Fuel contents gauging

[Dafydd Llewellyn]

The patter "I always dip the tanks" is the no-brains response usually applied whenever a potential fuel-starvation accident is reported. It's a sensible response - up to a point - for a Victa Airtourer*, A PA-28, an Auster, and a few other aircraft whose manufacturers provided a calibrated dipstick.

 

*Provided the rubber bag has not come loose from the buttons that retain the top of the bag.

 

However, let's see you get a sensible result trying to dip the tanks on a Bonanza, or almost any aircraft with long, shallow tanks in the wings. Or anything that uses a rubber bag, actually; if the bag collapses, it can give a "full" reading with almost nothing in the tank.

 

So "dip the tanks" is not really a valid answer, except in those cases where the designer catered for that method. It has been used for decades in transport-category aircraft, where one can walk around under the wings, in the form of "drip-sticks", and in that form there can be a number of them in each tank, and the combined reading is very accurate in most cases. I've never seen a drip-stick on a GA aircraft.

 

Fuel contents gauges conventionally come in a variety of types; the "old faithful" float - arm - rheostat (which puts electrical power into the fuel tank vapour space, but let's not worry about that) ; the capacitance type - which also puts electrical power into the vapour space - and gives wildly incorrect readings if it gets some water in the sensor; Sight-tube types - which work very well on tall, compact fuselage tanks, but vary wildly according to roll and pitch attitude, on wing tanks; Mechanical float gauges that can be read from the cockpit - these come in float/arm and twisted-strip types; they can be quite accurate if located at the geometric centre of the liquid surface area of the tank, but in a dihedral wing, that changes as the fuel is used.

 

The simple fact is, fuel contents gauging is not simple. There is hardly a single aircraft type with wing tanks that extend over more than a short length of the wing, that has a satisfactory form of fuel gauge, because no such thing exists. One can improve on it by using a number of senders, distributed along the length of the tank, but adding their outputs correctly is by no means simple - and not fail-safe.

 

So in practice, about the only practical approach for aircraft with long flat wing tanks, is to use a calibrated fuel flow meter, and start with full tanks.

 

 

[cheyenne]

I have the old vertical sight glass with restricters and the fuel dose not bounce around in my J160 when I built it, with the plane level I put 10lts in each wing then set the clear measurement transfer at the 10lt mark so now I have a known amount of fuel in the tanks, in flight they show 10lts less because of the attitude of the aircraft.

 

 

[jetjr]

I have both sight and electrical in jabiru, both as dodgy as each other

 

For some reason used to be pretty good but currently gets some type of air lock and RHS tank reads very low frequently.

 

At some point in flight this is broken and they correct themselves. Other tank seems pretty good

 

Dip stick on jabiru is near useless

 

Whole issue of leaving fuel behind to stay under arbitrary category stall speed (then battling this well known measurement problem) is absurd and a serious safety issue

 

 

[frank marriott]

In RAA most? or at least a lot of pilots fly their own aircraft - in that case you need to know your guages.

 

By that I mean -

 

In my case a J230. When filled I know I have 135ltrs. I run a fuel log, always, not just on a trip. I use a fuel burn of 25ltrs/hr. As I average better, then I know what the minium amount is, and confirm this against the guages.

 

Every fill I check the guages and compare with the amount of fuel it takes to fill. In my case the guages consistently under read by 10/11 ltrs. i.e. has 10ltrs more then guages read.

 

For a weight limited load, I use the "minimum fuel log amount" and add sufficient for the trip.

 

The recommended 45min fixed reserve is a guide. With the small [relative] fuel burn of RAA types I use a min of 60min fixed reserve.

 

I appreciate my approach doesn't address the flight school situation with multiple pilots, but a fuel log against the tacho hrs flown since last fill might go part of the way.

 

I will not accept a fuel guage reading alone on anything I fly.

 

 

[Guest Andys@coffs]

......Whole issue of leaving fuel behind to stay under arbitrary category stall speed (then battling this well known measurement problem) is absurd and a serious safety issue

especially when you know damn well that the engineering MTOW is vastly higher than the legislated MTOW.

 

The reality given the accuracy limitations is to be conservative in your best interests, if that means in good faith you end up exceeding the legislated MTOW then I wonder how that would go, but realistically as fuel unless you take it out is and remains to a degree an unknown I wonder if the ramp checkers are any more informed than we would be.....

 

The Para above is just me musing aloud, don't read it to be a recommendation to exceed legislated MTOW, there be only pain and grief at some point down that path!

 

Andy

 

 

[Dafydd Llewellyn]

I wouldn't put it past a ramp check to drain the fuel and measure it. However, your defence would be that there is no means to accurately verify the fuel gauge readings other than to start with full tanks. In theory, one of the jobs to do on a periodic inspection is to re-calibrate the fuel gauges and replace the fuel gauge calibration card. How one can in reality do that for an aircraft with long, thin fuel tanks and not have it a polite fiction, eludes me.

 

I agree the situation as it stands is unsatisfactory, and not only for the Jabirus that have wing tanks. This is an area in which the design standards have not caught up with reality.

 

 

[turboplanner]

I would agree with collapsed bladder tanks, but not with Jab tanks; give some people the slightest excuse and they will take the easy way out, and then you will see more fuel exhaustions.

 

Personally I liked the LSA tank which was tall, and you could visually inspect the fuel level for flight planning. If you know what the fuel quantity is before taxy, then you have half a chance by getting the fuel burn as accurate over time as you can.

 

By going to wing tanks there's a safety improvement in terms of a collision, but you then have to deal with the very small height.

 

If you know the fuel burn accurately, and your fuel input is greater than the duration of the flight plus the 45 minutes reserve, which would be most of the time, then you are working with a known minimum, so it's not an issue.

 

If you want to go touring, then you need to establish a dipping regime and do your own calibrations, marking a dipstick, and starting with an empty tank, and always, including calibration, if the aircraft is facing east when you do the fill, you need to wheel it around and park on the same spot facing west to check the difference.

 

The gauges should only ever used as a rough guide, and as Dafydd has said, have no chance of being accurate in such a slim tank, so there's no point in making a song and dance about it, you just have to develop a fail safe way of completing your flight without a fuel exhaustion.

 

 

[Guest Andys@coffs]

Im sorry guys but dipping is at times a useless exercise in Jabs, the refill point is at the highest point outboard, the dihedral significant and it can be the case that its completely empty or mere mm's at the refuel point yet there is still useful fuel left. furthermore unless you are dipping at a surveyed point at the airfield with guaranteed flat concrete/bitumen surface or where you have a small hanging spirit level on a stringline from winglet to winglet I still don't know what you would do with the results you obtain if you are looking for accuracy. Anything other than removing the fuel and measuring it as its put back in is going to be an approximation. Understand that, deal conservatively with it (Fuel log as per Franks comments) and don't try and work on less than 20+litres of reserve and generally you wont run into issues, anything else is going to have too many if buts and maybes that are all a function of the physical realities involved.

 

 

[crashley]

My RV 12 has a tank with a sight glass built in so you can always see the actual level it also has an electric float guage and a flow meter which shows fuel left, fuel used, fuel per mile , fuel per hour and fuel calculated to be left at next waypoint so I can cross check easily

 

 

[SDQDI]

I wonder how hard it would be to put two tank drains in one slightly higher than the other a bit like a motorbike setup that has a three position fuel switch, off, on and reserve. We all plan for a reserve but we don't know exactly when we start burning that last 20 litres or so, I know if we keep a time-fuel log you will have an idea but there are so many variables that can affect it.

 

On the motorbikes you run them till they run rough then flick over to reserve and you have enough to get home. (Unless you forgot to turn it back off reserve after the last time you ran out.) but my point is if we had a physical reminder/warning that we had half an hour to go it would help an awful lot.

 

I have a flashing light setup on my header tank and I test it by switching my wing tanks off after I've landed and by the time I am ready to turn off it is starting to flash I have about ten minutes of fuel left after the light starts (five litres). But with all these points they are still prone to failure or human error maybe we need to think hybrids with a battery powered backup even if it was only good for five minutes

 

 

[AVOCET]

I know of at least one instance where the fuel tap was turned off before lift off and at 500 ft the engine stopped ,

 

The resultant hot engine wasn't able to be restated and the plane impacted the ground , the flaps were still retracted , so I assume all effort was focused on trying a restart .

 

 

[Dafydd Llewellyn]

I know of at least one instance where the fuel tap was turned off before lift off and at 500 ft the engine stopped ,The resultant hot engine wasn't able to be restated and the plane impacted the ground , the flaps were still retracted , so I assume all effort was focused on trying a restart .

That's a bit off-topic for fuel contents gauging - but it's a constantly-recurring cause of accidents; possibly the greatest single cause of EFATO, in fact; especially with the carburettors used on Lycomings etc, which have carburettor bowls the size of soup bowls, and let the aircraft get to a couple of hundred feet before the engine stops.

The best answer I've seen for that, is to mount the starter button behind the "apron" of the fuel selector, so you can't access the starter unless the fuel is on. (There's at least one recreational type that does that). The downside of that is that if you get an engine fire due to a backfire on startup, you can't shut off the fuel and then crank the engine with the starter, which is the standard drill; you have to start the engine and then turn off the fuel. (No matter what the designer does, somebody is going to argue that it caused a problem; he can't win.) However, on the balance of benefit, I'll take an engine fire on start-up in preference to an EFATO, any day.

 

 

[fly_tornado]

Seen this? http://www.beliteaircraft.com/avionics/fuel-probe-system.php

 

 

[Louie]

I know of at least one instance where the fuel tap was turned off before lift off and at 500 ft the engine stopped ,The resultant hot engine wasn't able to be restated and the plane impacted the ground , the flaps were still retracted , so I assume all effort was focused on trying a restart .

What are the implications of not turning the fuel tap off on shutdown? If no negatives takes a potential error out of the equation.

 

 

[Dafydd Llewellyn]

Thanks, FT; looks very interesting. I'd be most interested to hear of people's results with it.

 

There are a variety of industrial devices, for tank contents gauging, some of them ultrasonic, and various other principles, but this is the first one that I've seen that looks potentially suitable for "difficult" aircraft tank shapes; most of them are obviously for industrial plant applications.

 

 

[AVOCET]

$199.00. Not bad value

 

For wing tanks you'd need 2 senders

 

Maybe hard to fit to jab wing tanks ?

 

 

[Dafydd Llewellyn]

What are the implications of not turning the fuel tap off on shutdown? If no negatives takes a potential error out of the equation.

Depends on the details of the fuel system. In many aircraft, it results in fuel crossfeed from one tank to the other, with fuel leaking out the tank vent, so you leave the aircraft with the tanks full, and come back to find it half empty (or worse) and a big patch of dead grass. Also, if the tanks have a positive gravity head to the engine, you're relying on the carbie float needle to prevent fuel loss. If the float needle doesn't seal perfectly, you're likely to have fuel dripping inside the engine cowl - which is an invitation to a fire if some idiot walks past with a cigarette.

 

 

[AVOCET]

What are the implications of not turning the fuel tap off on shutdown? If no negatives takes a potential error out of the equation.

This was not a case of turning the fuel tap off after shut down !

brf , say no more .

 

 

[DrZoos]

The mark 1 eyeball is a very good gauge of whether the stick or gauge is correct...even if it requires a ladder..

 

 

[Guest Andys@coffs]

FT the webpage says that the fuel tank can be from 6" to 48" in depth.......Jab wing chord isn't 6 inches let alone the fuel tank depth so no banana for a Jab fit, unless they have other units that will work

 

Andy

 

 

[AVOCET]

FT the webpage says that the fuel tank can be from 6" to 48" in depth.......Jab wing chord isn't 6 inches let alone the fuel tank depth so no banana for a Jab fit, unless they have other units that will workAndy

You could make something similar to the jab fuel drain and cover it with those fairings

I don't think it would be that hard for jabiru to work it out . Unless 6" is the absolute min thickness the probe will work .

 

 

[Camel]

Seems to me if the filler cap was closer to the wing root at the deepest part of the tank it would be a lot easier to dip. I have a calibrated stick I dip with, but have found if I can't see fuel visually in both tanks there isn't enough, I think it holds a fair amount of fuel before you see it.

 

 

[SDQDI]

What are the implications of not turning the fuel tap off on shutdown? If no negatives takes a potential error out of the equation.

If I don't turn the fuel off before shutdown the temp fluctuations of a day night day can cause the fuel in the header tank push out the breather onto the wing, turn the fuel off after landing and run it down a smidge and no troubles at all

 

 

[AVOCET]

Seems to me if the filler cap was closer to the wing root at the deepest part of the tank it would be a lot easier to dip. I have a calibrated stick I dip with, but have found if I can't see fuel visually in both tanks there isn't enough, I think it holds a fair amount of fuel before you see it.

Won't work , dihedral creates air pocket

air pockets

 

 

[DrZoos]

Camel i have the same. Empty the tanks. Fill 5 L at a time and calibrate your own stick for each tank.......works a treat.