Originally Posted by
achimha
That "in this case the oil pressure will be normal so it is likely just the gauge" case seems to be a standard thing that PPL students learn. I'm not convinced it's a smart thing to teach students. How likely is it and what is the chance of the pilot drawing the wrong conclusions because he remembers that the oil temperature gauge is very likely to fail?
Another case of rough running engine that most pilots are probably not aware of is mixture too rich. In hot and high conditions, you can often kill the engine by setting the mixture too rich. From my experience, students learn that in case of engine roughness, the mixture should be enriched.
The oil temp gauge failing is more likely then the engine actually suffering a catastrophic internal failure. It would be a pity if a student on seeing a failed gauge, wrote off an airplane on an unnecessary precautionary landing, an event I know has happened. If the oil pressure is normal there is essentially no way you can have an above redline oil temp. Obviously you still land at the nearest suitable airport though.
This does beg the question of what is "normal" oil pressure and temperature. Another area that usually isn't addressed in flight training. Oil temp and oil pressure effect each other.
I you have just started your engine on a cold morning it would be normal to have the oil pressure every high, probably close to the upper redline limit with the oil temp at the bottom of the gauge. As the oil warms up the oil pressure will slowly reduce to a more normal mid gauge (ie 75 ish PSI) indication. However if you restart the engine after a short break on a 30 C day, you will should a very high oil temp and the oil pressure will probably be in the yellow arc at low RPMs. Therefore a low oil temp at start up on a cold day with low oil pressure or a very low oil temp after a short shut down on a hot day should be of concern.
I agree with your comment about the mixture which is why it is addressed in my checklist. Since the problem could be too lean a mixture as well as too rich a mixture it is best to start with full rich and then lean to maximum RPM.