Notching Drain Lines?

"I may have been using "vent" and "drain" lines interchangeably....  These are the fuel drain lines from the fuel pump, drip trays, and airbox.  Maybe they should not be notched?"

912 - The only hoses (drain/vent) that should be notched (or alternative) to prevent unwanted vacuum impacting on the attached system are;

Fuel Pump Drain

Oil System Breather Hose

Note; If not using an airbox, the Carburettor Float Bowl Breather/Drain pipes should be terminated in the same air pressure as the carburettor air inlet.

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Sean, I see in the Rotax 912ULS Installation Manual where it says the oil drain hose should be notched, but I can't find where it says to also notch the fuel pump drain hose... Maybe I'm just overlooking it?  Thanks, Mike.

Michael

The concept of a notch in any vent line is standard practice for aviation.  You can find it called out in the FAA AC-43-13 current issue for aviation repairs.  It is  massive document, downloadable for free from the FAA government web pages.  

Cheers

"Sean, I see in the Rotax 912ULS Installation Manual where it says the oil drain hose should be notched, but I can't find where it says to also notch the fuel pump drain hose... Maybe I'm just overlooking it?  Thanks, Mike."

As RW has intimated, above - you don't want a vacuum acting on a system that has not been specifically designed to have a vacuum.

The fuel pump is a mechanical device that contains a diaphragm and a crankcase seal - neither, in my opinion, should be subject to a vacuum.

Similarly, the oil reticulation system has been carefully designed to operate at ambient pressure. The introduction of a vacuum adds a variable that the designers have not compensated for. A vacuum, through the breather, is unlikly to cause a great problem - likly increased oil exiting the breather and being deposited on the belly of the aircraft.= increased oil consumption and cleaning effort

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Lets not get carried away with vacuum concerns here. Read the Rotax instruction for the notch, it only mentions it as an alternate opening in case condensation ice plugs up the end of the tube. Don't add more than what it says.