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Fuel Flow DECREASE when Aux Pump On

samgaddis

Member
Anyone know why I would see a .5-.7 gph DECREASE in fuel flow when the aux fuel pump is turned on?

For what it's worth, I actually posed the question to ChatGPT4 and it gave the credible sounding answer below. But I still trust you guys more for now!

If you observe a decrease in fuel flow when the auxiliary fuel pump is turned on, it could indicate a few potential issues:

Auxiliary fuel pump malfunction: The electric auxiliary fuel pump may be failing or malfunctioning, causing it to impede the flow of fuel rather than augmenting it. This could lead to insufficient fuel flow to the engine, potentially causing power loss or even engine failure.

Fuel system obstruction: There could be an obstruction or blockage in the fuel system, such as debris or contamination, which is restricting the flow of fuel. This can also lead to insufficient fuel flow, which could cause engine performance issues or even engine failure.

Fuel pressure regulator issues: If the fuel pressure regulator is not functioning properly, it could be causing an imbalance in fuel pressure when the auxiliary fuel pump is turned on, leading to a decrease in fuel flow.

In any case, a decrease in fuel flow when turning on the auxiliary fuel pump is not normal and should be investigated. If you encounter such a situation, consult the aircraft's documentation, consult with an experienced aircraft mechanic, or seek assistance from a qualified aviation maintenance professional to diagnose and address the issue. Remember, it is crucial to ensure the proper functioning of the aircraft's fuel system to maintain safe and reliable operation.
 
Are you talking about a true fuel flow measurement (‘red cube’) or a fuel pressure gauge attached to the spider that is calibrated to read gal/hr? If the former, is your cube located downstream but close to the aux pump? It’s not unusual for turbulence just downstream of the pump to cause fluctuations in the FF readings. Mine reads a bit high. If your fuel pressure remains high or even a bit higher with the aux pump on I wouldn’t worry about it.
 
Collapsing hose?

Not a mechanic but First thought to me: is it possible there is a hose collapsing due to the increased suction of the electric fuel pump causing a decrease in the pressure to show down stream? I wouldn't think there is any soft hose before the aux pump, but maybe a dirty fuel filter?
 
Fuel or fuel/air mixture

If you have the red cube and the normal fuel flow has bubbles in it, this will
result in certain ft/min fuel speed, wheel rpm and flow reading.
When the electric pump is turned on, the bubbles disappears. Only fuel now.
When there is no air bubbles, the fuel speed ft/min will be reduced.
This causes the wheel to spin slower and the fuel reading will be reduced.

Air bubbles comes from pressure drop/ high temp in fuel lines, to many 90 deg fuel fittings, fuel lines without insulation, no air cooling of the mechanical fuel pump.

Good luck
 
If the red cube is installed between the elec pump and the mech pump, it is common to get erroneous readings while the elec pump is on. This is why the mfg recommends not putting it there.
 
Yes it's a red cube, but I will have to get a more detailed understanding of how it was installed. This gives me something to go on though. Thanks all.
 
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