Low fuel warning light

[1Phils]

Thanks for the reply. I will see what I can find was thinking of an old motorcycle carby float and cutting it down.

 

 

[Jaba-who]

That would probably work.

 

I recall my original bing carby on the jab had the foam type float. Later ones have a hollow plastic type.

 

But it was coated with a layer of some sort of paint or something that presumably kept the fuel out of the air cells.

 

you'd probably want to coat it with something - maybe epoxy or epoxy paint. My original floating balls were not sealed and they lasted about a month or so and they absorbed the petrol and just sank.

 

I found the uncoloured ones worked well but were virtually invisible in the fuel. I found fluoro yellow was equally invisible in AvGas.

 

I run exclusivley on AvGas which is 100Low Lead formulation its coloured blue-green.

 

I would guess what colour works best will depend on the colour of whatever fuel you use.

 

I tried different fluoro paints but they had all sorts of effects on the epoxy. Making it not cure etc.

 

I would guess if you sand down some carby float foam to little balls and then paint it then seal with epoxy they would probably work.

 

I don't know why I didn't try foam myself instead of having to make the Q Cell balls. Target fixation I guess.

 

 

[rankamateur]

Thanks for the reply. I will see what I can find was thinking of an old motorcycle carby float and cutting it down.

Don't use a modern Rotax float! Apparently they sink, I have to change mine before I can start my new engine.

 

 

[Old Koreelah]

Wish I'd read this thread three years ago. I wasted mobs of time trying to find some brightly coloured plastic that floats in petrol. I never thought of cutting up an old plastic float. Eventually I ordered three red float bubbles from an American firm (sorry, name long lost). They were about 10mm long by about 4mm and fitted neatly inside my sight tubes. Why three? I tested one to destruction, which wasn't hard- they're hollow and flimsy.

 

One is working well, even though the sight tube is at about 30 degrees off horizontal. The other gave up and sank.

 

I'd never thought of making my own with filler, but it's a great idea. Vinyl Ester is probably the best resin to use. It copes with most fuels better than epoxy.

 

 

[Yenn]

If you want to see the level in a plastic tube you could try having diagonal yellow and black lines painted in the background behind the tube. 25mm thick lines at 45 degrees would work. Where the fuel level is shows up as a change in the slope of the lines.

 

I don't know the theory, but it does work.

 

 

[Geoff13]

I don't know the theory, but it does work.

Refraction.

 

 

[Blueadventures]

Hi Phil

 

I fitted the clear 'Pulse fuel line' 1/4" ID and have no trouble seeing the fuel level. Its listed in the Aircraft Spruce cataloge. Have a look at mine any time. Even if i'm not there have a look. Cheers Mike

 

 

Hi jaba-who,I have a sav and I was thinking of putting some floating balls in my fuel level sight tubes for easier fuel level sighting but could not find any to use. Do you know where I can get some from please?

Phil

[AndyDrain]

Appreciate this is an old thread but....

 

Was flying over the mountains at 7500 ft when the fuel warning light came on...

 

I have four individual tank feeds and was running on the right inner with about 15 liters remaining.  I had been level for about 3 minutes.  Power around 5200 for the last 15 minutes.

 

Vent pipe goes from top of 6 liter header to left inner tank.  Tank vents are hockey sticks in caps with thick non original cap seals. Vents regularly blown through.

 

Selected left inner along with the right and fuel light went out.

 

I regularly run on right inner only without issue.  With left and right inners selected left tends to drain first.  

 

Have since test run 15 minutes at 5200 at 2000 feet without issue so must assume altitude had something to do with it.

 

Left inner had 25 liters so the vent pipe was well above the fuel level. All tank feeds checked as part of build all gave adequate flow.

 

My physics is not up to an explanation....

 

Savannah S, 15 months 230 hours..

 

 

[Kyle Communications]

Most likely it could be the altitude but what you think you have left in the tank in real terms of fuel amount at the pickup maynot be so. Savannahs tend to fly level in a nose down attitude. check the underside of your wing when flying level (no loss of altitude) and look at the position of the underside to the horizon. I bet it will be nose down. With that attitude your 15 litres is now running very close to the tank pickup feed. The pickups are not at the very bottom of the tank. If you had shifted the nose up to have the wing tip level then most likely the light would have gone out

 

 

[AndyDrain]

Thanks Kyle -  That would mean each tank would have around 15 litres of unusable fuel.  ( 4 x 15 = 60) Descent with less than 15 litres would require frequent leveling off.  Based on the trials I have conducted I don't believe that is the case.  I became reasonably familiar with the fuel system during the build and have not experienced the fuel light coming on, when not expected, at other times.  It is actually quite a useful way of ensuring I use most of the fuel in a tank.

 

I was just wondering how the reduction in pressure a bit higher up might cause the issue..

 

Thanks again. 

 

 

[Kyle Communications]

The higher you are the more chance of a vapor lock and the liquid is lighter than water and even lighter at altitude 

 

I have much better than std sight gauges and when the aircraft was on the ground I measured quantity in the tanks and marked the gauges accurately. The tanks when you install them are usually never actually flat with the bottom of the wing they are slightly kicked up at the back so you will never get any where near all the fuel out of them. I have had the fuel light come on many times on a slow decent yet I was sure I had plenty of fuel but realistically you have about 10 litres that are not useable on level flight to decending flight..well that you can count on. Its very manageable though just slow up and raise the nose and the sump will fill back up again. Well thats what happens in mine anyway. You can always measure that amount thats there of course with the 4 taps. As soon as the light comes on in level flight turn that tank off and then use your others. When you get back turn all the taps off then drain the sump. Then turn on that tank and drain the fuel out and measure it with the aircraft sitting on its tail to make sure. I think full tanks are 35 or 36 litres to the top. Its the only real way to be sure what you actually are carrying on board but is it actually useable. I havent actually measured what was left as I always take well more fuel than I need anyway. But it is a bit of a trap in the savannahs

 

 

[AndyDrain]

Thanks..

 

 

[mpeter14]

On 29/05/2014 at 8:53 AM, facthunter said:

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

 

 

  Happened to me. A very scary experience. The light went on though I had 4 gallons on my right tank. I had to make a emergency landing with the engine off. I am convinced about air bubbles blocking the fuel.

[IBob]

mpeter14 it would be really interesting to know the details of your aircraft where this happened.

Do you have the standard 2 wing tanks?
What sort of vents do you have on the fuel tanks??

[mpeter14]

On 04/09/2022 at 2:21 AM, IBob said:

mpeter14 it would be really interesting to know the details of your aircraft where this happened.

Do you have the standard 2 wing tanks?
What sort of vents do you have on the fuel tanks??

Hi IBob, I do have 2 standard wing tanks and the ventilation I have on the fuel caps is the one looking forward to the wInd. What happened to me was that one tank wasn't showing fuel but the other still had 4 gallons. This unequal consum of the tanks happens often, some times it consumes more the left tank some times the right. So I supposed that still having fuel on one tank was enough to ensure the feeding of the collector. To my surprise the collector "reserve" light went on. My theory is that bubbles of air goes from the empty tank to the one that still has. This because of the pressurization of the tank by the vent lines looking forward to the wind. I think that this pressurization is necessary to push the fuel to the collector. My solution to this problem was to install fuel valve selector that allows me to choose right,left and both tanks. When I see that one tank has less fuel than the other I just change the selector.

On 29/05/2014 at 8:53 AM, facthunter said:

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

 

 

  Happened to me. A very scary experience. The light went on though I had 4 gallons on my right tank. I had to make a emergency landing with the engine off. I am convinced about air bubbles blocking the fuel.

[IBob]

Hi MPeter14, thank you for your clear description.

I have a 4 tank Savannah S, but rarely use the outer tanks, so normally fly with the two inner tanks valved on. I have the tank vents piped to the underside of the wing, cut off at 45degrees facing forward. Like you, I have always had uneven feed from these two tanks, though I have not had a low fuel light as a result.
I have tried various things since I noticed this:
I saw a Savannah with forward facing fuel vents under the wings, like little pitots, thought this looked very neat and converted my own vents to this setup. The result was massive cross-feeding of fuel from one tank to the other...the LH tank was visibly going down, while the RH level went up, I promptly returned and landed. On landing, I found fuel across the RH upper wing. I think the pressure had been enough to cause that fuel cap to bleed air and fuel. It also occurred to me that even modest pressure in the tanks would cause them to swell or bug out, with pressure on the tanks supports and upper wing skin. So I converted back to the standard underwing setup, with the pipe cut at 45 deg to forward.

 

Since then I have done what I can to remove any undulations in the fuel lines (my lines all go to valves on the RH side of the baggage area), especially the lines coming from the LH tanks, which I have tied to an aluminium angle 'splint' to hold them straight where they pass across the upper fuselage. My thought is that any undulations may hold air, so requiring a small amount of pressure to drive the fuel up and over (this is a known phenomenon with gravity feed of water through long undulating hoses: the necessary head of water to cause it to flow is the sum of the undulations).

I have also experimented with adjusting the 45degree cut on the vent lines to more or less than 45deg.

My fuel feed is now improved, but still uneven, and I have learned that most aircraft with multiple tanks have uneven feed. Which is why many of them are flown with timed use of alternate tanks.

In summary, I think it is important to minimise undulations in the fuel lines: in a perfect world they would track always downwards from the tank, but this is not possible in the wing.
And I think the tank vents should provide only a very small increase in head pressure: any larger pressure may prevent a tank emptying as happened to you.

 

By my calculations, your 4gals of fuel would be approximately 65mm in the tank.

This would require an excess pressure of just .07PSI from the other tank to prevent flow.

 

[facthunter]

I've found with a school plane the assymetric fuel varies depending on who's been flying it. I'd be worried if one tank was empty and you still have BOTH selected that yu might get air.. For most of these planes the preferred situation for takeoff and landing is select BOTH and have (enough) fuel in each. Otherwise select only the fullest one and PROVE it at a safe place before relying on it. Flow rates must be adequate for Full power plus a fair bit of extra  flow. Nev

[IBob]

All good points, Nev.

As mentioned elsewhere, the Sav has a 6litre receiver tank in the fuselage, which I'm sure goes a long way towards ironing out irregular fuel flow, while giving plenty of warning if fuel flow is continuously inadequate (that is, assuming the low level switch and indicator are working, and which mpeter14 can check now he has valves on his tanks).

 

FWIW, in case I missed it here, the reason for the false low fuel indication that started this thread was that the earlier Savannah's had no vent line from the receiver tank. So any air there tended to stay there, and would then also expand with altitude, which would trigger the low fuel indicator. Later builds have a receiver vent, which goes to the upper LH inboard tank.

It's also worth considering what Cessna do for their singles:

They have a single tank vent, but situated to the rear of where the strut meets the wing. presumably this reduces the resulting pressure, also helps protect the vent tube from damage, maybe also reduces ingress of bugs.
They then have the upper tanks cross-ported, resulting in equal air pressure in all tanks. Which must go a long way towards providing an even feed from all tanks.

 

 

[facthunter]

IF you have a pump in each tank and a Non return valve at each  selector you can just turn all  valves on and all pumps on for a fuel emergency. Pumps don't always like running dry so for longer times that has to be addressed. Your fuel system design must allow reliable access to nearly all the fuel on board every time or it's not fit for purpose.. Some tanks expose ports on the go around when fuel is low also.  Nev