stuff like performance changing according to engine hours, wear and tear, random engine or magneto/pump failure should all be there. The idea of the percentage is just to find a balance in the alleged lack of accuracy. You wouldn't know what plane you have, whether it performs as it's supposed to or not, just like in real life, and this could iron out the "perfomance to chart" issues in a compromise form that might find everybody happy. We all would have the same fair chance of a failure or a particularly performing / not performing plane (within minimum percentage of course, which would be so marginal not to make it worth to restart or do any other silly choice).

I don't see why you guys immediately point at it as a whine/cheat factor. It's not the mentality that would help us develop further. Whiners can be politely asked to either take the rule or find another game (since we're aiming at accuracy here), while cheats can be solved by giving a limited number of planes available, which you have to take care of and manage like the real thing, instead of yanking it in the air like an air racer.