Static Discharge

[gman]

Hi Folks,
Here's an interesting phenomenon I observed recently and I was
wondering if anybody else has seen anything like it.

About two weekends ago, I was doing a cross country in actual (IMC)
with one of my Instrument Rating students near Cheyenne, WY. We were
at 11,000ft for a while and the temperature was near freezing but
there was no precipitation in the area. All of a sudden, there was a
sound like a pebble hitting the windshield. I explained to my student
that it was either an isolated hail pebble or ice coming off the
propeller. About ten minutes later, I heard the same sound but this
time it sounded more like an electric discharge similar to the one you
hear when you touch the door knob after walking on the carpet. I
rechecked the fuses and the alternator but everything was working
fine. The sound was definitely coming from the outside. Then about
five minutes later, we heard the same sound. But this time, I was
trying to read the mag compass which is mounted up high on the
windshield. I'm pretty sure that I saw a flash of light this time near
the top center of the windshield where the Plexiglas meets the
aluminum . This made me wonder: When flying in IMC over an extended
geographic area (Cheyenne to West of Denver) is it possible for the
airplane to start discharging by arcing? Has anybody else seen
something similar to this? I should mention that the winds were not
especially fast that day (about 160 @ 30kts).

[Jim Macklin]

Most slow airplanes do not have static discharge wicks, but
in the correct conditions, they can build static which can
build to very high voltages. It is also possible that the
grounds [bonding] between components is not good and that
can lead to arcing across hinges and between ailerons and
wing, etc.

Snow and ice crystals cause the build up, the fix is proper
bonding and static wicks, even on a C172.


"gman" > wrote in message
ups.com...
| Hi Folks,
| Here's an interesting phenomenon I observed recently and I
was
| wondering if anybody else has seen anything like it.
|
| About two weekends ago, I was doing a cross country in
actual (IMC)
| with one of my Instrument Rating students near Cheyenne,
WY. We were
| at 11,000ft for a while and the temperature was near
freezing but
| there was no precipitation in the area. All of a sudden,
there was a
| sound like a pebble hitting the windshield. I explained to
my student
| that it was either an isolated hail pebble or ice coming
off the
| propeller. About ten minutes later, I heard the same sound
but this
| time it sounded more like an electric discharge similar to
the one you
| hear when you touch the door knob after walking on the
carpet. I
| rechecked the fuses and the alternator but everything was
working
| fine. The sound was definitely coming from the outside.
Then about
| five minutes later, we heard the same sound. But this
time, I was
| trying to read the mag compass which is mounted up high on
the
| windshield. I'm pretty sure that I saw a flash of light
this time near
| the top center of the windshield where the Plexiglas meets
the
| aluminum . This made me wonder: When flying in IMC over an
extended
| geographic area (Cheyenne to West of Denver) is it
possible for the
| airplane to start discharging by arcing? Has anybody else
seen
| something similar to this? I should mention that the
winds were not
| especially fast that day (about 160 @ 30kts).
|

[[email protected]]

On Mar 23, 2:48 pm, "gman" > wrote:
> Hi Folks,
> Here's an interesting phenomenon I observed recently and I was
> wondering if anybody else has seen anything like it.
>
> About two weekends ago, I was doing a cross country in actual (IMC)
> with one of my Instrument Rating students near Cheyenne, WY. We were
> at 11,000ft for a while and the temperature was near freezing but
> there was no precipitation in the area. All of a sudden, there was a
> sound like a pebble hitting the windshield. I explained to my student
> that it was either an isolated hail pebble or ice coming off the
> propeller. About ten minutes later, I heard the same sound but this
> time it sounded more like an electric discharge similar to the one you
> hear when you touch the door knob after walking on the carpet. I
> rechecked the fuses and the alternator but everything was working
> fine. The sound was definitely coming from the outside. Then about
> five minutes later, we heard the same sound. But this time, I was
> trying to read the mag compass which is mounted up high on the
> windshield. I'm pretty sure that I saw a flash of light this time near
> the top center of the windshield where the Plexiglas meets the
> aluminum . This made me wonder: When flying in IMC over an extended
> geographic area (Cheyenne to West of Denver) is it possible for the
> airplane to start discharging by arcing? Has anybody else seen
> something similar to this? I should mention that the winds were not
> especially fast that day (about 160 @ 30kts).

In the right atmospheric conditions, insulators like plexiglass can
build up a large static charge as air rushes past it. This is
basically an air-driven Van De Graff generator. The sound you were
hearing was most likely the charge arcing from the plexiglass to the
metal frame as it reached a high enough voltage to break down the air
and arc over.

[Bob Gardner]

Google Saint Elmo's Fire. It is a fairly common phenomenon.

Bob Gardner

"gman" > wrote in message
ups.com...
> Hi Folks,
> Here's an interesting phenomenon I observed recently and I was
> wondering if anybody else has seen anything like it.
>
> About two weekends ago, I was doing a cross country in actual (IMC)
> with one of my Instrument Rating students near Cheyenne, WY. We were
> at 11,000ft for a while and the temperature was near freezing but
> there was no precipitation in the area. All of a sudden, there was a
> sound like a pebble hitting the windshield. I explained to my student
> that it was either an isolated hail pebble or ice coming off the
> propeller. About ten minutes later, I heard the same sound but this
> time it sounded more like an electric discharge similar to the one you
> hear when you touch the door knob after walking on the carpet. I
> rechecked the fuses and the alternator but everything was working
> fine. The sound was definitely coming from the outside. Then about
> five minutes later, we heard the same sound. But this time, I was
> trying to read the mag compass which is mounted up high on the
> windshield. I'm pretty sure that I saw a flash of light this time near
> the top center of the windshield where the Plexiglas meets the
> aluminum . This made me wonder: When flying in IMC over an extended
> geographic area (Cheyenne to West of Denver) is it possible for the
> airplane to start discharging by arcing? Has anybody else seen
> something similar to this? I should mention that the winds were not
> especially fast that day (about 160 @ 30kts).
>

[BDS]

"gman" > wrote

> About two weekends ago, I was doing a cross country in actual (IMC)
> with one of my Instrument Rating students near Cheyenne, WY. We were
> at 11,000ft for a while and the temperature was near freezing but
> there was no precipitation in the area. All of a sudden, there was a
> sound like a pebble hitting the windshield. I explained to my student
> that it was either an isolated hail pebble or ice coming off the
> propeller. About ten minutes later, I heard the same sound but this
> time it sounded more like an electric discharge similar to the one you
> hear when you touch the door knob after walking on the carpet. I
> rechecked the fuses and the alternator but everything was working
> fine. The sound was definitely coming from the outside. Then about
> five minutes later, we heard the same sound. But this time, I was
> trying to read the mag compass which is mounted up high on the
> windshield. I'm pretty sure that I saw a flash of light this time near
> the top center of the windshield where the Plexiglas meets the
> aluminum . This made me wonder: When flying in IMC over an extended
> geographic area (Cheyenne to West of Denver) is it possible for the
> airplane to start discharging by arcing? Has anybody else seen
> something similar to this? I should mention that the winds were not
> especially fast that day (about 160 @ 30kts).

Had a similar experience while flying in the midwest during a pretty decent
snow storm.

It was a night flight and we were cruising along when suddenly the charts I
had placed up on the glareshield jumped up and stuck to the inside of the
windshield. I could feel the strong static charge when I took them off, and
there was also a nice discharge from my fingertips. For the next 30 minutes
or so you could put your fingertips up near the windshield and draw a
constant corona discharge from about 1/4-in away. I had never seen anything
like that before and have never seen it again since.

BDS

[BDS]

"Jim Macklin" > wrote

> Most slow airplanes do not have static discharge wicks, but
> in the correct conditions, they can build static which can
> build to very high voltages. It is also possible that the
> grounds [bonding] between components is not good and that
> can lead to arcing across hinges and between ailerons and
> wing, etc.
>
> Snow and ice crystals cause the build up, the fix is proper
> bonding and static wicks, even on a C172.

Bonding and static wicks will not prevent a static charge from building on
an insulator such as the windscreen itself, but they certainly will keep the
entire airframe at the same potential as you mention. In fact,
theoretically you could have a ground plate on one side of the windscreen
and still build a static charge on the other side of the windscreen under
the right conditions. That's the nature of static charges on insulators.

BDS

[[email protected][_1_]]

On 23 Mar 2007 14:54:07 -0700, wrote:

>In the right atmospheric conditions, insulators like plexiglass can
>build up a large static charge as air rushes past it. This is
>basically an air-driven Van De Graff generator. The sound you were
>hearing was most likely the charge arcing from the plexiglass to the
>metal frame as it reached a high enough voltage to break down the air
>and arc over.

Hawker 800's have plastic windscreens and a marginal ground path.

Think about cold, dry air at altitude.

They will indeed light you up if you aren't careful. Snap, Crackle and
Pop. BTW, you're usually standing on a stepladder when you get
poked...

TC

[Morgans[_2_]]

<toecutter> wrote
>
> Hawker 800's have plastic windscreens and a marginal ground path.
>
> Think about cold, dry air at altitude.
>
> They will indeed light you up if you aren't careful. Snap, Crackle and
> Pop. BTW, you're usually standing on a stepladder when you get
> poked...

You might need to explain that one, a little more.

What are you doing up around a windshield at altitude on a stepladder? <g>
--
Jim in NC

[Jim Macklin]

The term is capacitor. Most plastic has an anti-static
coating and plastic airplanes have conductive elements
built-in.

Franklin must have said it, the key to electricity is ,
ouch!



"BDS" > wrote in message
...
| "Jim Macklin" > wrote
|
| > Most slow airplanes do not have static discharge wicks,
but
| > in the correct conditions, they can build static which
can
| > build to very high voltages. It is also possible that
the
| > grounds [bonding] between components is not good and
that
| > can lead to arcing across hinges and between ailerons
and
| > wing, etc.
| >
| > Snow and ice crystals cause the build up, the fix is
proper
| > bonding and static wicks, even on a C172.
|
| Bonding and static wicks will not prevent a static charge
from building on
| an insulator such as the windscreen itself, but they
certainly will keep the
| entire airframe at the same potential as you mention. In
fact,
| theoretically you could have a ground plate on one side of
the windscreen
| and still build a static charge on the other side of the
windscreen under
| the right conditions. That's the nature of static charges
on insulators.
|
| BDS
|
|

[Morgans[_2_]]

"Jim Macklin" > wrote

> The term is capacitor. Most plastic has an anti-static
> coating and plastic airplanes have conductive elements
> built-in.
>
> Franklin must have said it, the key to electricity is ,
> ouch!

You know, the electricity is not what usually hurts me, though.

It is losing all of the skin off my knuckles, when I get jolted, and
involuntarily try to remove my hand from behind the panel, or from in ...
(insert your favorite location where your hand is when it gets shocked) far
too rapidly!

Same thing goes with being on a ladder and getting shocked. The shock will
not hurt nearly as much as falling off of the ten foot ladder you are
standing on, but it is hard to remember that, at the time it happens! <g>
--
Jim in NC

[TheSmokingGnu]

Morgans wrote:
> <toecutter> wrote
>> Hawker 800's have plastic windscreens and a marginal ground path.
>> BTW, you're usually standing on a stepladder when you get
>> poked...
>
> You might need to explain that one, a little more.
>
> What are you doing up around a windshield at altitude on a stepladder? <g>

Holding on for dear life! :D

TheSmokingGnu

[Jim Logajan]

"Morgans" > wrote:
> <toecutter> wrote
>>
>> Hawker 800's have plastic windscreens and a marginal ground path.
>>
>> Think about cold, dry air at altitude.
>>
>> They will indeed light you up if you aren't careful. Snap, Crackle
>> and Pop. BTW, you're usually standing on a stepladder when you get
>> poked...
>
> You might need to explain that one, a little more.
>
> What are you doing up around a windshield at altitude on a stepladder?
> <g>

He must be one of these people:

http://www.youtube.com/watch?v=wBm8I5JRNlw

;-)

[Bertie the Bunyip]

On Mar 23, 8:48 pm, "gman" > wrote:
> Hi Folks,
> Here's an interesting phenomenon I observed recently and I was
> wondering if anybody else has seen anything like it.
>
> About two weekends ago, I was doing a cross country in actual (IMC)
> with one of my Instrument Rating students near Cheyenne, WY. We were
> at 11,000ft for a while and the temperature was near freezing but
> there was no precipitation in the area. All of a sudden, there was a
> sound like a pebble hitting the windshield. I explained to my student
> that it was either an isolated hail pebble or ice coming off the
> propeller. About ten minutes later, I heard the same sound but this
> time it sounded more like an electric discharge similar to the one you
> hear when you touch the door knob after walking on the carpet. I
> rechecked the fuses and the alternator but everything was working
> fine. The sound was definitely coming from the outside. Then about
> five minutes later, we heard the same sound. But this time, I was
> trying to read the mag compass which is mounted up high on the
> windshield. I'm pretty sure that I saw a flash of light this time near
> the top center of the windshield where the Plexiglas meets the
> aluminum . This made me wonder: When flying in IMC over an extended
> geographic area (Cheyenne to West of Denver) is it possible for the
> airplane to start discharging by arcing? Has anybody else seen
> something similar to this? I should mention that the winds were not
> especially fast that day (about 160 @ 30kts).

Don't know what the wind would have to do with it, unless you were on
a boat and getting it.
St Elmo's fire , it's called, and it even built up on ships masts (I'm
sure it still does) but it's very common on airplanes. With a plexi
windshield, you can draw the littl elightning bolts into the cockpit
with the tip of a graphite pencil! But it's not supposed to be good
for your windshield, though! In jets we get it all the time. It can be
little lightning bolts across your windscreens (pencil trick doesn't
work through that much glass) and you can also get long fingers of a
dull plasma-like glow coming from the wipers and other bits or even a
long (maybe 20 foot long) cone of light built up on the nose,
depending on conditions. If there's CB around, you're quite likely to
get a lightning strike as well. I don't know if the fire makes you
more atractive to lightning or it's just co-incidental, but I've been
hit about a dozen times and had St Elmo along for a few minutes just
before we got smacked. Not to say it's common to get hit by lightning,
BTW. But it's unusual to go for more than a month's flying without
seeing St Elmo's fire at least once or twice in that period. Gotta be
wet, of course..!


Bertie