But they are just passive tubes, aren't they? Why would they not drain in the air just the same as on the ground?They 'vent' ...mostly OUT after engine shut-down, ON the ground....AFTER a flight. They drip stuff, so when a pilot does a "walk-around", he/she doesn't walk directly beneath, lest he/she get oil on his/her uniform shirt.
(The dry cleaning expenses are high enough, already!!).