What triggers the automatic engagement of the AC fuel pump 1 or 2?

The automatic engagement of AC fuel pump 1 or 2 is primarily triggered by the failure of the ejector pump. The ejector pumps are designed to maintain fuel flow from the tank to the engines by using engine or APU bleed air to create suction. If an ejector pump fails, the system recognizes that it can no longer rely on the ejector’s capabilities to transfer fuel effectively. As a result, the fuel system automatically activates the corresponding AC fuel pump to ensure there is a continuous and reliable flow of fuel to the engines, thereby preventing an engine fuel starvation scenario.

This mechanism ensures that the aircraft can still operate safely and efficiently even in the event of an ejector pump failure, providing redundancy to the fuel delivery system.

In contrast, the other options do not trigger the automatic activation of the AC fuel pumps in the same way. APU operation may use the fuel system but does not specifically necessitate the engagement of the AC pumps. Crossfeeding of fuel is a manual operation intended to balance fuel loads between tanks and does not automatically engage the pumps. Finally, while engine shutdown does create situations where fuel management becomes crucial, it does not inherently engage the AC pumps without a failure of another system like the ejector pump.