There is a fundamental reason we perform preflight run-ups and engine checks before takeoff: It is a whole lot better to find problems at 1G, 0 feet agl and 0 knots airspeed than it is while airborne. Making sure a power- plant will work as we intend before taking off is just good airmanship. A good run-up doesnt mean every- thing is perfect, however, and we train for airborne engine problems, including full use of its controls and instruments. Sometimes, though, the problems were looking for dont reveal themselves when it is convenient for us, and we have to diagnose engine issues in the air. Urgently. And fly the airplane at the same time. It is not a comfortable experience.