[FUEL SYSTEM] Fuel Flow needle bug

Hello, I’ve tried to use the correct Simvar ENG FUEL FLOW GPH or ENG FUEL FLOW
PPH, but it doesn’t work. With ENGINE OFF, battery ON, Fuel tank selector set
to the fullest tank, boost pump ON, and throttle 1/4inch advanced, I don’t
have any Fuel flow. This is required to perform the engine start procedure. By
the way the fuel flow gauge is only working when the engine is started, not
before. For information: Using legacy fuel system, fuel flow gauge is working
great with engine OFF but it’s not the correct solution. We have to use the
modern fuel system. Do you have any idea please ? Thank you.

  1.  Name="ASOBO_INSTRUMENT_Needle_FuelFlow_Template">
  2.        1
  3.        Needle_FuelFlow
  4.        Needle_FuelFlow
  5.        ENG FUEL FLOW GPH:1
  6.        gallons per hour
  7.        0
  8.        26
  9.        200

Before the engine starts you will have NO fuel flow as this variable (ENG FUEL
FLOW GPH) is the gallons per hour being burned by the engine when running. How
can it display GPH when the engine is not running? This variable is not for
what you are describing.

Hello, thank you for your feedback. But What variable can be used to show
current fuel flow with Engine OFF ? Thank you so much.

Hello @Flysimware &
@PESIM_Thomas The Eng fuel flow gph simvar
will return a value even if the engine is shut off, launch a flight with a
c172 for exemple, cold and dark, start the battery, activate the fuel pump and
set the mixture to rich and set the throttle to 100%, you will see the fuel
flow gauge rise and the eng fuel flow gph (variable used by the
ASOBO_INSTRUMENT_Needle_FuelFlow_SubTemplate) increase, here’s an example from
my side :

This is the expect
behavior of this variable and the behavior that
@PESIM_Thomas want on his aircraft if I
understand well, I’m sorry that my suggestion on the [previous
fuel-flow-gauge-bug.html) didn’t solve this issue for you,
@PESIM_Thomas can you send me your package (
see [3) provide private
bug-or-crash.html) ) I will investigate it. Best regards, Yohan

Hi @Yoanito, Interesting, I was not aware. I actually tested first but I never
moved the throttle levers in my test. Having the fuel pumps on must be
circulating the fuel or how would there be any fuel flow. So this makes sense
now. Thanks for the info.

Hello @PESIM_Thomas After running some test
on your aircraft, I’ve found the issue. The issue here is that your aircraft
don’t burn any fuel and you don’t have a system to return the unburned fuel to
a tank, so when you activate the pump, for merely a second your lines are
being filled, but once they’re all filled, the pump will stop the fuel flow
and won’t sent any more fuel until the lines are empty again.
@Flysimware did have the right answer, my
example only work because I was using the C172 which use the legacy fuel
system, sorry for that. Here’s a video showing the issue : The flow quickly
increase until the 0.3 Gal limit of the lines is reached, then stop until the
lines are empty again. If you want to have a constant fuel flow without the
engine running with the modern fuel system, you need to make a system to
return any unburned fuel to a tank, to create a constant stream of fuel. Best
regards, Yohan

That is good to hear that the modern system and not legacy will NOT show fuel
flow/burn per hour once the lines are filled. I would expect this behavior
because until the engines burning fuel it should not matter if you have the
fuel pumps on or off the fuel is not flowing or burning based on an hourly

Yeah ive also noticed its impossible to set the fuel flow with a piston
engine. Not matter the simvar or unit, it just gets ignored. It doesn’t work
with input events either.

Hello @Yoanito After some test, I’ve create a
junction after engine to send the unused fuel to the tanks. But with Engine
OFF, boost pump ON, and max throttle, I don’t have the fuel flow. It doesn’t
work. Maybe a limitation from ASOBO. Do you have a solution ? Or it’s
definitely a limitation ? Best regards, Thomas - PESIM