Spark plugs should be removed for inspection and servicing at the intervals recommended by the manufacturer. Since the rate of gap erosion varies with different operating conditions, engine models, and type of spark plug, engine malfunction traceable to faulty spark plugs may occur before the regular servicing interval is reached. Sep 10, 2013 · “The typical TBO recommendation on a Hartzell constant-speed propeller is divided into two parts: a six-year calendar limit and/or 2,400 hours of operation—whichever comes first. (Yes, there’s a TBO recommendation for fixed-pitch props, but we’ll get to that later.)