Fuel pressure flactuations

[oren_rokach]

My fuel pressure , with fuel pump off, is fixed at 25 PSI. w/fuel pump at 27.5psi.
Very stable. my hazard is set at 24. and never reached this hazard.
Yesterday, while performing some manuvers, When staightening after climb (maybe just a bit less than positive 1G). Fuel pressure dropped to 20-21.
Is it normal? if mechanical pump is still operational (obviously), does it mean that fuel is not pressent at suction point at the wing?
Engine was stable, If i wasn't too strict about the threshold, I wouldn't even know that the drop exists.
Your thoughts? is it common?

 

[RVDan]

I have had discussions with Tempest on the mechanical fuel pump pressure fluctuations and their low limits. Their criteria is 20 psi on the low end. I was seeing 22 PSI on a new mechanical pump. I set my low pressure caution at 20 psi. The servo will function down to 15 psi. My Airflow Performance electric pump runs at 31 psi.
As has been noted in other threads, any leak in the fuel system prior to the mechanical pump will cause pressure fluctuations in the mechanical pump. I had a fuel selector valve leaking ever so slightly that would cause pressures to momentarily go low enough to affect engine operation.

 

[oren_rokach]

I have had discussions with Tempest on the mechanical fuel pump pressure fluctuations and their low limits. Their criteria is 20 psi on the low end. I was seeing 22 PSI on a new mechanical pump. I set my low pressure caution at 20 psi. The servo will function down to 15 psi. My Airflow Performance electric pump runs at 31 psi.
As has been noted in other threads, any leak in the fuel system prior to the mechanical pump will cause pressure fluctuations in the mechanical pump. I had a fuel selector valve leaking ever so slightly that would cause pressures to momentarily go low enough to affect engine operation.

Thanks!
And do you believe the thigs minor loss of positive G, can cause such loss?

 

[Avanza]

The fuel pressure transmitter compare the pressure in the fuel line with the surrounding air pressure and present this as Gauge fuel pressure. (psi (g).
Fuel pressure vill change depending on fuel temperature, bubbles in the fuel line and surrounding air pressure.
Taking of from low altitude climbing to high altitude with warm fuel may cause the fuel to boil in the tanks and bubbles in fuel line.
After 20 min or so at altitude the fuel will cool down and and the boiling stops.
My fuel pressure warning is set to 15 psi and the engine runs fine at 7 psi.
Use electric fuel pump if required.

Good luck

 

[RV10inOz]

Climbing through around 5000' all aircraft are in a zone of vapour bubbles being easy to form, especially with a hot fuel pump during a climb. And yes set your yellow range to 15-20PSI and red (alarm) at 15.

 

[RVDan]

Thanks!
And do you believe the thigs minor loss of positive G, can cause such loss?

The diaphragm type pumps displace a very small volume with each stroke. Any air, which is compressible reduces the pressure that the pump can develop.