What must be done prior to shutting down the APU?

Turning off the air conditioning systems is essential prior to shutting down the Auxiliary Power Unit (APU) to prevent potential damage to the APU or the aircraft's systems. The APU generates power and provides air to support various functions, including the air conditioning system. By disconnecting the air conditioning systems before shutting down the APU, you ensure that there is no sudden loss of air flow or pressure that could cause issues with the components involved. This procedure promotes safety and system integrity and ensures that all systems cease functioning in a controlled manner.

The focus on proper shutdown procedures, such as turning off the air conditioning systems first, aligns with standard operating protocols designed to maintain the longevity and reliability of critical aircraft systems.