AF3400 EFIS Fuel quantity

[DRead]

A couple of questions about fuel quantity. I have an AF3400 EFIS that has a disclamer on start up, "Do not solely rely on this instrument for aircraft fuel quantities". Do I need another quantity indicator, and if so can I just run another lead off of the same resistive fuel float sender to another gauge?

[Mike S]

I suspect the statement is motivated by a lawyer----but dont know for sure.

I also suspect that adding a second instrument to the same sender will make the readings goofy.

Do you also have a floscan or other form of totalizator?

How about a dip stick to actually check the fuel level when you do a walk around?

[Bob Martin]

Fuel quanity is an important question. I had to figure this out when I added a fuel totalizer. I had fuel gauges too. I looked around and saw other planes with electronic totalizers opting to NOT have fuel gauges so I wondered myself what was correct.
Fuel gauges show what the float or capitance reads.
Totalizers show what SHOULD be there based on math.
Looking in the tanks is actually what is there.
The story I think about is a fuel leak while flying, the fuel drains out, but the totalizer shows me to have plenty......not good. A set of gauges would reflect the leak....
You have to decide for yourself what you want in your plane.

[mikerkba]

DRead:

The AFS equipment reports both fuel gauge and fuel consumed info with great accuracy. There are two means of monitoring fuel levels with their equipment.

First, the fuel level gauges, which can be up to four in number, get calibrated by you during installation. I recall that you need at least 4 data points, (e.g. 5-5-5-6 gallon intervals in my RV7A), though many builders use 2 gallon intervals and may achieve better results. I did not find it necessary to do the 2 gallon intervals. The calibration procedure is very much like that required in the other EFIS systems. Start with an empty tank, do a "set for zero," add gallons, set for that amount, add more, set again, etc, until full. I have an alarm set 4 four gallons in each tank, and I've tested that by burning fuel to that level, switching from that tank, and confirming that 4 were remaining when I refueled.

Second, the fuel totalizer will "count" down the fuel consumed from full, or any other amount you report as "on board" at the start of a flight. (When you refuel, it takes 2 button pushes to inform the EFIS of "Fill Main" or "Fill All" after engine start. It will remind you if you forget to inform it because it notes the discrepancy between the fuel gauges and the fuel calculations. (See next paragraph.) I did not have to adjust the calibration on this instrument, as it was reporting within .2 gallons just as it came from the factory. The AFS EFIS (like other EFIS hardware) reports all kind of info off of this side of the instrument: fuel consumed since last refueling, fuel remaining at destination (if you are GPS interfaced), fuel required to destination, MPG in statute and Nautical, fuel burn rate per hour, etc. All data is reported to .1 gallons. I found this all quite remarkable, particularly since no calibration of the totalizer was needed.

On top of that, the two systems (fuel gauges and fuel "computed" to be on board are compared by the EFIS. If the EFIS notes a discrepancy, it gives you an alert (aural or visual, or both, our choice). So, I think this is a MUCH better system than any factory plane I've flown.

Regardless of the fuel gauge system you use, you'll find that the tank angle/float interface will not report the fuel levels with accuracy at 16 gallons or above. The float cannot "reach" up there. I think the capacitance type fuel tank sensors can be installed to so detect. I didn't use them on my RV7A since they are supposedly "confused" by mogas, which I run up to 50-50 with avgas. Other than that, the capacitance sensor, which replace fuel floats, reportedly work quite well. AFS and other EFISs can work with either the float or capacitance-type sensors.

The "do not rely solely" language is to remind us to visually confirm fuel levels during pre-flight.

Hope this helps.