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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."
...have been using similar technology for some time.
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...
Those who live by the sword, get shot by those who live by the gun...
>>Running a live wire into a passenger jet's fuel tank seems like a bad idea
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.)
Why bother with electricity at all. A piece of fiber to an optical encoder would do the job just fine. I can't think of any sensor that couldn't be implemented optically.
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 In other words, the problem was solved long ago.
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.
Millivolts. Most level sensors are variable resistors, so you only need to exceed the forward min. bias of the resistor (see the spec. sheet) to have accurate results. Above that, it's just a matter of calibration and maintaining a well-regulated power supply.
Good point. Note that electronic sensors are also used in underground (and above-ground) storage tanks. Electric turbine pumps, too.
I am not a crackpot.
How does a purely mechanical device transmit the information back to the pilot/cockpit?
It can be done, but doing it electronically is much easier and lighter.
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.
It's not wasting time, I'm educating myself.
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.
Tank probes are capacitive and use a very weak signal for excitation. The spec is 25uJ maximum which is WELL under the energy required to ignite fuel. Typical systems use way less energy to make measurements. The problem is more that wiring for OTHER more power consumptive things is routed through the tanks in some designs. Also I agree, optical isn't any better or worse of a method.
Unless they have changed something very recently (in the last couple of years), the guage controlling unit is inside the tank, wires and all. The only thing outside is the plug to go into the wiring harness. I've changed plenty of sending units.
The wires for your electric fuel pump are inside the tank too.
Gone!
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.
You're going to overestimate your remaining fuel if you are relying on a capillary. Unless, of course, your ocular spectrogram can automatically correct for the capillary rise.
-- Cave quid dicis, quando, et cui
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 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.
http://en.wikipedia.org/wiki/Category:Fuel_exhaustion_on_commercial_airliners
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
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.
Battery monitors are trying to do an incredibly tricky job. For all practical battery chemistries, the fully-charged voltage is only a tiny fraction more than the to-all-intents-and-purposes-spent voltage.
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.
Je fume. Tu fumes. Nous fûmes!
Biodiesel has a very high flash point... Definition of "flammable" being: "easily set on fire" (Oxford American Dictionary), Biodiesel fails that definition, thus meriting the "non-flammable" designation.
Gimli Glider.
Flight 800?
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.
I think you are getting confused with TWA Flight 800 which went down in 1996. It is thought that some electrical fault created a spark in the centre fuel tank of the aircraft (which was warm and almost empty) causing an explosion. It is likely that due to the age of the aircraft that the wiring was degraded (or had been altered in an unauthorised manner) resulting in ignition taking place.
http://en.wikipedia.org/wiki/TWA_Flight_800
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 http://en.wikipedia.org/wiki/TWA_Flight_800