Posted for REHBINC@aol.com:
In a message dated 2/23/2007 1:56:40 PM Eastern Standard Time,
"Bryan Burr" wrote :
What you have to realize is that the Lancair ES that had the explosion
problem also was parked on top of a blue plastic tarp. There was
absolutely no contact with the ground and the tarp acted as an
additional insulator. While there are still risks associated with
static creating activities if you make efforts to create a grounding
event such as touching the airplane, connecting the airplane to a ground
when fueling, wiping it with a damp rag frequently when waxing, buffing,
polishing, the risk is minimized. Also, the explosive event happened by
blowing the wing off using an air nozzle over an open fuel filler inlet.
And the tank had fuel or fuel vapors escaping. All of the factors lead
up to the explosive event. Blue tarp, long period of sanding, blowing
off the wing using an air nozzle, open fuel port, no frequent attempt to
discharge the static.
As you stated below, the ES structure is non conductive. Parking it on a
plastic tarp would therefor have virtually no effect on static discharge from
the
wing surfaces. I have no knowledge of this event beyond what I have read
here,
however I will go out on a limb and suggest that the tank had probably been
emptied. Otherwise, the result should have been a small fire at the mixing
zone
surrounding the open fuel filler.
Touching a plastic, or other "nonconductive", surface will only equalize the
surface charge of the area you touch. It will have negligible effect on the
surrounding area. Actually, it is difficult to ignite a flammable mixture
with a
spark discharge from a nonconductive plastic as the capacitance of the small
area discharged typically doesn't store enough energy to light the fire (this
is why plastic gasoline containers are safe). I would also suggest that the
likely point of ignition in the above accident was from the metal ring at the
fuel filler.
During the painting of my airplane (I had flown 250 hours so fuel had
been introduced to the system) I made absolutely sure the painter
touched the wings frequently. I made sure he hosed down the floor and
used water and wet sanded during the sanding process. I made sure
during polishing that he did not go for long periods of using an
electric polisher/buffer. We always had safety wire or something
touching the ground and the airplane surface he was working on.
As long as the fuel caps are closed, and I would think you would want them
that way to keep dust out, I don't see a hazard here. Not that it hurts
anything
for it to be grounded.
During fueling I make sure the grounding cable is attached either to the
exhaust or to the tow bar plate and I also make sure that the refueling
nozzle is contacting the metal ring that seats the fuel cap. Refueling
trucks that use certified aircraft refueling hose has a ground wire
embedded in the rubber hose that connects to the nozzle.. By touching
the nozzle to the fuel cap seating ring another grounding is being made
in addition to the regular static grounding cable.
The main area of concern regarding electrostatics during refueling is the
flow of nonconductive gasoline against the fuel hose wall and the break up of
fuel droplets upon discharge into the tank. A metal tank filled with
electrically
charged fuel creates a very effective capacitor that can quickly discharge a
large amount of energy. In a metal airplane, as with your car, the ground
wire
in the fuel hose completes a circuit between your vehicle, the hose, the pump
and ground, so long as the nozzle remains in direct contact with the filler.
This prevents any difference in electrical potential from developing between
these components and thereby eliminates an ignition hazard. (so long as the
wire remains intact) As I understand it, the separate ground cable is there
to
ensure that the aircraft is electrically discharged at a point remote from
any
fuel vapors. This prevents the grounded nozzle from arcing to a charged
filler
ring when the nozzle is inserted into the fuel tank. The cable also serves as
a backup in the event that the wire in the refueling hose breaks.
In a nonconductive aircraft, I would suggest that the ground cable be touched
to the filler ring prior to opening the cap. The filler ring, truck, hose and
nozzle should then all be at the same electrical potential and no spark will
be generated when the nozzle is inserted. Of course you could touch the
nozzle
to the ring before the cap is removed and achieve the same result so long as
the hose wire isn't broken. Grounding to the exhaust or tow bar doesn't seem
terribly useful to me as those areas are not electrically connected to
components that are likely to provide an electric spark in the presence of
flammable
fuel vapors on a nonconductive aircraft.
Remember that in non carbon fiber airplanes (IE Lancair normal ES) there
is no conductivity. E glass does not conduct. So there is no way to
fully discharge static. That is why static wicks don't work on an ES.
They only discharge the static created by the wick itself. They do
nothing for the wing surfaces. Perhaps there is another topic of
discussion for using metal based primer during painting and then
installing static wicks.
You may say that the rubber tires touching the ground are also an
insulator. This is true. When I asked the same question the answer was
that there is enough dirt, grime, moisture etc. on the tires that static
is discharged. This is also why using a grounding cable when refueling
is necessary.
I think you asked the wrong person. If the tires were squeaky clean they
would still conduct enough to prevent hazardous buildup of electrostatic
charge
from virtually any source. The reason is the carbon black used to make them
black colored. Carbon is conductive.
When you hear of people setting themselves on fire at the auto gas station
from electrostatic charge obtained when sliding off of a vinyl seat, they are
nearly always wearing white or very light colored shoe soles. Black soles
will
typically safely discharge your body to the same earth the vehicle is parked
on
before you reach the filler. With white plastic soles and a dry day, maybe
not.
I also prop my tow bar against the wing being refueled as
added measure of safety.
Do you also prop your tire iron against your plastic fuel can when you buy
gas for your lawn mower? Why one and not the other?
Rob
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