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Electricity Over Glass
guddan writes "Running a live wire into a passenger jet's fuel tank seems like a bad idea on the face of it. Still, sensors that monitor the fuel tank have to run on electricity, so aircraft makers previously had little choice. But what if power could be delivered over optical fiber instead of copper wire, without fear of short circuits and sparks? In late May, the big laser and optics company JDS Uniphase Corp., in San Jose, Calif., bought a small Silicon Valley firm with the technology to do just that."
I've replaced the gauge on a mid-eighties Buick a number of times and I can tell you live wires go into the gas tank. The transducer was a one-piece unit. Did you ever consider there is more than one way to design something? Your point, therefore, is invalid.
Intrinsically safe circuits can ignite gasoline when they are hit by lightning. The concern in aircraft applications isn't that the fuel ignites in normal operation. Rather, it is suspected that some airplanes have exploded after being hit by lightning.
If enough power hits just the right wire, and the tanks are near empty (with lots of explosive fuel vapors), and enough planes get hit by lightning in flight in a sensitive location, then potentially disaster can happen. The accident data says fuel tank explosions occur, and this might be a possible cause. Safety problems demand a precautionary approach. Hence the desire to eliminate the wire going to the fuel tank.
Further resources:
http://www.epa.gov/fedrgstr/EPA-GENERAL/1997/April/Day-03/g8495.htm
http://easa.europa.eu/doc/Events/fueltanksafety_24062005/easa_fueltanksafety_24062005_large_transport_ppt.pdf [pdf]
Note: a widespread consensus exists that many possible ways for fuel tanks to ignite exist. As such, most of the focus is on minimizing the likelihood of ignition, rather than one specific cause, like the fuel tank wires themselves.
Speaking as a former USAF Avionics Specialist, who worked on C-5's, C-141's, and C-130's, and who personally saw a parked C-141 burst into flames on the ramp because of a fuel probe maintenance accident, let me explain things simply.
Design considerations:
JP4, the fuel that makes most jets run, is difficult to ignite. It needs a heat source. You could run a bare wire into a full tank and not have a problem. However, heat that wire up, and get the fuel/air mixture just right, and you have a problem. Big Boomba Problem, to quote JJB.
The big problem is the mostly empty tank and exposed heat sources. The C-5 has a nitrogen purging system. Basically, as fuel empties from a tank, it is replaced by nitrogen. The only way that wing is going to explode is if something other than a bare wire acts on it. Then, you've got bigger problems.
The big problem comes when you open the tank for maintenance. So, there are massive safety considerations. The C-141 that exploded in the mid-90's at Travis AFB in California blew because a jackass tech did not follow lockout/tag out procedures. The 141 doesn't have the nitrogen purge, but the tanks were open anyway. Two senior specialists were standing on top of the aircraft when the wing blew. Several others were in the cargo box. Luckily, aside from bumped elbows and bruised body parts, everyone got out o.k. We towed nearby aircraft to safer distances. There was precious little left of the burnt aircraft that identified it as such.
Most amatuers could make a good guess at a practical design for fuel sensors, but most of the solutions developed as such will end up being to costly, too heavy, will introduce other problems such as high maint., or simply won't work in 3-d, or extreme temperatures.
Politics is the art of looking for trouble, finding it everywhere, diagnosing it incorrectly and applying the wrong fix.
http://en.wikipedia.org/wiki/Category:Fuel_exhaustion_on_commercial_airliners
Most jets (the largest quantity number of them, civilian commercial and private aircraft including everything from jetliners to small turboprops) burn Jet-A, which is a completely different formulation from the old JP-4. JP-4 had a significant amount of lighter molecular weight hydrocarbons (e.g. more of the constituents of gasoline) blended in.
JP-4 was also phased out of use by the USAF over ten years ago. JP-8 is used now, which is a completely different formulation from JP-4 and has much higher flash point than JP-4. JP-4 was a naptha-based fuel and JP-8 is a kerosene-based fuel. Today's Jet-A and JP-8 have very similar base formulations, but they have very different additive packages blended in. JP-8 has a much higher flash point than Jet-A too, since it is tailored for use in military aircraft that need to handle supersonic operations.
Gimli Glider.