Which appliance must always be replaced, rather than merely cleaned and inspected after a major internal engine failure?

The oil cooler and prop governor must always be replaced after a major internal engine failure due to their susceptibility to contamination by debris and metal fragments generated during such an event. These components play critical roles in the engine's operation: the oil cooler is essential for maintaining proper oil temperature, and the propeller governor regulates engine speed.

After a significant failure, the risk of residual debris damaging or obstructing these components is substantial, which can lead to further complications or failures in the engine system. Cleaning these parts may not guarantee the removal of all contaminants, potentially putting the entire engine operation at risk. Thus, replacement ensures the integrity and reliability of these essential components, thus maintaining safe and effective engine performance post-failure.

In contrast, the other options involve components that may be thoroughly cleaned or inspected without the same level of risk associated with contamination. For example, while the fuel pump and oil filter are crucial to engine function, they may not necessarily require replacement as a routine practice after an engine failure, provided they can be adequately inspected and cleaned.