Inadvertent Icing and Engine Breather Obstruction

[MEH]

I recently heard of a Flight Design which encountered inadvertent icing resulting in obstruction of the breather tube coming from the engine. This obstruction resulted in an increase in the crankcase pressure such that an oil seal was "blown" and the engine puked out most of its oil before landing. A low oil pressure alarm resulted before the engine could be shut down. This loss of oil and oil pressure resulted in suspected fatal engine damage.

In the "Certificated" world, stock engine breathers must contain a vent hole within the engine compartment for such a contingency.  (See attached photo.)  Should icing develop, the vent hole at the edge of the engine compartment would be a "safety", allowing the engine blow-by to be vented without increasing crankcase pressure should the end of the breather get clogged by ice. 

Similarly, fuel vents that hang out in the slipstream, and are susceptible to icing on their leading edges, have a hole on the trailing surface for just such protection. (See attached photo.)

Although the Flight Designs are not permitted "Flight Into Known Icing Conditions", it would seem to me that perhaps we should consider adding a vent hole to the engine breather at the bottom of the engine compartment as a point of safety should inadvertent icing be encountered.

Just thinking out loud.

[Jacques]

the 912 installation manual calls for a opening in the vent tube to get some warm air in.

just make sure the installation is 'by the book '

[Tom Baker]

You can simply cut a slit through one side of the rubber hose about mid way. That will put it in the engine compartment where it shouldn't freeze,  and the pressure will open the slit if the end freezes shut.

[Jacques]

exactly,,,,,, the engine being factory installed,,,,the slit ´ should ´be there,,,,

[FlyingMonkey]

Or you could just cut the vent hose a few inches shorter so that it terminates inside warm under cowl air instead of in the cold airstream.  It might get a little more oil on your belly on average, but it's like having "oil vent heating" on continuously. 

Also just thinking aloud.

[Roger Lee]

Rotax doesn't want any of their tubes out in the air stream and want them cut back inside the cowl. The outside air movement causes negative or positive pressure depending on its position which you don't want.

[DHeal]

"....Although the Flight Designs are not permitted "Flight Into Known Icing Conditions", it would seem to me that perhaps we should consider adding a vent hole to the engine breather at the bottom of the engine compartment as a point of safety should inadvertent icing be encountered...."

It is not just "Flight Into Known Icing Conditions" or "inadvertent icing" that is the concern -- flying along on a beautiful but freezing VMC day can also be of concern.  The crankcase vent spews out water as a normal by-product of engine combustion. When this exiting water gets cold enough near the exit point of the vent line it may freeze thus causing blockage of the line leading to increased crankcase pressure and potential failure of the crankshaft/crankcase seals.  I suspect that for the Rotax 900-series engines with its remote oil tank this increasing back-pressure would additionally interfere with the proper return of residual crankcase oil to the oil tank and possibly result in other internal oil flow issues.  

[Anticept]

I would be interested in seeing how cold that air and moisture could get, but I find it very unlikely to freeze over itself without being in arctic conditions. The oil circulating in the tank is easily 150 degrees on subzero F days, those are crankcase blowby gasses dumping out of that breather tube, and there is a LOT of air blowing out of that tube. Finally, the reason the cowling has that separation down there is that is where the formerly cold from the cylinder fins and radiator are exiting.

Whistle slots are cut into breather tubes on certified aircraft as a last resort, but you have to be in some pretty nasty ice.

That's not to discount the concern, the tube really should not be whipping around in the wind carelessly.