View Full Version : Fuel computer under reading
Last week I did some training for night rating flying more than 4 hours on the tanks. After refuelling I find out that:
- the amount of fuel on the bill was 146.36 litres = 38,67 US Gal
- the indication on G1000 of my C172 was GAL USED: 34.6
The difference is 11.7% on the dangerous side, because the gauge is under reading.
Normally I fill the tanks full and set the fuel computer 3 gallons less (to 50 gal instead of 53) just in case... For me it looks like to set the computer to 53-11.7%=46 USG is the "first aid treatment" for this problem. Would be better to find a real solution. Ideas? Is the gauge adjustable?
Do you have similar experience?
Miroc
Genghis the Engineer
3rd Apr 2011, 09:58
Inline fuel flow gauges are almost invariably wildly innacurate - 11% out I'd say is actually pretty good.
Use the manual, plus safety factors. Anything in the cockpit of a light aeroplane can only be regarded as indicative
Also do bear in mind that just because the G1000 is pretty advances, it doesn't change the fact that the sender in the fuel line is probably 30 year old technology and cost a couple of dollars.
G
englishal
3rd Apr 2011, 10:03
You have to calibrate them over time by adjusting a constant (K factor) based upon how much fuel is ACTUALLY used and how must is measured to have been used. If one takes the time to do this properly then accurate readings can be achieved. It takes a number of flights where the fuel is measured by the meter and also by refueling, then a formula is used to adjust this factor which is entered into the fuel computer.
You have to calibrate them over time by adjusting a constant (K factor) based upon how much fuel is ACTUALLY used and how must is measured to have been used.
......then a formula is used to adjust this factor which is entered into the fuel computer.
Thank you, this is interesting. I expected that the flow meter will be adjustable. However, it is probably not user adjustable. What will the shop need to know as I will go for the ARC renewal in October?
Miroc
I think the G1000 installations use a fuel totaliser which in GA is usually a Floscan 201 turbine transducer, feeding what is basically a pulse counting instrument. A bit like this stuff (http://www.peter2000.co.uk/aviation/misc/fuel.pdf).
What I found curious is that not one of the pilots I have ever spoken to who flies behind a G1000 knew anything about the system. So maybe the totaliser function is optional? But if it is missing there is no way the system should be showing what appears to be a "fuel on board" figure because it will be way off no matter how derived.
There are also installations which never read right. These normally result from the transducer being in the wrong place, or the wiring being unshielded and picking up e.g. ignition interference. Example (http://www.peter2000.co.uk/aviation/shadin/index.html)
Assuming a proper totaliser is installed, you indeed have to calibrate the instrument for the actual "K factor" (as Shadin call it) which you determine according to the pump fillup, following starting with the tanks topped-up accurately to the brim, some flying, and another topup to the brim. You also need to do it at a pump which is regularly checked by Trading Standards, which rules out a lot of airport bowsers and rules out many fixed pumps in N Africa (i.e. anywhere south of the Alps or the Pyrenees) as those pumps tend to be adjusted for a few % gain to the airport.
It cannot be done legally by the pilot, which is a massive irritation because one usually has to do it several times to get it just right. I used to fly to Air Touring to get it done :ugh: In reality most people with the old instruments just do it themselves. On the Shadin instruments it is either rotary hex switches or a front panel adjustment (and you need the easily obtainable installer manuals). I have no idea what it is on a G1000; presumably it is in one of the installer pages. I have very recently acquired a ton of G1000 install manual PDFs from some kind soul but haven't bothered looking through them....
A and C
3rd Apr 2011, 12:56
On the G1000 it is adjusted on the maintenance pages, for this you will have to get hold of the instalation manual and (if you want to retain your sanity) an eight year old to make the adjustments.
A correctly calibrated Floscan unit can work to + - 2%. It took me a number of adjustments but mine is very good on Mogas, but not quite so good for avgas.
Rod1
The Floscan 201 transducer should be "to spec", or nothing. The "pulses per USG" is engraved on the side; e.g. 298 means 29800 pulses per USG. This is for avgas. There is no reason why one should get a different figure unless there is something seriously wrong with it and then there will probably be no output at all.
There is no "calibration" whatever on the transducer. The calibration is in the displaying instrument, but in any decent installation just setting "29800" (or whatever the engraved figure is) in there should give you good accuracy; within 1-2%.
Floscan sell them for about $100 to Shadin who sell them for over $600 with an 8130-3, and claim to test each one on an avgas flow test fixture.
The exception to the foregoing is where there is flow turbulence or interference on wiring, as described in one of the articles linked earlier. This can give varyingly weird results, and many people, and aircraft dealers too, have (in ignorance) tried to compensate for that by setting a fudged K-factor.
IO540
The instructions for my unit gave a K factor and a note to say that calibration was required for most installations. Having used them in a professional environment in a previous life, the company had to calibrate each one individually (this was not in aviation).
Rod1
Jan Olieslagers
3rd Apr 2011, 20:21
Anything in the cockpit of a light aeroplane can only be regarded as indicative
Thanks for the reminder. I feel like this "anything" includes many pilots, certainly including my poor self.
The instructions for my unit gave a K factor and a note to say that calibration was required for most installations. Having used them in a professional environment in a previous life, the company had to calibrate each one individually (this was not in aviation).
Perhaps yours was a non-certified unit.
Shadin told me that most Floscan 201Bs they process come out around 29800 (IIRC that was the figure).
The one used in a G1000 aircraft would be a certified unit, with the K marked on it.
But as I wrote earlier these things are very sensitive to turbulence. The inlet pipe has to be straight (or have a very gradual curve at most) for the last 1-2ft, and it seems that a lot of installations ignore this. The result is a reading which is ~ 10% out, but the error varies.
Mine was a certified Floscan 201. The info with it said something about calibration being necessary to cope with installation variables. The unit was 10 – 12% out initially but has been consistently accurate (2% ish) on Mogas for several years now I have calibrated it. It was time-consuming getting it absolutely right.
Professionally, we fitted over 800 of the things with remote links back to base. The data was compared with ECU info, fuel added etc etc for predictive health monitoring and fuel planning (non aviation application). All the units required calibrating, but were fine for some considerable time after.
Rod1