Fuel Pumps, Fuel Bypass Check Valve, Vapor Lock
Hello all,
I have yet to find a thread on this from previous, so I thought I would mention...
I have come to realize that the mechanical fuel pump on the 912 is very robust, but still offers a remote chance for failure as noted from previous threads. This mechanical diaphram pump is designed to push no more that 5psi to the carbs of the 912ULS. Any pressures much greater than 5psi can potentially push past the needle valves and jetting.
Currently planning a back up pump (boost pump) to operate at the same max pressure, as offered by LEAF Aviation. (Facet Electric fuel pump, #H7278, 2.5 to 4.5psi). I am aware that a boost pump is not necessary, but is a nice to have option in the event of a vapor lock scenario in the fuel line above the engine. I have been informed I could run this boost pump in line with NO secondary bypass and check valve, and be just fine. John McBean informed me none of his factory airplanes have a secondary boost pump, they are not needed on this gravity feed system.
Where the boost pump shines to my understanding, is when people will use auto gas with or without ethanol, that the likelyhood of vapor lock is greater. With a boost pump, the pilot has the ability to clear the vapor lock and keep the engine running. Same holds true for overcoming a the mechanical fuel pump from Rotax if it were to ever fail. The other net benefit as noted in previous threads, is the boost pump will fill the carbs' float bowls full of fuel for quicker cold starts.
So with all this on the table, the question I want ask is: What have the majority of builders installed in their projects? I am inclined to run a secondary boost pump, without the the by pass check valve. Thinking underneath the flap console, just after the fuel valve, and before the firewall. Boost pump from LEAF is a free flowing pump with no check valve, max pressure at 4.5 psi. I have been informed the likelyhood of this restricting fuel is highly unlikely on an in-line install. Installing a one way fuel check valve would be for redundancy.
Other considerations is under the seat pan with plenty of space, but if the seat pan were to ever collapse, it could rupture/compromise the hard fuel line.
In addition, to alleviate vapor lock in fuel line above the engine in high heat scenarios, I plan to run a t-fitting from the fuel line after the Rotax mechanical pump, and before the carbs, t-fitting installed preferably closer to the carbs to get away from the heat generated from the engine. The t-fitting would reconnect to main fuel line, forward of the firewall. Question here is, should this fitting and hose connection be of a smaller inner diameter than the fuel line to create a higher back pressure to the main fuel line? Or can this be of the same inner diameter as the existing fuel line. Any thoughts or pics of installs would be greatly appreciated!
Without a t-fitting installed in a high heat scenario, followed by parking the aircraft, it has been said this can create over-pressure within the fuel line, possibly flooding of the engine on restart, creating hard starts.
Thank you everyone, I think everyone needs to think seriously about best practices here!
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