1-If the fine filter plugs up, the 1 bar spring-loaded check-valve will open allowing unfiltered fuel to the engine. it may cause roughness, and eventual power loss but it should be a gradual & noticeable and get you home.
2- If you do not have a spring-loaded checkvalve the filter can plug up and the engine will stop.
3- If you don't have a checkvalve the plugged filter will cause more and more resistance and the fuel pumps will draw more and more voltage until the B generator cannot keep up. Then you will get a Gen B failure lamp and the ECU will attempt to switch to Gen A. As Gen A cannot provide this amount of current you get a double generator failure.
4-If you do not have a spring-loaded checkvalve: As the filter plugs up the fuel pressure will rise to the maximum the pumps can put out (13 bar/180 psi!). No problem for any proper hose and fitting assembly but still not a good situation.I have looked at some aircraft and the hose and fittings look to be poor quality; I wonder if they are rated for 13 bar/ 180 psi?
Why does the 1 bar spring-loaded checkvalve not open when the system pressure goes to its normal 3 bar operating pressure? As the filter and checkvalve are in parallel the pressure in the checkvalve must exceed the pressure in the filter by 1 bar before the ball opens.
You should have a fuel pressure sensor installed BEFORE the fuel filter. Then you will see a RISE in fuel pressure as the filter becomes plugged and you will know you need to replace the fine filter. Some of the engine instruments have a "change filter" warning" when it detects a rise in fuel pressure. I know the Stock Flights Sytems EMU and Dynon Skyview have this feature, no experience with the others.
Also: do not put the fuel pressure sensor after the fine filter (I know this sound more intuitive: lower pressure: Danger!) but this location has been proven to be less accurate and stable.
If you do not have this warning feature you should change your fine filter if you see the fuel pressure gradually go into the red (more than 3.2 bar). If you have a proper sized fine filter (with lots of element surface area) it will give you lots of time to see this upwards trend.
