How can closing of the start air valve be confirmed during engine start?

Confirming the closing of the start air valve during engine start is best indicated by an increase in air manifold pressure. When the start air valve closes, it prevents any additional starting air from entering the engine's combustion chamber, which leads to a momentary increase in air pressure within the air manifold. This pressure increase signifies that the airflow dynamics are changing as the starting process transitions to normal engine operation.

The other options do not directly correlate to the confirmation of the start air valve's closure. A decrease in fuel flow typically indicates a different issue related to fuel supply rather than directly observing air flow changes due to valve positions. A decrease in exhaust gas temperature (EGT) is more indicative of combustion changes rather than confirming air valve status. Monitoring RPM can show the engine's acceleration but does not specifically reflect changes in air manifold pressure due to the closure of the start air valve. Thus, the increase in air manifold pressure is the most definitive indicator of the valve's closure during engine start.