If an engine catches fire during start, what should be the immediate action?

The most appropriate immediate action if an engine catches fire during start is to immediately abort the start and secure the engine. This is crucial because the safety of the aircraft and crew is the top priority. Shutting off the fuel flow prevents additional fuel from feeding the fire, and securing the engine can involve shutting off the engine controls to prevent further ignition sources.

Continuing to motor the engine can unintentionally worsen the situation by allowing more fuel to enter the combustion chamber while the fire is already present. Increasing throttle to dissipate the fire is a dangerous and counterintuitive action, as it only risks escalating the fire. Activating fire suppression systems only without first aborting the start may delay critical actions required for a comprehensive emergency response and won't effectively address the immediate needs of stopping fuel flow and securing the engine.