June 1, 2012
By Rod Machado
Our story begins in Tulsa, Oklahoma, on a sunny spring morning. The pilot of a Beech Baron prepared to depart on a cargo flight at 9:30 a.m. After starting his engines, he did something each of us does on most flights. He reached for his radio switch and turned it on. This time, however, his right wing exploded. Kaboom!
Care to guess what happened? Wrong frequency, maybe? A right-wing conspiracy?
Investigators found fuel stains along the inside and the outside of the lower wing skin. Their report said, “The aux tank vent and the anti-siphon line had become disconnected where it joined the vent line. All the hose clamps on the vent system lines and vapor return line were found loose.”
While dihedral aids in lateral stability, it also aids in pooling leaked fuel near the wing’s root. Most likely our pilot, “Kaboom Man,” smelled fuel the moment he entered the cockpit. Most of us who’ve flown older airplanes that have seen action in the flight training and cargo theaters know it’s not uncommon to sniff a whiff of fuel vapor after the machine sits overnight. But this airplane must have had a terrible case of the vapors.
Kaboom Man survived, and I’m sure some mechanic placed a placard on his newly wounded Baron that read, “Limited to right traffic only.” There is a valuable lesson to be learned here, and “Don’t turn on the radio” isn’t it. This teachable (and combustible) moment suggests that if you smell fuel in a closed environment, don’t provide it with an ignition source.
Most likely, rotating the radio switch created a spark between its metallic contacts, or a spark somewhere along the radio’s circuitry, that ignited avgas fumes. In a closed environment where a proper mixture and density of fuel and air exist, it only takes a tiny spark to ignite the mixture. With the exception of spark size, the process is exactly what happens inside your engine’s cylinders. Atomized fuel is set upon by a plug that sparks, and combustion results. Unfortunately, the kaboom plume doesn’t know it’s not inside a cylinder, and there are lots of other things in lots of other places can spark an explosion.
Static electricity can certainly be an ignition source. Every time I pull my socks out of the dryer, I know our house shows up on some pilot’s Stormscope. Then there’s the YouTube video of a young lady who is fueling her car at a filling station, then reenters and exits her car (building up a static charge on her clothing). Without grounding herself to the car’s frame first, she touches the pump handle, ignites the escaping fuel vapors, and adds new meaning to the term, “Warming up the car.” Gasoline fumes have a very low minimum ignition energy. It’s an energy requirement easily met by static electricity or an electrical contact spark.
Fortunately, everyone has a fuel vapor detection device attached to the front of his head. It’s your nose. While I might be willing to tolerate a tiny “hint” of fuel vapor in older, fabric-type airplanes, it’s definitely unwise to enter (or remain in) an airplane hosting strong avgas fumes—if there’s a fuel smell, run like hell.
Separated or broken fuel lines or a ruptured fuel bladder or tank from an accident can be another inflammatory opportunity. Several years ago a fellow we’ll call Tom arrived at an airplane accident scene after the pilot stumbled away from the wreckage. Hearing that the master switch was left on, Tom entered a cockpit that reeked of fuel vapor and turned off the master switch.
The 64,000-peso question is: Good move or bad move? Peso for your thoughts? Bad move, unless your goal is to become a hot Tom-ale.
Without thinking about it, Tom was playing a high- risk game of Donkey Bomb and betting all his body’s molecules on the outcome. Given the strong plume of fume, the only likely payoff on this maneuver would be from a life insurance policy. Without anyone in the airplane to rescue, it’s much wiser to move away from the wreckage and phone those for whom fire is in their job description. Let the firefighters handle the problem.
If a master switch is to be turned off, the pilot should do it before he crashes, if possible, or immediately after the crash, if able. Returning to an empty cockpit in the presence of strong fuel vapors just to turn off the master switch is to risk sending yourself into a low Earth orbit. If you need some space, that isn’t the way to get it.
Combustible fuel vapors can be deadly and deserve our respect. That’s why my grandfather would never enter a house hosting gas-burning appliances and flick the light switch without first sniffing for gas vapor (suppliers add a noxious scent to natural gas for your protection). When I asked him why he did this, he replied, “Because if there is a gas leak, then the lights will come on real bright all at once, then they’ll go out for a long, long time.”
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Aircraft Power and Fuel,
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AOPA thanks our members for their continued support in protecting the freedom to fly.