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TerryWighs

Well Known Member
Ordering wing kit...any threads/thoughts on advangtages of type of fuel sending units to choose??? Thanks
 
* From what I hear (completely uninformed opinion here) the capacitance senders sometimes have problems with alternative fuels, but they're more accurate through the entire range.

* The float senders aren't accurate for the first few gallons because of the dihedral of the wings. This is why the gauges Van's sells are the same for the 4, 6, 7 and 8, even though the 7 and 8 have more fuel capacity. The Van's gauges only indicate the remaining 15 gallons. I'm not sure how much it indicates in reality as I haven't calibrated my fuel gauges (EMS D-10) yet, but it seems like you could indicate a little more out of the 21 gallons available in the 7 and 8.

* The capacitance requires you to run some wires through the tank. If you have maintenance issue with the senders you're basically screwed. You're gonna have to cut access holes in the rear baffle.

* Capacitance system has no moving parts

* Capacitance is (IMHO) probably the better solution if you're using the aerobatic mods (uneccessary unless you're installing inverted oil). If you use floats + flop tubes, the floats have to be located to the second bay meaning they won't indicate as accurately when low of fuel...but you really shouldn't be cutting it that close anyway.

As for me, I went with the floats. Easier installation IMHO and if you ever have a problem, it's just a matter of removing about 6 or 7 screws and the sender is out. From my bench testing they are quite precise throughout their range.
 
If you use the Search function here on VAF, I seem to recall that there were a long thread or two specifically on this subject already.
 
capacitive

Not claiming which is better, or even which is better for you. Here is what I find with my airplane which has capacitive plates installed in the wings and a EI gauge. The readings are extremely accurate if you take the time to set everything up in the beginning. If we land for fuel and the gauge says that we have 9 gallons in the left, and 7.5 in the right I know that we have 16.5 gallons total, which the guage also tells me, and with a capacity of 36 gallons I can program the pump after swiping my card for 20 gallons and it will put both tanks exactly to the bottom of the filler necks. There are 2 ranges however where the plates simply cannot measure properly and that is until the fuel is burned down to the top of the plates. In other words, any fuel above the top of the plates will not measure and the guage will say both tanks are full when they are actually down slightly. This is also true at the extreme opposite when the tanks are empty. Everything in between you can count on to be dead nuts.

FWIW, the plates in the tank are the shape of the ribs, but about 1/2 inch shy of touching the skin all around the inside of the tank. These are mounted in such a way to isolate them from the airframe electrically. Very, very accurate as long as the fuel is between the top and bottom of the plates.

Regards,
 
Just as an update: I spoke to GRT about the prebent Princetone probes (I wanted to use those instead of Van's). They indicated that they would be a problem with flop tube due to the tube banging around in there, so I'm back to Van's units.

edit: I should mention that this wouldn't be a problem if you're not using flop tubes. I like hanging upside down so flop tubes for me.
 
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TerryWighs said:
Ordering wing kit...any threads/thoughts on advangtages of type of fuel sending units to choose??? Thanks
Click to expand...

In 1996 TWA flight 800 (a 747) out of New York exploded with the loss
of all on board. The accident investigation was the most expensive
investigation in the history of aviation. The NTSB concluded that the
most likely cause was a spark in a central fuel tank arising from a
capacitance type Fuel Quantity Indicator System (FQIS). Subsequent
research by other parties has suggested that four other airliners may
have previously suffered the same fate.

Electrical problems aside, I wouldn't personally trust any fuel tank sender to
provide accurate fuel level readings so I'm installing the float type
but also installing a fuel flow transducer which should be accurate to
a few litres if correctly installed.

The beauty of a transducer coupled to a decent fuel computer is that
regardless of changes in power settings enroute it will provide not
only fuel remaining but time remaining. This can then be checked
against any GPS ETE output to instantly provide reserve endurance in
minutes at the destination. Hard to beat that for accurate fuel
management.
 
Floats and Fuel Flow!

I agree with Bob! I have float-type sensors, but never pay much attention to them, since my totalizer (fed off the flow transducer) is acurate to within a gallon in a tank full. Isn't technology grand? The only other aircraft fuel gauging system I've every flown that was as reliable was the cork float and rod in my old J-3..... :rolleyes:

Paul
 
I find it interesting how many RVs are being built with fuel flow/totalizers. When I completed my -6 in '93, fuel flow indicators were almost unheard of in small airplanes. I installed one more as a conversation piece than anything. After getting used to it, I wouldn't have an airplane without one. It makes setting the mixture faster and I know my remaining fuel within a 1/2 gallon. Of course I still reference my gauges (float type) because the fuel totalizer does not recognize a leak. It only knows what goes through the transducer.
Mel...DAR
 
Bob Barrow said:
In 1996 TWA flight 800 (a 747) out of New York exploded with the loss of all on board. The accident investigation was the most expensive investigation in the history of aviation. The NTSB concluded that the most likely cause was a spark in a central fuel tank arising from a
capacitance type Fuel Quantity Indicator System (FQIS). Subsequent
research by other parties has suggested that four other airliners may
have previously suffered the same fate.
Click to expand...

Aside from the obvious differences between a 747 and an RV, the fact is that AVGAS in a fuel tank is nearly impossible to ignite because the mixture is just WAY too rich at normal temperatures. Jet A, on the other hand, will ignite very happily at normal, everyday temperatures. They have absolutely NOTHING at all in common.

Floats, caps....use what you want for whatever reason you want, but TWA 800 is absolutely irrelavant.

Good point about the flow meter, though. Definately one going into my plane.
 
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"the fact is that AVGAS in a fuel tank is nearly impossible to ignite because the mixture is just WAY too rich at normal temperatures".

True of the liquid in the tanks, I wouldnt bet on the concentrations of the gas phase above the liquid or around the filler tube when opened under all temperature variations seen.
 
Actually, I was talking about the fuel/air mixture of the gas, not the liquid. It's just too rich to burn. You're right, this isn't always true of all conditions but it's true in general and doesn't even take into account that the energy going into the capacitive plates themselves is tiny. My only point is that there are lots of reasons to use floats over capacitive gauges but safety is just not one of them.
 
Yeah, Cessna uses them in their big twins and stuff, if they weren't safe, the FAA wouldn't have bought off on that. The electrical problems with TWA were not in the senders themselves, but in the wires going TO the senders. Big difference. And from my understanding JetA is harder to ignite than AVGAS, but either will. There are reasons why we can't use electric screwdrivers to open the wing tanks up in a wet wing... a Mechanic at POC found that out the hard way when he killed himself and splayed the wet wing of a cessna 421C open because his Makita ignited fuel vapors in the tank. Granted the tank had been drained, so the vapors were probably at a more igniteable mixture.
 
Twa800

The TWA800 tanks were also heated well above the ambient temperature due to proximity to the air conditioners. If you're really worried about flamible fuel tanks, all of the TWA800 info is here.

As for Cap vs Res senders. I installed capacitive mostly because I'm a geek and I think they are cool. Bob isdead on in that senders are nearly worthless other than to indicate if you have a leak. A flow meter is the way to go.
 
As a matter of fact, I will be using flow meter/totalizer as my only gauge in my bi-plane. I'm not concerned about a leak going undetected since the tanks are in the top wing just above and forward of my head. And with just 14 gallons total, I WILL visually check before each flight.
Mel...DAR
 
That is interesting about the 747 explosion(s). I happened to be flying 747's at that time and the big deal then, was all about running the main tanks dry with the pumps left on. The speculation was that the pumps, running dry, and left on, got hot and ignited the fuel mixture in tanks. A procedure was developed to preclude this happening by shutting these pumps off with about 400 lbs still in tanks. (400 LBS is chump change for a 747) Of course, it often happened that we would look up and find the pressure lights flickering for the pumps and zero fuel remaining in tanks. Since these were a/c pumps, they would not overspeed when run dry so we were all skeptical at the time.......

One thing I will say about capacitive plates, is that if you ever get any amount of water in your tanks, it throws the measuring way out of line. This was common in the 737 and caused no end of head scratching. Of course, in an RV, if you have that much water you have more problems than the fuel quantity gauge indication. Somehow, when the water gets in between the plates it raises hob electrically with the sensing.

BTW, I used to drive a fuel truck for Shell while waiting to get my first flying job. As an aircraft refueler, we would be shown movies of the inside of a fuel tank being refueled. I was astounded at the amount of static electricity being discharged inside the fuel tank while refueling was in progress. It seemed there was small shots of lightning firing every second. The presence of water in the fuel aggravated this discharge. The point being however, is that there are sparks inside the fuel tanks when refueling is in progress. Today there are additives to somewhat alleviate this phenomena.

Cheers, Pete
 
Overheating fuel pumps were an issue in Boeing's military airplanes...there were a couple of inflight explosions in the 80s and early 90s that were attributed to that.

About the safety of capacitive senders, they've been in transport-category airplanes for forty-plus years. I've seen the capacitive-probe FQISs in other Boeing airplanes get abused with lots of electromagnetic energy. Not a recommended thing to do, but there were no explosions...otherwise I wouldn't be writing this! :D

As far as my RV-7's tanks are concerned, I'll have no hesitation in using them.

Cheers,

Dave
 
RV7 with capacitive fuel sensors. I have seen that Princeton electronics and Blue Mountain supply capacity to voltage converters but cannot find either on the net. Anybody help with an address or email?

Ian Beadle Western Australia
 
One thing not mentioned here is adequate planning. I do have float senders with analog gauges, and it's just a reference, and not very accurate. I have fuel flow which I am calibrating now (mainly for leaning and engine diagnosis), but the thing that blows me away is that people get wrapped around the type of fuel senders they are using. Go with the float senders and save bucks, etc. Dipstick your tank or fill up everytime you fly. You should KNOW the fuel burn of your airplane, regardless of senders, gadgets, etc. If your plane at the most burns 10 GPH, and you know you have 36 gallons, then you have 3.6 hours of fuel worst case, or 3 hours of fuel and a reserve. Plan your flight to include stops before you get to your reserve (Don't forget the winds!) and you'll always be ok. Unless you have a leak of course, but how many times have you had a *significant* fuel leak? Build the tanks right, the hoses, the fittings, etc and you will not have a fuel leak.

Bottom line, is plan your fuel, don't depend on gauges and gadgets.

Off the soapbox now.
 
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