Tuesday, May 25, 2010

Bonding straps and safety wire

Today, we did something that scared me a little - we worked on gascolators, which are essentially fuel filters. To work on a gascolator, one must first shut off the fuel supply and the crossfeed between fuel tanks, then drain the remaining avgas out of the gascolators and the lines this side of the shut-off valve.

Did you ever notice those "NO SMOKING" signs by the gas pumps, and "no cell phones"? It's not because the for-your-own-good busybodies got there first. Fuel fumes, in enough concentration, are explosive - and any ignition source is enough to set them off. The reason the public can usually get away with yakking on their cell phones, getting into and out of their car, and wearing clothes full of static electricity, is that the gasoline pumps are bonded to your car to reduce the chance of explosion ignition via static electricity buildup. (There's a wire in that hose). When fueling the airplane at the pump, we run a separate static line from the pump to the airplane to bond the two, and minimize any chance of explosion.

On the other hand, sometimes we forget. And sometimes we use plastic gas cans, which distribute the charge on the periphery of the can and have no way to be adequately bonded. And every couple years, someone's plane burns to ashes because of this. Well, if the static potential is bad going into the plane, it's just as bad coming out - and liquid moving through air, especially dry cold air, will build a potential. We've lost a few planes up here from draining fuel from the gascolator into a bucket.

The search for the bonding strap from plane to bucket made the younger mechanic annoyed and impatient, while the much older and more experienced mechanic smiled into his work with a pleased expression. I would not have been surprised, as I rolled under the plane on the creeper, to hear "You are beginning to learn, young grasshopper."

Did all go well? Actually, no. Something is wrong with the fuel system on this plane, and instead of dumping roughly a cup of gas before dripping dry, a steady stream of gasoline just kept pouring out. The young mechanic climbed up to the cockpit to check the shutoff valves and recheck the controls - and still, it kept pouring out. Not content with the bonding strap, I touched metal to the plane's aluminum skin, and called out "Still pouring out. Steady stream. Still pouring. Oh, shut off for a moment - back to pouring."

You might scoff at me as overcautious, but I was the person on a creeper, unable to get quickly away if the gasoline detonated. I prefer caution.

After we shut off the fuel drain on the gascolator and noted the fuel system upstream as definitely broken but not subject to easy diagnosis (the more experienced mechanics will check it tomorrow), we moved to the other gascolator on the plane - and this time, the young mechanic did not look hurried or impatient as I secured the bonding strap carefully from sloshing bucket to airplane, and waited a moment for any pent up electrons to migrate before cautiously reaching for the pliers to cut the safety wire holding the nut in place.

I'm only beginning to learn, though. I still can't change the oil in a Cessna 180 without leaving a puddle on the ramp to mark my activity. Sigh. More experience, more practice - and right now, more cleanup. Well, his plane had a dirty belly anyway, and the customer will be just as happy to get a cleaner airplane back than he left at the shop.

1 comment:

Old NFO said...

Smart of you... And excellent reasoning too!