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How Are Planes Grounded (electrically)?  
User currently offlinePlanelover From United States of America, joined Dec 2001, 321 posts, RR: 0
Posted (12 years 1 month 3 weeks 2 days 19 hours ago) and read 7521 times:

Hey all,

Not sure if this is a dumb question or not, but here goes.

When a car is struck by lightning, the lightning just goes through it because it is, of course, in contact with the ground. What happens when a plane is struck by lightning? What keeps a plane from being damaged since it isn't in contact with the ground?

Anyhow, that's it. Have fun.  Smile/happy/getting dizzy
PL

9 replies: All unread, jump to last
 
User currently offlineFBU 4EVER! From Norway, joined Jan 2001, 998 posts, RR: 7
Reply 1, posted (12 years 1 month 3 weeks 2 days 10 hours ago) and read 7490 times:

Basically,a plane is divided in two:the interior is positive while the exterior is negative.These parts are isolated from each other.Control surfaces are bonded to the aircraft skin and a lightning strike will propagate to the extremities of the plane,i.e. wing tips,stabilizers,fin tip,etc.Around these locations you will see static dischargers,spikes consisting of cobalt,that will dissipate the electrical charge from the lightning to the atmosphere.If the plane is well maintained there are no major problems if it is struck by lightning.
Sure,a mighty flash and a big "booom" followed by a post-flight inspection.That's all.



"Luck and superstition wins all the time"!
User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 2, posted (12 years 1 month 3 weeks 2 days 9 hours ago) and read 7481 times:

Actually, take a look at a car. You might find it is standing on four round rubber things called 'tyres'. As most things made of rubber, they're not very good to use if you want to ground something as they're non-conductive. IOW, the car isn't all that different from the aircraft...  Big grin The sheet metal in the skin forms a Faraday cage though, so the interior will be rather well protected from the strike just like FBU described.

However, a bad strike might just end up damaging the skin in the entry or exit points. Having a hole blown in the radome isn't all that uncommon. If you're unlucky, a strike can take out avionics on a large scale. They're well shielded and tested but there is a lot of hard-to-control energy in a thunderbolt. :/

Cheers,
Fred




I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offline737doctor From United States of America, joined Mar 2001, 1332 posts, RR: 38
Reply 3, posted (12 years 1 month 3 weeks 1 day 17 hours ago) and read 7412 times:

I have seen some pretty ugly lightning strikes in my day, requiring a sheetmetal repair. The area often looks like a little spot weld. Some are pretty minor, only requiring the area to be burnished. Deeper ones often have the area drilled out and filled with a rivet. Occasionally, I've seen larger ones in which a more surface area was cut out and patched over. And yes, lightning strikes can wreak havoc on avionics systems.


Patrick Bateman is my hero.
User currently offlineAndroid From Japan, joined Jun 2002, 80 posts, RR: 0
Reply 4, posted (12 years 1 month 3 weeks 1 day 17 hours ago) and read 7409 times:

I have seen a Dash-8 with over 750 exit wound from lightning strike-mostly burned out rivets. It usually comes out the belly on the Dash-8 .Another hazard is lightning will actually magnetize the Engine and reduction gearbox which causes rapid spalling of bearings . Some manufacturers are reviewing the use of aluminized paint to help with the bonding and reduce lightning strike damage.

User currently offlineAvionic From Denmark, joined Nov 1999, 111 posts, RR: 1
Reply 5, posted (12 years 1 month 2 weeks 3 days 12 hours ago) and read 7289 times:

just to correct FBU4ever (sorry  Smile) the static dischargers you see in the wing and the tail are mainly for static electricity, caused by the friction of the airplane in the wind....I have rarely (or never) seen an airplane with a lightning exit point over the static dischargers....You always have an entry and exit point on the airframe......the jobs gets funny when you find either, and have to find the other, it can be a big airframe to search on  Smile

User currently offlineFBU 4EVER! From Norway, joined Jan 2001, 998 posts, RR: 7
Reply 6, posted (12 years 1 month 2 weeks 3 days 7 hours ago) and read 7268 times:

I stand corrected,Avionics!
"My" exit points have always been in the tailplane tip area.Not near the wicks.
But reducing the static from an airframe will reduce the likelyhood of a lightning strike,so the wicks are an integral part of the protection.



"Luck and superstition wins all the time"!
User currently offlineAvionic From Denmark, joined Nov 1999, 111 posts, RR: 1
Reply 7, posted (12 years 1 month 2 weeks 2 days 7 hours ago) and read 7218 times:

korrekt.....and i can hear you have experience with dc9/m80/90 from your exit points....  Smile .....static electricity can be very powerfull....i have seen the plastic shaft of a screwdriver explode when touching a 767, and i have seen blue "flames" when grounding an md90........powerfull stuff, not to be fooled around with.... Kan du ikke lide gardemoen?

User currently offlineAirplay From , joined Dec 1969, posts, RR:
Reply 8, posted (12 years 1 month 2 weeks 1 day 20 hours ago) and read 7193 times:

FBU 4EVER

How do static wicks help prevent lightning strikes? What do you mean by "the interior is positive"?

Objects don't have to be "grounded" in order to attract lightning strikes. Lightning simply finds the path of least resistance in it's quest to equalize the charge. Some strikes actually originate from the ground and rise to the clouds.

Aluminum is an excellent conductor, so it is not uncommon for it to pass through aircraft in the vicinity.

Damage is minimised by the use of electrical bonding. Flexible bonding straps are connected between moving control surfaces to allow any current due to static build-up or lightning to pass harmlessly from the airframe to the control surfaces (like an aileron) instead of the current attempting to pass through a bearing or other part that would cause serious damage.

The entire exterior and critical interior components are electrically bonded including the composite and rubber parts such as tires and de-ice boots. This is achieved by manufacturing this "normally insulative" products using conductive elements mixed in or using special coatings.

Static wicks are used to slowly bleed the electrical airframe charge caused by friction as the airframe flys through precipitation. In this case, the bonding allows continuity to the moving control surfaces to static wicks. Many wicks are actually mounted on the control surfaces. Others are on the non-moving airframe parts.

The wicks are not just pieces of wire or metal. There are many types. Carbon impregnated cotton is used on small low-performance aircraft. Alloy rods with platinum tips located in a low pressure inducing structures are used on large fast aircraft. There are many many different types. All of them share one characteristic: The electrical resistance from the airframe to the tip is very high. In the order of Meg-Ohms. Some as high as 20 Meg-Ohms. This high resistance allows the static to bleed very slowly. This avoids arcing and the associated radio noise. Because of this slow discharge rate, larger, faster aircraft need more wicks than smaller slower aircraft to allow them to bleed the enormous amount of static electricity generated.

One last thing. Properly maintained aircraft will try to "ground" themselves at touchdown to equalize the charge it built up while flying. They do this either through their "conductive tires" or by a steel flexible rod that is usually suspended from the gear bogies. Of course there are some factors that prevent the airplane from achieving this goal, such as very dry conditions on the ground.



User currently offlineAvionic From Denmark, joined Nov 1999, 111 posts, RR: 1
Reply 9, posted (12 years 1 month 2 weeks 4 hours ago) and read 7117 times:

As a little note, we do check these (roughly every 1 to 6 months depending on a/c type) with a megaohmmeter. The bonding from airframe to static disc. retainer to be less then 0,1 ohm (0,4 if the material is composite) and from the tip to the retainer roughly between 2 to 20 Mohms (i think there are 2 types, Chelton and another, don´t remember the name, it is the old yellow ones.....wait i think they are Granger, yeop that´s it). Anybody tried to burn their fingers on the megaohmmeter?

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