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Fuel Gauge Calibration: really level?

bill.hutchison

Well Known Member
I've got a nifty (and accurate) EI fuel computer on the airplane and I like it, but I'd like to have a proper fuel gauge. EI makes a nice one of these as well and I've got one on the way.

Reading the manual (because I do that sometimes) it indicates that the fuel gauge must be calibrated with the aircraft in a "cruise angle of attack."

Okay. I get this. But I also know that my cruise AOA can vary a few degrees, and I also know that the -6A sits a little nose-high compared to a level flight attitude.

So what I'm wondering is just how much effect this slightly-nose-high attitude really has on the fuel calibration. Is it significant enough that I really need to consider leveling the airplane to do this?

I'm happy to do this - I'm just wondering how much error it would introduce if I didn't.
 
If we were flying formation, I'd likely radio you: "Alpha 2: wiggle your toes!" Relax! :D

Your angle of flight will vary depending on multiple factors including baggage load, fuel loading....... I doubt any airplane ever flies "level". I would set your airplane about where it flies most of the time and calibrate it for that angle of attack. Fuel gauges are there for a backup reference anyway, regardless of their accuracy. Your timer/watch and fuel flow are going to be of more benefit more or less confirmed by what your fuel gauges are telling you. AND: doing it at your usual AOA will likely be easier............

IMHO..........
 
2 place RVs are assumed to be at level attitude for fuel calibration (and w&b) purposes when longerons at the canopy deck are level. And, yes, you should try to get as close as you can while doing the calibrating. This may mean putting blocks under the mains and/or deflating the nosewheel some. And, yes #2, level flight attitude varies slightly depending on airspeed and weight, but not enough to matter significantly for fuel level indication purposes.
 
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My 6A registers up to a couple gallons different fuel quantity in flight compared to sitting on the ramp (assuming I have less than about 15 gallons in the tank, fuller tanks than that doesn't matter)
 
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Best way of answering the question is simply to calibrate it as recommended at cruise AOA, then move the airplane up and down a few degrees and see how much (if at all ) the reading changes.

(On my RV-4 I just finished reprogramming my engine monitor to include a "tail-down" mercury switch input. So even on the ground it should show correct fuel levels (up to the fuel sender/tank limit -- with tail down you hit the limit at lower gallons.)

Finn
 
Fuel gauge calibration.........

When I was calibrating SuzieQ years ago, I had it level to the cockpit longerons and was putting fuel in the tanks in 1 gallon increments. (Yes, I am a little OCD.O) I did tail up and tail ground to note the differences (and to mark my preflight fuel quantity measuring probes). The gauge diagrams from that process are hilarious with red lines (indicating where the gauge was pointing) EVERYwhere! So they were closely calibrated with the old style float indicators. During the process I would let the gauges settle, then rock the wings to make sure they still read the same; they did.

Once flying, my flight log on my knee board is updated every 30 minutes with likely fuel consumption and switching tanks. I would look at the gauges and think 'Yeah, that's about right.' My reliance on fuel gauges which have a less than stellar reputation of accuracy meant less than what was written on my flight log.

Calibrate your gauges to your level of comfort. Cruise AOA and ground attitude to note the differences would be good information. Then rely on your brain, not on a gauge.:):)
 
Level flight attitude story

Years ago I purchased the CIES sending units and their recommended Hi Tech combined gauge for my 77 Cardinal 177B. In my attempt to be anal I filled many 1 gal milk jugs with 100LL to be poured 1 at a time for a total 50 gal capacity. Drained the tanks into 5 gal jugs to be transferred to 1 gal jugs and began the trips up and down a step ladder 1 gal at a time. Oh Yeah, before starting I rolled the mains up on ramps and then adjusted the nose wheel up/down to get level flight attitude. The gauge has two sides 1 for each tank. The problems came from adding a gallon run down the ladder enter the cockpit press right the button or buttons (before they timed out) and run back up the ladder REPEAT! Each time I poured it sloshed and each time I entered cockpit it sloshed! Now you gotta wait. Looked like a "Keystone Cops" movie. after several mistaken button pushing (Ooops Darn wrong side tank again!) Start OVER! One whole day and eventually it all worked perfectly! Loved it ! Swear by it! Hate ladders! :eek: Ed
 
Years ago I purchased the CIES sending units and their recommended Hi Tech combined gauge for my 77 Cardinal 177B. In my attempt to be anal I filled many 1 gal milk jugs with 100LL to be poured 1 at a time for a total 50 gal capacity. One whole day and eventually it all worked perfectly! Loved it ! Swear by it! Hate ladders! :eek: Ed

GREAT story! I can identify completely! And I only had 32 gallons to contend with! And no ladders!:D:D
 
When I was calibrating SuzieQ years ago, I had it level to the cockpit longerons and was putting fuel in the tanks in 1 gallon increments. (Yes, I am a little OCD.O) I did tail up and tail ground to note the differences (and to mark my preflight fuel quantity measuring probes). The gauge diagrams from that process are hilarious with red lines (indicating where the gauge was pointing) EVERYwhere! So they were closely calibrated with the old style float indicators. During the process I would let the gauges settle, then rock the wings to make sure they still read the same; they did.

Once flying, my flight log on my knee board is updated every 30 minutes with likely fuel consumption and switching tanks. I would look at the gauges and think 'Yeah, that's about right.' My reliance on fuel gauges which have a less than stellar reputation of accuracy meant less than what was written on my flight log.

Things I wish I'd added for context.

What you describe is exactly how I fly, and I keep both a stopwatch and a clock in the airplane to manage the tank switching and expected burn.

The burn rates in my PoH are pretty accurate, (I am one of the lucky ones who bought a flying RV with a pretty good PoH that the builder did) and I have the bonus of an EI fuel computer on board. It's very accurate on totals and flow....usually to within half a gallon if my receipts and fill experiences are any indication.

However, it's a fuel computer, not a fuel gauge, and thus, if I DO make a mistake on either when I switched tanks or enter the wrong input while fueling, then everything is off.

Then there's that whole 91.205 thing. My op limitations don't allow me any night ops without full 91.205 compliance and that means adding a fuel gauge. And I'd really like to have one anyway.

So - yeah, I'm with you that the clock and known burns are your best friend, and this is always what I teach my students. Plus a general best practice that landing with at least an hour of gas in the tanks is a pretty good idea.

And it's also why I'm less concerned about being super-mega-accurate on the gauge calibration. Because between the clock, the fuel computer and the good ol' Mk-1 dip stick, I feel pretty well-covered here. :)
 
Fuel Computer VS Fuel sending units

This is mostly a problem for High Wings but the reason I see for wanting to have accurate fuel gauges is if you leave off a fuel cap and the siphon effect takes most of your fuel in flight but the computer says you only burned 16 of 25 Gals that were available: HOW CAN IT BE EMPTY ?? :rolleyes:
 
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