What is the maximum altitude for starting the APU?

The maximum altitude for starting the APU is 37,000 feet pressure altitude. This limit is specified as part of the aircraft's operational guidelines to ensure reliable APU performance. At altitudes above this threshold, the air density decreases, which can affect the APU's combustion efficiency and its ability to start. Operating the APU within the prescribed altitude limits ensures that it is provided with adequate airflow and fuel properties necessary for smooth operation. This is particularly important since the APU is crucial for providing electrical power and environmental control when the engines are not running, especially during ground operations at high altitudes. Understanding these operational limits is essential for safe and efficient aircraft management.