Regarding the Bonanza accident, I think my questions would be:
- How did the instructor come to take off with one tank nearly empty, and the other full? There may not be a max fuel imbalance figure for the Bonanza, but it is very poor airmanship to take off with such an imbalance. He did check the fuel... didn't he...?
Even if the briefing given to the student was erroneous - all he had to do was turn the selector to the OTHER tank. How hard is that?
As far as fuel cocks on 150s jamming in the "on" position is concerned - one has to wonder at the efficacy of the maintenance the aircraft is receiving. It takes a long time for the cock to jam, and if it is stiffening up, it should be picked up on a 50 hour check or whatever they have in Oz.
I can't think of any good reason to turn the fuel off when the aircraft is not in use, simply as a preventative maintenance measure. Just maintain the aircraft properly in the first place.
I can, however, think of some serious consequences to forgetting to turn the fuel on prior to engine start...