Correction of fuel gauges ?

[wanabigaplane]

Correction of fuel gauges ??

 

A friend has an aircraft with the normal two wing tanks, and two standard VDO fuel gauge arrangements to read the contents. Not only are the gauges hopelessly non-linear, but the empty and full reading on the gauge have nothing to do with the contents of the tank.

 

“I think I can help you with that “ I says, thinking all I had to do was add a resistor or two to bring the endpoints into line, and maybe even fudge a more linear response over the span. Simple.

 

I poked around with my trusty multimeter while he was filling the tank, and I discovered things aint so simple after all. The circuit did not even work the way I expected such a simple circuit had to work. Everything goes in the wrong direction! In fact, adding a few resistors would make it worse, not better.

 

So, forum members, before I launch into designing an electronics gizmo that corrects the reading of the gauges, I ask if any of you know of a simple fix, or a commercially available correction device that will solve the problem? Remember - the senders are already exist. They are already mounted in the tanks, and the gauges are already in place on the dashboard. They cannot be changed now!

 

A friend in need is a pest indeed!

 

Jack.

 

 

[BigPete]

I'm amazed that manufacturers bother with fitting fuel gauges. (electric ones in the dash)i_dunno I mean, we're taught not to trust them right? dip the tanks laddie, know how much fuel you use per hour and do the sums, etc etc. :confused:

 

My Jabby :heart: and quite a few older/different aeroplanes have a sight gauge, :thumb_up: what you see (on the level ground) is what you have. :thumb_up:

 

Sorry Jack - doesn't help you one little bit.

 

regards

 

:big_grin::big_grin:

 

 

[Guest ozzie]

OK in the GA world fuel gauges need to be calibrated.

 

This is done in the following manner.

 

fill the tank from empty unusable and write down how much is now in the tank and observe where the needle is in relation to full. Now drain the tank from the engine outlet. not from the drain points. measure how much fuel has been removed. if the tank holds 100lts drain off 25lts and note the needle position another 25lts note the needles indication.keep doing this until the fuel stops flowing. what is left in the tank is marked as unusable. now write down the indicated amount and next to it write down the actual contents and place this next to the revelant gauge. this will give you an accurate reading on how much fuel you really have.

 

Ozzie

 

 

[facthunter]

Fuel Gauges- accuracy.

 

Notoriously unreliable in just about every light aircraft that I have ever flown, so never trust them on their own.

 

Knowledge of the ACTUAL rate of fuel usage for your particular aircraft is essential to operate it safely (& legally). Ie you are required to use the established rate or the operating handbook figures, so what Pete says operates as a back-up in any case, and is pretty reliable unless you have a fuel leak or something changes to alter your rate of fuel usage.

 

I prefer a visual sight tube, or failing that, a direct mechanical linkage on the end of the tank (from the float to the gauge) As in the Citabria. It does not indicate the correct figure on the ground, however because of the attitude of a taildragger on the ground.

 

A properly graduated dipstick is the best check and was once "par for the course" in a pre-flight check. It should still be. ( Not necessary in the case of full tanks).

 

Ozzie's comments re calibration gives you the way to see how the gauge indication compares with a known quantity of fuel, and this relationship must be proven. It presumes the integrity of the sender, and the gauge, and the effect of voltage changes (power supply) will allow you to get a consistent result. This consistency (reliability) is essential, and is what is generally lacking. An unreliable fuel gauge is about as usefull as an ashtray on a motorbike.. Nev..

 

 

[Guest ozzie]

Oh by the way most (older) cars gauges run on 8 volts not 12vts so when you have a lge current draw they will still read correctly. check the gauges voltage rating.

 

 

[facthunter]

Voltage.

 

Think that some incorporate a voltage regulator in the system. Would a diode do that? Seem to remember something about a wheatstone bridge, too, where it is not voltage sensitive. I don't like complexity... Nev..

 

 

[Yenn]

Ive never heard of 8V car guages. How would they work? If 12V is cut down to 8V by a resistor and the base voltage then falls to 8V surely the feed to the gauge would be less than 8V then. My experience with them is that they are really a variable resistor and a volt meter.

 

 

[Guest ozzie]

If you have gauges running on 12vts and you load up the system by say turning on your lights wipers and fan.(it's a crappy night). then the gauges will not read correctly so the design like in old holdens use a 8vt circuit using a length of resistor wire, this is shown as "pink" in the wiring diagram and the wire is marked as such along it's length, this resistor wire also runs the coil at 8vt.

 

ozzie

 

 

[Guest Maj Millard]

There are only two ways not to run out of fuel !!!!!!!!

 

1. know what your engine burns per hour....

 

2. Know how much fuel is in your tanks prior to take off, by checking contents

 

with a calibrated dip stick.

 

It always works, dipsticks don't lie.............

 

Somethings don't change, - they are just made more complicated !!!!!!

 

 

[wanabigaplane]

So, real pilots don't need ....

 

You are right, Big Pete, that didn't help one little bit ! Nor did any of the other posts. The consensus seems to be that real pilots don't need fuel gauges. What this really means though, is that no one knows of an after market add on device for correcting fuel gauges!

 

The fuel gauge in my own aircraft does work, and it seems to be quite accurate right round the scale. I always start with a full tank and do my calculations anyway, but I really like being able to cross check as I go. I really appreciate being able to reliably read the fuel level at any time, even if that doesn't make me a real pilot.

 

Cars have been using the float and meter type fuel gauges for years with excellent results. The only reason they are a problem many aircraft is that the shape of the tank is not the same as in the car, and the aircraft manufacturer has no means of correcting the reading. Since so many pilots have this problem with their fuel gauges, someone must have come up with an after market device that makes the gauges read correctly !

 

Jack.

 

 

[facthunter]

Fuel gauges.

 

Jack, Don't think I would agree that fuel gauges are not needed by "real" pilots..

 

Wouldn't a re designed face cover what you need?

 

I suggest non-reliance on fuel gauges, not their removal.

 

Proper fuel management legally requires that you determine the fuel contents by a means independant of the gauges, and the usage of fuel rates specified by the maker or a method of determining ACTUAL documented rates, is used for flight planning purposes. Nev..

 

 

[Guest ozzie]

electronic sensor

 

This sensor is used on the water supply container on the steam cleaning units that i service. it simply is stuck to the outside of the plastic tank and when the water level drops below the sensor it trips a relay and shuts down the machine. this will work on any liquid but is not suitable for metal tanks. i would work ok for a plastic fuel tank. just glue it several inches above the base and wire it to a micro relay and light buzzer ect for a low fuel level warning. If someone wants to try one i can pass one along. wiring is simple 12v supply and one wire to the relay. it also is adjustable for the sensitivity of the liqud and thickness of the tank.

 

Ozzie

 

 

 

 

[Gibbo]

There are essentially two types of fuel level sensors,

 

4-20ma. which is basically 4mA at empty and 20mA when full. These are normally powered and also tend more accurate. $, VERY linear in output.

 

Variable resistance type. These are common in the motor industry. VDO make a huge range of them. Note the resistance when empty and also full. These will tend not as actuate or as Linear in the output. Swing arm type fuel sensors aren't worth the money. VDO make a great tubular fuel level sensor but they most likely won't fit into an aircraft fuel tank. VDO state the full and empty range readings on their website.

 

Calibration should be as per earlier GA tread.

 

Gibbo

 

 

[sain]

Capacitative sensors are probably pretty good, though you'd have to make and install them yourself.

 

Jim Weir (he writes for kitplanes) has articles about them here:

 

KP0000

 

and

 

KP0000

 

 

[jcamp]

Regarding 8 and other odd voltages in cars. A common stetup in a 12V vehicle was to have a dropping resistor so the ignition normally worked on 8V, there was also a direct feed from the starting switch/circuit so that on starting the ignition had 12V (actually 10 or 11 depending on how sick the battery was) for easier starting.

 

 

[Guest Maj Millard]

I don't think it's a case of 'real' pilots not needing fuel guages, it's more the case

 

that 'real ' pilots have learnt not to trust fuel guages.

 

Maybe that's how they got old enough to be considered 'real' pilots ??.. :thumb_up:

 

 

 

[youngmic]

Buy a fuel flow meter?

 

 

[Yenn]

Knowing your consumption and what you start off with, is not a insurance against running out of fuel. What happens if you develop a fuel leak in the piping somewhere, or there is a fault in the carburettor.

 

Both of those have happened to me in a Thruster. The pins on the float were worn and gave a richer mixture and the pipe to one of the carbies from the pump broke. That resulted in fuel spraying back over the cockpit, so was immediately apparent to me. It resulted in an out landing. The pipe was only fitted a few months earlier and had seen 18 hours of run time and it was pure luck that it wasn't the pipe from tank to filter, which would have stopped me dead.

 

 

[youngmic]

Yenn,

 

You'll look long and hard before you find someone suggesting that a fuel flow gauge is insurance against running out of fuel.

 

As for your "what if scenario" you answered your own question.

 

Nothing will ever stop idiots from crashing, it's Darwins way.

 

M

 

 

[Guest ozzie]

Just calibrate the them.!

 

 

[lazerin]

It's interesting to hear the comments on the unreliability of fuel gauges. In the sportstar, all we have to go by is the fuel gauge...:confused:

 

There's no other way to check quantity.

 

 

[Guest Maj Millard]

You mean you can't check fuel qnty at the fill point with a calibrated dip-stick on the Sportstar?. ..how so ?......And if anyone is looking for a slightly used Navman fuel flow, I have one in a box somewhere complete....it's the first thing I removed from the Lightwing when I bought it.

 

And frankly folks, I have no problem at all if anyone wants to fly around using electronic fuel guages as thier sole indication of fuel remaining. As Ozzie suggests they better be calibrated correctly. I calibrate them all the time working as a GA maintenance engineer, and probabily have done hundreds. Personally, I use a fool-proof dipstick, as do many GA pilots. What is the sound of silence ???

 

 

[lazerin]

The sportstar we fly at parafield only has a single tank situated directly behind the seats. The filler cap is located on the side of the fuselage and as such, is at an angle. I imagine this is the reason that a dipstick wouldn't work very well.

 

With the fuel cap off, it's still pretty much impossible to see the level of fuel until it's reached the brim (and overflows).

 

I'm told the fuel gauge is calibrated and very accurate :big_grin:, here's hoping!

 

 

[facthunter]

Fuel gauge.

 

Well it wouldn't be good enough for me and I would doubt if that method of quantity determination is LEGAL.

 

When you have empty tank(s) and add a known quantity of fuel you know what quantity you have.

 

When you fill them right up you know what quantity you have. IF you remove a KNOWN quantity of fuel from a Full tank, you know what quantity you have.

 

There is a principle in flying that you NEVER depend on ONE source of information,(confirm it with a back-up), if you want to die old. You have to be able to verify it. Look what happened at Mt Erebus.. Nev..

 

 

[Guest Maj Millard]

It dosen't matter if the stick is on an angle. As long as you can put it in so that it touches the bottom of the tank. Assuming it would be put in the same way every time, once the cal is done and the stick marked, it should work. Don't know what sort of an angle you've got, as I'm not intimate with the Sportstar.