seems to one of the more credible reports about positive lightning. Damn interesting.


David
Keeping calm is terrorism against those who want to live in fear.
 
tdscanuck
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RE: Lightning Strike

Sat Sep 01, 2012 2:25 pm

Quoting Starlionblue (Reply 6):
I've heard the figure in docus that every airliner is struck by lightning once a year on average.

That's a good rule of thumb, and is used often in the industry. It varies wildly on an airline-by-airline and fleet-by-fleet basis because certain regions of the world are far more prone to lightning, and lightning risk scales with cycles rather than hours (you're almost never struck in cruise).

Quoting Starlionblue (Reply 6):
On a related note, how do the static wicks help?

They don't, other than ensuring continued employment for the airframe techs.

Quoting Starlionblue (Reply 6):
Do they sacrifice themselves and burn off to discharge powerful strikes?

They do tend to get blown up by lightning strikes, but that's a side-effect rather than a desirable condition. In order to do the job they're designed for, they also are very popular choices for the lightning exit point. David L has it right:

Quoting David L (Reply 7):
I think they're there to dissipate a build-up of static charge from friction on the airframe. I assume they're vulnerable to damage by lightning strikes rather than being designed to sacrifice themselves - more a case of "accidental death" than "suicide"?

Exactly. Due to airspeed, under many atmospheric conditions, the airframe will pick up static charge. This will ruin the ground plane for the radio antennas and cut off communications. The static wicks are designed to shed excess static electricity. They are almost totally helpless in the face of large direct currents like lightning. However, the features that make them good at shedding static (small, pointy, located at the extremities) also make them very good exit points for lightning. Hence they tend to get shattered or blown off during lightning strikes.

Quoting Aaron747 (Reply 8):
This I have always been a bit unclear about. The vast majority of lightning is negatively-charged, but rare positive strikes are incredibly powerful by all accounts and usually eminate from the anvil of a CB. These strikes have been known to travel great distances from the generating cloud, so in theory, it should be possible for them to strike aircraft that are well clear of the weather itself.

A pilot I know and trust absolutely got struck in completely clear air; his experience does match the description of positive lightning. The aircraft (a 737) was OK. The conductive capability of large jets is extremely high due to the shear amount of material and extremely rigorous bonding. Small aircraft are, perversely, probably at more danger here.

Tom.
 
MD11Engineer
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RE: Lightning Strike

Sat Sep 01, 2012 6:55 pm

I have a piece of belly skin of a 737 in my desk drawer, which got punctured by a lightning strike. The strike left a crater of about 5 mm diametre which went right through the skin (about 3 mm thick), but a layer of PRC sealant prevented it to melt into the frame beneath. The damaged area got cut out and repaired with a patch and doubler.
We counted more than 50 impacts on this aircraft. Entry was near the nose, but the arcing went from rivet head to rivet head, across pax window frames.
The exit, as usual, was a static wick and it´s diverter strip oon the horizontal stabilizer.

Jan
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larshjort
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RE: Lightning Strike

Sat Sep 01, 2012 7:37 pm

Quoting fr8mech (Reply 3):
The airframe on the other hand, is a different story. Usually, there will be burn marks. Markedly, at the entry point (usually small) and at the exit point (usually big and ugly). Quite frequently there will be strikes all along the airframe between these 2 points. Each burn must be addressed in some fashion. I've seen aircraft out-of-service for a couple of hours and I've seen them out-of-service for several days.

I have seen pictures from a QI CRJ that lost 2/3rds of one of the winglets.

Quoting tdscanuck (Reply 10):
Quoting Starlionblue (Reply 6):On a related note, how do the static wicks help?
They don't, other than ensuring continued employment for the airframe techs.

They are evil things trying to destroy the eyes of technicians. 

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