912 ULS fuel pump pressure

Hi David,

RV advised, to RV 12 owners, to fit a higher pressure fuel pump, that addresses a very similar fuel delivery problem in my Sonex, I replaced a 5 psi Facet back up pump, with a 7 psi.

I routinely see the pressures you report. 

The fuel return line restrictor jet will, to some extent, govern your fuel system pressure. From my perspective your pressures are good. Fuel pressure sensors, as fitted to most small aircraft, are not precision equipment - as long as you are comfortably above minimum pressure, at full throttle - all should fine.

If you continue to be concerned about "high pressure" consider fitting a larger (higher flow) restrictor jet to your return line. WARNING this may cause low pressure at partial throttle settings.

As I understand Rotax advice - Fuel pressure recomend to Max 7 psi to prevent the float chamber valve from being overcome (leaking under pressure)

5-6 psi isn't an issue. If you start going up over 7 psi time to take a look and figure out why.

The mechanical fuel pump, like most diaphragm type fuel pumps, is designed to limit the pressure it provides to a level which won't overcome the carburettors' float valves. P/N 893115  (introduced in 2013) is the current design of pump, so the 0.5 bar / 7.26 psi limit in the 912 OM is the one applicable to your pump. It might be worth asking Evektor if it's permissible to change the gauge marking on your aircraft to the new limit.

On my aircraft - which does not have a fuel return line - I've been using the new type pump since 2018 with no issues at all. (I've no idea what my fuel pressure is - I don't have a fuel pressure gauge to check it with.)

There's a very good article which explores the construction and operation of the 912's mechanical fuel pump here on this very site:

https://www.rotax-owner.com/en/rotax-blog/item/77-rotax-912-fuel-pumps

(Illegitimate of a system, just lost my post, forgot to copy before posting)

Tim,

You are correct however failed to mention the return lines other, important function - reducing the risk of  the fuel system developing "Vapour Lock" (VL) otherwise known as fuel vaporisation.

Rotax did not recomend the fitting of a fuel return line to early Rotax 9's (carburetted engines) and as you state, they run just fine without this feature.

Unfortunatly Rotax 9's are prone to VP in hotter climates/seasons, usually after the engine has been flying (got hot) and restarted after a short ground spell (insufficient time for engine & systems to return to ambient temperature).

Vapour/bubbles form in the fuel, due to high undercowl temperatures. The vapour inhibits fuel movement and can not be effectively used by the engine to deliver power - can be difficult to start and/or dies shortly after hot start.

The return line allows the vapour to be drawn off and be replaced by relativly cool liquid fuel. The cool fuel also cools the hot fuel system.

Rotax, recognising the potential risks, now recomend the fitting of a fuel return (to tank) system

From my perspective, there are is no "downside" to the fitting of this simple system, just a relativly small plumbing job.

Part of my pre engine start practise, is to turn on my Boost Pump (BP) - Watch for the expected pressure rise (inferring good fuel  flow).

If hot day & BP rattles, as if no fuel, followed by a slower than usual pressure rise, I suspect VL.

Pressure will rise as vapour & bubbles are forced into the return line, out of the system, back to the tank. Cool liquid fuel replacing the hot vapour, right through to the carburettor float bowls. To assist in this, I run my BP during start,taxi, run up, do not turn off until safe altitude has been reached (normally turn off before start and on again during run up & TO/Climb OUT).

A good instant hot start is almost assured however the system has warned me to be alert to a potential power loss/engine fail on TO and even to Climb Out - have a plan!😈