Electricity Over Glass 187
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."
Light Sensors in cameras... (Score:2, Informative)
however, there is a problem with what is called dark current. that is when there is no light hitting the transducer, and there is still a current being developed...
Re:Is this needed? (Score:3, Informative)
Why? it's extremely difficult to ignite liquid gasoline, or jet fuel. An air-fuel mix ignites quite easily, however. So moral of the story: if you're paranoid that wires in your fuel tank are freyed, keep your fuel tank full. Or get your crappy car fixed.
(In fact, nearly every automobile built in the past 20 years has not one, but two powered devices in the fuel tank -- a fuel pump and a level sensor.)
They could go farther (Score:2, Informative)
Having said the above, the product seems like a solution in search of a problem. I can't recall any incidents where a fire or explosion was caused in an airplane because of faulty wiring in the fuel tank. There are lots of places where an electrical spark could cause an explosion. For instance in a mine, or factory, dust explosions are an ever present danger. To deal with that, we have explosion proof wiring. http://en.wikipedia.org/wiki/Electric_actuator#Explosion_protection [wikipedia.org] In other words, the problem was solved long ago.
Re:Is this needed? (Score:4, Informative)
Could run off a watch battery for months (Score:4, Informative)
Re:Intrinsic Safety. (Score:3, Informative)
Good point. Note that electronic sensors are also used in underground (and above-ground) storage tanks. Electric turbine pumps, too.
Re:They could go farther (Score:1, Informative)
I can. [wikipedia.org]
Re:what exactly are they sensing? (Score:1, Informative)
It can be done, but doing it electronically is much easier and lighter.
The issue is power limiting (Score:3, Informative)
It's a lot easier to ensure the power is properly limited. Running a sensor is a low power application (you wouldn't be using a "steel cutting" laser), and the power is limited with the size of the laser diode. There's no other way to get power through the line.
With electric lines, the issue is whether the wire to the sensor is going to short to another wire somewhere else in the wiring harness that will accidently put a lot more power on the line. There are a TON of wires on an aircraft, going every which way, some of which can deliver a lot of power. Short one of those to the sensor line and you can get a spark in the fuel tank.
Re:Intrinsic Safety. (Score:5, Informative)
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 [epa.gov]
http://easa.europa.eu/doc/Events/fueltanksafety_24062005/easa_fueltanksafety_24062005_large_transport_ppt.pdf [europa.eu] [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.
NOT the issue! (Score:1, Informative)
Re:Light Sensors in cameras... (Score:4, Informative)
Re:Is this needed? (Score:4, Informative)
The wires for your electric fuel pump are inside the tank too.
Avionics Tech Saves /. (Score:5, Informative)
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.
Re:I've already invented such a gadget (Score:2, Informative)
Re:Is this needed? (Score:3, Informative)
Aircraft are required to operate at various altitudes (which have various temperatures and pressures) making compensating for differences in pressures and temperatures in a system that requires vacuum lines more difficult (and more difficult to maintain and keep calibrated). Early aircraft had a sight glass on the outside of the tank, but these are only good for reading volume and at a specific aTTitude (i.e on the ground) intrinsic safety [omega.com] is a well understood practice within electrical engineering and has proven to be extremely safe and reliable when proper maintenance and operational maintenance procedures are used.
Modern aircraft fuel quantity measurement is through it's capacitance, as this compensates for temperature / volume, when it is the 'mass' (and hence energy) of the fuel decides which just how far you will fly. You are only interested in the mass of the fuel.
I don't recall this happening very often. Last one i remember was the center tank on an airliner that they suspected had developed a fault, and also had NO fuel - (blamed the vapor) but IIRC the fault being pinned on the fuel measurement system was not conclusive... I think they looked more closely at the fuel pump which normally sits submersed in the fuel with the electrics outside the tank. Run a pump dry and it gets hot. Heat + Oxygen + ignition source (vapor) = boom.
Uh Oh ?! (Score:5, Informative)
Re:Sounds like a bad idea. (Score:3, Informative)
Most of the wing is the tank, but not all of it. There is room behind the 'leading edge' and the trailing edge (between the aft of the tank and the front of the flaps/ailerons. ) This is where other services go, such as air ducts for the leading edge De-Icing (heating) systems, and wires that run to those little navigation lights way out there on the wingtips. Not to mention all the wires to and from the wheelwell (undercarage).
Not a good picture - but it shows what I mean: here [bst-tsb.gc.ca]
Most jets do NOT burn JP-4 (Score:5, Informative)
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.
Re:Fuel Gauges (Score:3, Informative)
There are battery charging monitors that integrate the current over time to get an idea of how many amp-hours are remaining, but even these don't account for the tendency of most battery chemistries to self-discharge.
Re:Could run off a watch battery for months (Score:2, Informative)
Thus, such a system would be extremely cost prohibitive for next to no safety gains.
Re:Is this needed? (Score:2, Informative)
just 2 words: (Score:5, Informative)
Re:What about capacitance fuel sensors? (Score:3, Informative)
Flight 800? [wikipedia.org]
The NTSB investigation ended with the adoption of their final report on August 23, 2000. In it they concluded that the probable cause of the accident was "an explosion of the center wing fuel tank (CWT), resulting from ignition of the flammable fuel/air mixture in the tank. The source of ignition energy for the explosion could not be determined with certainty, but, of the sources evaluated by the investigation, the most likely was a short circuit outside of the CWT that allowed excessive voltage to enter it through electrical wiring associated with the fuel quantity indication system.
Re:Is this needed? (Score:2, Informative)
http://en.wikipedia.org/wiki/TWA_Flight_800 [wikipedia.org]
The flight that went down November 2001 was AA Flight 587. It suffered mechanical failure of the rudder.
http://en.wikipedia.org/wiki/American_Airlines_Flight_587 [wikipedia.org] http://en.wikipedia.org/wiki/TWA_Flight_800 [wikipedia.org]