Precise Fuel Mass Measurement in GA

[mirkoni]

Dear fellow pilots,
How do you practically solve the problem of precise measurement of fuel quantity in your tanks, in order to calculate the fuel mass that you have in them, needed for mass and balance calculations?
My particular Seneca IV has fuel gages, but how much can I rely on their correctness in estimating the mass of fuel in the tanks?
How can you know how many LBS of fuel you currently have in your Seneca, or for that matter, any GA piston airplane?

Thanks in advance,
M.

[Pilot DAR]

Hopefully, the fuel quantity indicators are reasonably accurate to the nearest quarter tank, then assume the weight to the greater quarter tank, and you'll be conservatively safe. Or, fill them right up, and you'll have a pretty accurate idea. If you have a calibrated dipstick, that can help too, though they are variable as to their accuracy, and airplane attitude can affect the reading.

[ahramin]

Do you have one of these?

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/220x225/fs_450_m_220x225_1e16268d319161babee7841afc239bc1c91d0812.jp g
https://cimg5.ibsrv.net/gimg/pprune.org-vbulletin/500x460/cgr_30p_premium_sized_test_500x460_2e01ae4de11761a805c6a3ba4 55420bb01cdfd15.png
With modern fuel flow instruments it is easy to be brutally accurate in your fuel quantity on board. Several aircraft I fly have these systems and landing with just over 30 minutes fuel (in other words less than 1/10th of a tank remaining) is a regular occurrence. Failing that, you can be fairly accurate starting from full tanks and using a stopwatch for fuel burn but it takes some practice. Otherwise you just overestimate for W&B and underestimate for FOB and hope you aren't too far off.

[TheOddOne]

All the above measure volume. If you want to be accurate, you must obtain the Specific Gravity of the fuel as it was delivered to the installation you draw it from, then re-calculate for the temperature etc conditions on the day. We do this upon accepting a delivery of fuel, against the SG declared by the fuel company.
We measure fuel burn against time over a period to establish an average burn volume. We measure everything down to 1/100th of a litre, then use a factor of 1.6 lbs/litre to calculate mass. We accept this is an approximation as each flight has a slightly different profile but if our calculated fuel burn is out by more than 1 -2 litres upon refilling to full, then we'll think something has gone wrong.

TOO

[mirkoni]

Hopefully, the fuel quantity indicators are reasonably accurate to the nearest quarter tank, then assume the weight to the greater quarter tank, and you'll be conservatively safe. Or, fill them right up, and you'll have a pretty accurate idea. If you have a calibrated dipstick, that can help too, though they are variable as to their accuracy, and airplane attitude can affect the reading.

Thanks for the answer.
The common practice is to fill the tanks right up, but with two of us in the cockpit and full tanks Seneca IV goes overweight. So I need to find a way to fill it to an amount that will not make it overweight, say around 200 lbs (mass) below full tank. The bowser has a totalizer in litres, and there is always some fuel left in the tanks, when we start refuelling ... obvuously if the tanks were empty at beginning of refuelling, we could calculate litres to fill it to desired lbs value, but with some fuel left...

[mirkoni]

With modern fuel flow instruments it is easy to be brutally accurate in your fuel quantity on board. Several aircraft I fly have these systems and landing with just over 30 minutes fuel (in other words less than 1/10th of a tank remaining) is a regular occurrence. Failing that, you can be fairly accurate starting from full tanks and using a stopwatch for fuel burn but it takes some practice. Otherwise you just overestimate for W&B and underestimate for FOB and hope you aren't too far off.
Thanks for the answer.
The common practice is to fill the tanks right up, but with two of us in the cockpit and full tanks Seneca IV goes overweight. So I need to find a way to fill it to an amount that will not make it overweight, say around 200 lbs (mass) below full tank. The bowser has a totalizer in litres, and there is always some fuel left in the tanks, when we start refuelling ... obvuously if the tanks were empty at beginning of refuelling, we could calculate litres to fill it to desired lbs value, but with some fuel still there...

[zambonidriver]

but with two of us in the cockpit and full tanks Seneca IV goes overweight
Not (at all familiar) with the type but my understanding is that it accomodates for five or six passengers. Do you mean overweight with 2 pilots + pax or just the two of you ?

[md 600 driver]

Not (at all familiar) with the type but my understanding is that it accomodates for five or six passengers. Do you mean overweight with 2 pilots + pax or just the two of you ?

rather than overweight do you mean it’s out of balance ?

[340drvr]

Lacking a fuel flow/fuel totalizer instrument, the aforementioned calibrated dipstick is probably the next best thing, original installed fuel gauges are bottom of the list as far as accurate measurement. Common practice for the full tanks/two-up-front out of balance in the Seneca (and other aircraft that have similar weight envelopes) is to put some ballast in the rear baggage compartment, two cases (12 packs) of engine oil, or a bag of lead shot, or weight(s) from a gym set, anything will work, properly secured or tied down, of course.

[john_tullamarine]

Good on you for worrying about the problem. However, there really is no point trying to get the fuel on a lightie accurate to the last poofteenth of a kilogram.

Why ? Because you are starting with an empty weight and empty weight CG which is nothing like that sort of accuracy. If the aircraft has been weighed by someone who knows what he/she is doing, and has exercised considerable care, you might, with a goodly bit of luck, get the empty weight good to, say, 5-10 kg and the empty weight CG good to 5 mm. On the other hand, I've seen quite a few a lot worse than that ....

How long ago was it weighed and how much rubbish has accumulated in the bilges and elsewhere since ?

Then, when you start loading it, how accurately do you calculate all the stuff you put into the aircraft, including the occupants ?

Are you starting to see a picture, here ?

If you have reasonably large tanks, you can invest in an hydrometer for a few dollars to check the specific gravity. If you really want to play with it, you can check the fuel temperature and run the appropriate sums. When you are measuring the volume, how closely can you reproduce the level which related to the last fuel calibration the maintenance guys did ? And it just goes on and on.

So what do I do ? I'm an experienced pilot, experienced consulting engineer, and regarded as one of the leading weight and balance guys in Oz.

I figure the volume as accurately as I reasonably can, considering the earlier comments. Either I check the hydrometer reading (yes, I have one in the bag) or, more easily, check with the local fuel supplier what their figure is (that is relevant to what they pay so they keep a good eye on things). Your answer will have an error, certainly, but the error will be within the noise of the total weight value for the aircraft and it just isn't worth worrying yourself too much about the last gram.

I see that my colleague, earlier, has made similar observations. He, also, has an engineering background.

[Pilot DAR]

I have found over the years, that aircraft weight and balance reports are often the most likely documents on board the airplane to have errors. I have done flight testing where precise weight and balance (including considering the specific gravity of the fuel) was a requirement. The first thing I did was insist on a reweigh of the aircraft, which I witnessed. Then, although a qualified maintenance person would calculate and complete the weight and balance report for the airplane, I would calculate my own. If they did not agree, we'd discuss. I've had a number of discussions over the years...

Your first step in assessing the potential accuracy of the present weight and balance report for an airplane is to see how it was created: Was it created by an actual weighing? Or, was it calculated? When a radio was changed out for a new one, was the plane reweighed? (usually not) Or the difference in the radio weights simply calculated? I ferried home a newly purchased Cessna 182RG for a friend years ago. He had new radios installed. During the process, old and unused wiring from a pre previous avionics installation was removed - 18 pounds of it! 'Bet that had not been considered in the calculated weight and balance I used to fly it home!

During a proper weighing, an airplane's fuel tanks must be either completely empty/unusable fuel correctly considered/full, and specific gravity considered, engine oil quantity accurately considered, the plane should be clean in and out (gunk under the floor), seat pockets, baggage compartments, and glove box all empty, and the airplane conforming to an equipment list, so what's being weighed is the configuration it'll be flown in: things like removable heated windshield - there? Or not? What type of battery (some weigh different for the same size), what tires (different plys different weights) etc.

Then, as yo fly it, have you weighed what you're carrying? The towbar and extra oil in the baggage compartment? The collection of things in the seat pockets, the headsets etc.

It's great that you want to be right on, and I will never suggest deliberately flying the airplane out of W&B limits. But, I can also say that during some W&B critical flight testing, the airplane is deliberately tested 5% out of limits, and some Cessnas, under some circumstances are approved for overweight flight. My most important comment will be, that if you think that you may be flying with an W&B error, assure that it's not a "too far aft" C of G error, that's the dangerous one. If you're within C of G limits, and accidentally 1/8 fuel capacity too heavy for a cross country flight, having tried to be right on, that will not become a point of regret for you later.

Sometimes your best still can't be perfect, just do a really good "best"!

[IFMU]

https://www.sportys.com/universal-fuel-gauge-26-in.html

Here is one with Seneca II markings.
https://www.aircraftspruce.com/catalog/pspages/piperfuelstik.php

Knowing that most wing tanks are more or less rectangular, a wooden dowel might be close enough. Dihedral might mess with that approximation though.

[Pilot DAR]

Yes, the position of the airplane on the ground, and wing dihedral can fuss the accuracy of dipsticks. Dipsticks are a 100% certain means of knowing your fuel quantity, provided that the dipstick is accurate. I have never seen an "approved" dipstick. I've seen lots of different dipsticks, and some very good ones, but no one actually taking responsibility for approving one as a "flight instrument" which a fuel quantity indicator in the cockpit is approved to be. The only dipstick I trust, is one I have made or checked myself. This, by leveling the airplane on the ground, and filling a tank from empty, pausing along the way to mark the dipstick at specified quantities, as pumped in.

Similarly I like to personally assure myself of the accuracy of any fuel quantity indicator I rely upon. This includes those of the original airplane, and very certainly, an STC'd aftermarket system. At my client's request, I had the shop install an EDM930 engine monitor in his 182 amphibian - it included fuel quantity indicators. The fuel tanks were newly overhauled, and their capacity confirmed by the fuel tank shop. The fuel quantity transmitters were new, and those specified for the EDM930. I took the plane to the pumps with the avionics tech, to do the initial calibration of the quantity indicators. We calibrated to within a liter. Everything seemed perfect. We also calibrated the "K factor" of the fuel flow, and I did all of the flying to validate the fuel consumption - it was right on.

I returned from on of my test flights in the plane, noting that the fuel quantity at landing was about 3/8ths on both tanks, both by the bar graph, and the numeric indication. The next afternoon, the avionics tech said to me: "You'd cut the fuel pretty close last flight!" Huh? I'd been indicating lots when I landed.... They had filled the tanks that morning, and from that quantity, I had landed with twelve liters usable!!! I go the ugly look from Transport Canada when this was discussed. I should have dipped the tanks to validate the fuel quantity..... I asked the Transport Canada pilot if he was aware of an approved dipstick for the 182, or anywhere in the flight manual where it tells you to dip the tanks to assure quantity - he was unaware of either, which I knew before made the point. And, I pointed out, that it was a completely STC'd fuel indication system, installed and calibrated as specified. Add to that, dipping the tanks of an amphibian, either on wheels, or floating, is not easy, nor particularly safe without a special set of steps or platform. But, the problem remained - the quantity was indicating way wrong, and we could not deliver the plane.

We recalibrated, everything had been right, and was right again. I observed that the fuel flow indication, and thus numeric fuel consumed were right on. So I always knew how much I'd used. What I began to notice that the plane "made fuel" in flight. I could take off with 300 liters, and after 90 minutes of flying, the sum of fuel burned, and numeric fuel remaining in the tanks was nearly 400 liters - something was not working. I phoned JPI, who made the EDM930. I asked if there was an internal check such that the sum of fuel burned, and fuel remaining in the tanks could not increase in flight - crickets on the other end of the line.

We manually recalibrated the fuel quantity to indicate correctly, by "lying" a little as we added fuel. After that, it was pretty close, and erred on the safe side. I still advised the owner to rely more on the fuel flow and burned than indicating. I flew the plane more than 100 hours after, and with the owner, and our manual calibration had been fine. But I was always, cautious, 182 amphibs glide poorly!

When I flew the 310, I learned that the factory fuel indicators for the main tanks were right on, that was reassuring. But, no matter the system, even dipsticks, I self assure before I trust!

[mirkoni]

Not (at all familiar) with the type but my understanding is that it accomodates for five or six passengers. Do you mean overweight with 2 pilots + pax or just the two of you ?

Two pilots + full tanks was calculated to be a little overweight... airplane measured on scales previously, got BEW from the maintenance organisation...

[9Aplus]

Can explain what we are doing to solve the fuel quantity problem on our oldtimer MS880B Rallye
Bad news:
Low wing, huge dihedral, no dipstick on earth can measure that..
No flow meter or fuel computer.
Good news:
Tank transparent sight gauge tube with small ball float, present on the side of the cockpit.

But indication/measurements are never the same, because aircraft is never in ideal
horizontal position on the both axis.
So what we are doing?!
We are using digital inclinometer and drawing cross calibration/correction diagram X-Y on *360o with known
near top quantity of fuel in both connected wing tanks.
*Turning the plane full 360o on the same position.

Experience shows that such diagram is than usable to any quantity
bellow, down to unusable fuel minimum.
Read the sight
Measure the X angle
Measure the Y angle
read the diagram, find out the "real" fuel quantity

Hopefully, may help to someone ;)

PS we are considering accurate fuel flow meter with ultrasound technology.
Mean no obstruction of fuel flow. The search is ON.
One known good solution, cost more than 11k GPB for sensor only!

[Less Hair]

We had wooden dipsticks with gallon markings or used fingers to verify the tanks being topped off. Worked well enough.

[IFMU]

Here is my calibrated, backlit fuel tank.
https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/1152x648/img_20210406_220004739_b0dbe45e4283bdaf906a013893df064d0005f 0fe.jpg

I can view it through a camera on a display in the cockpit. Ground on left, flight on right. It's a tailwheel airplane.
https://cimg6.ibsrv.net/gimg/pprune.org-vbulletin/401x726/img_20210406_220016046_2d4d20239ae3c339ed45dae4816929fe2de64 61e.jpg

Calibrated in gallons.