Ask Captain Lim

Good Day Captain Lim,

Thank you for your wonderful website and invaluable information that it contains. I would like to ask a question regarding piston engines.

May I know why the idle speed cannot be less or more than the specified range of an aircraft? For example, why can*t the Cessna 152 idle less than 500 rpm and more than 800 rpm?

Thanks

Kit Raakshan

Hi Kit,

Just like a car, the handbook normally recommends that the engine idling rpm should be between 700 - 800 rpm. This also applies to the Cessna 152 piston engine too.

In other words, the recommended 500 to 800 rpm is the optimum idling range. It solves the spark plug fouling, reduce valve guide wear and valve sticking problems.

For instance, if the idling rpm is too high, some fuel doesn*t vaporize and enters the cylinder as a liquid where it partially burns and forms carbon deposit.

The rich idle mixture (higher idle rpm) causes lead fouling. The less of carbon and lead stuff entering your engine at idle the better. The optimum idle setting is one that is rich enough to provide a satisfactory acceleration under all conditions and lean enough to prevent spark plug fouling or rough operation.

On the other hand, if the idling rpm is too low (below 500 rpm) the engine will run rough and eventually it will quit.