What is the maximum altitude for starting the APU?

The maximum altitude for starting the Auxiliary Power Unit (APU) is FL370. This altitude is specified in operational guidelines to ensure that the APU functions effectively and reliably at higher altitudes. Starting the APU above this limit could lead to insufficient airflow and inadequate performance, potentially resulting in failure to start or improper operation. Aircraft systems are designed to operate within specified parameters, and knowing these limits helps maintain safety and efficiency in flight operations.