What check needs to be performed before starting a piston aero engine after ensuring the ignition is "OFF"?

Performing a check for engine hydraulicing is crucial before starting a piston aero engine, especially after ensuring the ignition is "OFF". Hydraulicing occurs when fuel or oil accumulates in a cylinder, preventing the piston from completing its cycle and potentially causing serious damage if the engine is started in that condition. This check is an essential safety and maintenance procedure.

By checking for engine hydraulicing, the operator can mitigate the risk of engine failure or damage at startup. This involves ensuring that there's no liquid present in the combustion chamber that could lock the engine and cause catastrophic failure upon ignition.

In contrast, while checking the pilot's flying license, ensuring permissions from the Tower, or assuming no further checks are necessary could seem relevant to pre-flight checks, they do not directly address the immediate operational safety concerns related to starting the engine. Hence, prioritizing checks for engine hydraulicing is critical to maintain the integrity and safety of the engine operation.