Fuel Info On The JPI EDM Engine Monitors and Garmin GTN750s
There was a discussion at the club meeting yesterday about a "Fuel Qty Mismatch" warning that came up on the EDM900 engine monitor in 9RN. Some, dare I say, boisterous conversation ensued about what exactly we are being warned about when that pops up, and why it happens. Since some members seem to have a curiosity regarding how much fuel is contained in the airplane around them, I did some digging.
A little research and a call to JPI came up with the following info:
"Fuel Qty Mismatch" occurs when the measured fuel quantity in the tanks ("FUEL QTY GAL" display in 9RN and "FUEL QTY" in 26Y) is different from the calculated fuel remaining as displayed by the EDM ("REM" in 9RN and "Est REM" in 26Y). If these differ by more than 10%, it should trigger the warning.
The calculated fuel remaining is derived from the "REFUEL?" gallons number entered by the pilot on startup, minus the actual measured fuel that has passed through the fuel flow transducer to the engine ("USD" in 9RN, not displayed in 26Y) . Garbage in, garbage out, as those computer nerds say. If you don't stick the tanks and enter a good number at the "REFUEL?" prompt, you can ignore the "REM" and "Fuel Qty Mismatch" displays.
No fuel quantity or level (calculated or measured) is transmitted between the EDMs and the GTN750s, either direction. You can do fuel planning with the GTN but your fuel level must be entered manually, and maybe your fuel flow, too.
Fuel flow, measured at the transducer MIGHT be transmitted from the EDM to the GTN, depending on our particular installations. You may be able to see that connection on the "Utilities/Fuel" page in the GTN. I don't know if our systems are configured to do that.
Likewise, "distance to waypoint" and "distance to destination" MIGHT be transmitted from the GTNs to the EDMs, once again, depending on our installations. If this was enabled, the EDM will calculate how much fuel should remain when arriving at those locations and display them in the "scanner" section of the screen below the EGT/CHT bar graphs. Of course, this is also calculated from the number you put in when the EDM asks you "REFUEL?"
Thanks, Scott!
Anyone aspiring to be our heroic test pilot could experiment by inputting an exaggerated (very high or very low) fuel flow value at preflight, then see if you hit that 10% disagreement very quickly. That should tell us if the calculation is based on the "dumb" constant flow that we enter or a "smart" value coming from actual fuel passing thru the transducer. ...if I understand it correctly, that is.
Can anyone remind me (and save me from digging up the answer) how to update FOB in the EDM if you messed it up when first powering up?