Low fuel warning light

[fallowdeer]

Hi all

 

Over the mountains the other day at 7000ft and the low fuel light came on.

 

Immediately could see I still had what fuel I would expect on board by looking at the sight tube in the wing root, and knowing that both tanks feed individually into the 6l feeder tank I wasn't too worried, but still stayed high till I was near the field ten minutes later.

 

The light has been going on and off intermittently since (haven't been back up) and I can see that the feeder tank is totally full so it would seem it is a sender fault.

 

Not having built the plane I'm not familiar with the insides so just looking to find out what to expect before I drain everything and pull it to pieces.

 

Any advice/knowledge welcome.

 

Thanks

 

Peter

 

 

[facthunter]

Sight tube in a wing root is a class 1, reliable fuel gauge. No indicator comes near it. The light only increases anxiety unnecessarily in those circumstances. Nev

 

 

[rankamateur]

The light is a bit more use than that Nev, If your breather tubes are blocked or kinked you could have fuel in your sight tube that is no use at all because it is not getting to the feeder tank to feed the engine.

 

 

[facthunter]

The sky could fall in too. You would have to convince me, somewhat. I've had too many false warnings. Anytime you have a question about fuel quantity with a sight tube you rock the wings (without balancing with rudder) slowly, and check the result. Nev

 

 

[SDQDI]

But you have to remember nev that this fuel warning light is working on the header tank and if the fuel is blocked from getting to it then you could run out of fuel to the engine while having plenty in the wings.

 

It seems that this isn't the problem though as the original poster said his header tank was still full so more likely to be a sensor /sender problem.

 

 

[facthunter]

The fuel system should be designed properly. There should be NO chance of kinking a line. Where it might happen there should be some casing to guide it. It's THAT important. Nev

 

 

[SDQDI]

I agree nev but people have taps, filters etc it doesn't have to be a kink to block a tank outlet. I even saw an auto shutoff nozzle that had lumps of Teflon quarter inch in diameter coming out of it (it was at an car service centre) if something like that got in it could stop the flow.

 

 

[Scotty 1]

Mistakes can be made Nev, and our Hornets and the Savs have a header tank with a low level warning light . The fuel is feed to the header tank from whichever wing tank( or neither if you forget to turn one on) and then onto the engine. I can remember two incidents, one were the PIC when changing tanks forgot to turn the second tank on and the second incident was when a fuel tank cap was left off and the fuel was sucked out. Simple mistakes. But you must admit another warning system can't be bad. With the AAK range the fuel can been seen (or not)running through glass fuel filters when you are warned to a system problem with the header tank low fuel warning light. You then have the time (with mine 15 minutes or 5 ltrs ) to rectify a mistake or find a landing place. Yes the low fuel warning light might be faulty but at least it has warned you to a problem. But you are right, you must know your fuel system. And yes it will not warn of a problem after the header tank, but Ole always fits fuel pressure gauges to his range so so if the engine pump fails we then have an electric pump to switch to. Still not foolproof but if you know your system, better than nothing.

 

As far as the Sav's I think they are set up similar, so maybe it is just a faulty sender unit. But don't just guess find out the problem and fix it.

 

 

[Guest Maj Millard]

Fallow dear , That setup in the Savannahs is one of the safest I've seen. The press to test on the light is great and the whole sump Idea with the low fuel light is spot on ...all aircraft should have it.

 

Have heard in the past of avgas causing faulty sensor operation in the Savannah set up.......have you been running avgas ?....I don't know why it effects it.

 

I have also been wondering if the low oil warning on your standard Honda 5.5hp pump motor would work in a similar fashion, they seem to work pretty well on those engines..............Cheers .......Maj....

 

 

[rankamateur]

I have also been wondering if the low oil warning on your standard Honda 5.5hp pump motor would work in a similar fashion, they seem to work pretty well on those engines.

What are you suggesting Ross? rig it to cut the ignition when the level drops below the sensor, that would liven things up for sure.

 

 

[Guest Maj Millard]

No ob

 

What are you suggesting Ross? rig it to cut the ignition when the level drops below the sensor, that would liven things up for sure.

No obviously you would just wire it into the system as a switch, which could activate a warning light similar to the way the one works now.

 

Thinking they may be more robust and not so subsceptable to fuel damage, as they do have to operate for many hours exposed to hot oil and vibration, and still be reliable when needed.....I have a spare one here and I'll have a little play around with it....

 

If you go back a bit you may remember the early Honda pumps didn't cut your engines ignition , but instead activated a red light instead of the green one that was on when things were fine...............Maj.....

 

 

[Jaba-who]

Have no idea what the set up in your aircraft but the system I have in my jabiru consists of a contact float switch and a simple open/close circuit connected to the Dynon on one of the general contact switches.

 

The switch is located about two thirds or so up the vertical height of the header tank. When the fuel level is above the switch the floating half lifts up and is closed and the contact circuit reads tank "Full" on the Dynon. (OK its not quite full but close enough) when the fuel level drops below the switch the floating end drops away and the Dynon reads "Tank Low".

 

Pretty simple.

 

I must admit I believe almost diametrically opposite to Facthunter. The sight tubes in the jab are particularly unreliable. The jabiru ones are quite wide bore and they slosh around all over the place. This is especially a problem in turbulence and if you aren't flying properly coordinated.

 

I filled in the holes with epoxy when I built it and drilled out tiny ( 1/64th) holes for the fuel to dribble slowly in and out of the tubes. So it changes much more slowly.

 

Next the fuel level is often hard or impossible to see when the tanks are more than about 80% full so I made some coloured float balls.

 

Lastly I have never had the float switch give a false reading either false low or false full.

 

But that's just my experience.

 

 

[Ultralights]

this happens in my Savannah fairly regulalrly, but more often when i run a tank dry and change to the main tanks when the red light comes on, what appears to happen is a air bubble gets trapped in the header/sump tank as the level falls a bit before i change tanks, and the incoming fuel then stops the air escaping back up into the open tank to vent. its usually solved by opening both tanks on either side, a few slow turns and the air escapes, light goes out.

 

 

[planesmaker]

I would think the header tank should have a seperate vent back to one or both tanks. Tom

 

 

[Ultralights]

how would you stop fuel flowing back up the vent from the header/sump tank? when the tank is the lowest part of the fuel system and gravity fed from tanks above it?

 

 

[facthunter]

The jabiru is a special case . The thin wing section makes any fuel measurement difficult. Having an auxiliary central tank with about 40 minutes fuel or a bit less, is the best idea as the fuel from the other tanks will be able to find it's way there over time and the unusable fuel should be a minimum. This is probably the only way to know how much fuel you have left with any accuracy when you are nearly exhausted. Nev

 

 

[Scotty 1]

I would think the header tank should have a seperate vent back to one or both tanks. Tom

Or a separate vent on the wing

 

 

[planesmaker]

how would you stop fuel flowing back up the vent from the header/sump tank? when the tank is the lowest part of the fuel system and gravity fed from tanks above it?

The sump vent should be vented to the top of the tank or into the wing tank vent system. Yes the vent tube will have fuel in it but the top should be above the fuel level in wing tanks and should have continual rise from sump with no dips to allow the air to rise and not get trapped. Tom

 

 

[bobcharl]

Peter,

 

I wonder if Maj. is close to the mark with his avgas suggestion.

 

Refer to the article in the R A Aus magazine June 2010 ( before it was called Sport Pilot) on P.34.

 

In brief, a group of Savs travelling together each had this problem after filling with Avgas. The problem rectified itself after reverting to Mogas.

 

Does anyone know if there is a difference in the Specific Gravity of the two?

 

Bob

 

 

[fallowdeer]

Thanks for the replies everyone, food for thought everywhere.

 

I'm going to source a replacement (Aerokits??) and fit it, just trying to avoid having to unseal everyhing multiple times, would like to take it pieces just once.

 

Down the hangar today and turned on the master, no light, couple of hours later and on turning on the master light on.

 

No avgas has gone in during 40 hours I've put on since starting to fly the airframe in November and the previous owner only very, very rarely used avgas.

 

Can anyone describe the actual sender unit or post a pic?

 

Thanks

 

Peter

 

 

[Scotty 1]

This is the unit I have fitted to mine. Avgas shouldn't affect it.

 

http://www.aircraftextras.com/PDF-files/AD-AEFG-85x11b.pdf

 

This is their homepage. There are some interesting gadgets here, have a look.

 

http://www.aircraftextras.com

 

 

[Guest Andys@coffs]

Avgas has a defined conductivity requirement of 500 to 600 pS/m at 20deg C and additives are put into the batch so that static build-up is less than would otherwise be the case.

 

Mogas doesn't seem to have any conductivity specifications, but diesel does at 50pS/m. In fact the documents that I could find that discussed the conductivity said that some additives increased conductivity but in doing so made the fuel system more prone to corrosion, and that 50pS/m was the cuttoff point at which apparently static became less of an issue, less than 50 and it is an issue......

 

Anyway depending on how the sensor works such a change from 50 to 500 could well change the behaviour of the electronic device.

 

From my trade training background a Siemen inverted becomes resistance so 500pS/m is 2Mohms per meter and diesel by comparison is 20Mohms per meter or 10 times more resistive.

 

Its not immediately obvious why the difference should be an issue, in simplest terms air between 2 terminals is less conductive that either alternate but without seeing the circuit I wouldn't know.

 

Andy

 

 

[Old Koreelah]

...I made some coloured float balls...

That's got my attention. I imported some special red "floaties" for my sight tubes, but one has developed a leak and slipped below the fuel level. Can you tell us about yours?

 

 

[Skee]

Is the Sav fuel reservoir fuel meter a Contact/no contact meter?

 

say when i measure now without fuel it has a circuit, so when i will will it cut it?

 

 

[Ron Hoey]

Hi allOver the mountains the other day at 7000ft and the low fuel light came on.

 

Immediately could see I still had what fuel I would expect on board by looking at the sight tube in the wing root, and knowing that both tanks feed individually into the 6l feeder tank I wasn't too worried, but still stayed high till I was near the field ten minutes later.

 

The light has been going on and off intermittently since (haven't been back up) and I can see that the feeder tank is totally full so it would seem it is a sender fault.

 

Not having built the plane I'm not familiar with the insides so just looking to find out what to expect before I drain everything and pull it to pieces.

 

Any advice/knowledge welcome.

 

Thanks

 

Peter

Hi Peter,

I had a similar situation with my Savannah. It could be one of 2 things as far as I know. Firstly it could be a faulty sender as you suggested or secondly it could be the wire coming back to the sender earthing out (this is how the light is activated, by the negative side of the circuit being connected to earth through the closing switch on the sender unit. To find out which problem is yours, disconnect the spade connection on one of the wires coming off the sender unit (there is only one with a connection). With the master switch on, if the light stays on, it is an earthing problem, if it goes out it is the sender unit. Hope this helps. Ron.