The reporter has raised a safety concern regarding the accuracy of the checklists being used by [Operator]’s Cessna 152 fleet. The reporter stated that the Cessna 152 Approved Flight Manual (AFM) contains a limitation during engine run-up that allows for a maximum drop of 125rpm during the magneto serviceability check, however the checklist used by [Operator] states that a 150rpm drop is permissible. The reporter is concerned that as a result of this discrepancy, the C152s of [Operator] are not being operated in accordance with the limitations of the AFM, and that this increases the likelihood of a latent fault not being detected prior to flight.
[Operator] has advised the ATSB that the Cessna C152 AFM states that the maximum allowable RPM drop observed during the pre-flight magneto check "should not exceed 125rpm" and therefore does not constitute a 'hard' limitation. [Operator] has further advised that the relevant Lycoming engine manual states that the pre-flight magneto check "...should not exceed 175rpm." Given that neither the Cessna C152 AFM nor the relevant Lycoming engine manual contains a hard limitation that must not be exceeded during the pre-flight magneto check, the [Operator] position is to adopt a 150rpm drop as the maximum permissible during this check on the C152 type.
[Operator] is operating in accordance with a CASA accepted operations manual. As the wording in the flight manual says ‘should’ not ‘must’ this is not a hard target. This therefore does not represent a regulatory breach and CASA will not be taking any action in relation to it. The Pilot In Command (PIC) has the final say as to the serviceability of the aircraft, and if to fly or not, the flight manual offers the opportunity for the PIC to check the serviceability of the magnetos at a higher RPM if doubt exists. As stated above, CASA does not view this a regulatory breach, and therefore not a safety concern.