What is the maximum altitude for assisting engine start with the APU?

The maximum altitude for assisting engine start with the Auxiliary Power Unit (APU) on the ERJ 170/175 is 21,000 feet. This limitation is established to ensure that the APU can provide sufficient pneumatic pressure for the engine start process at higher altitudes where air density decreases. Above this altitude, the engine start capability may be compromised due to insufficient APU performance, which could lead to unsuccessful engine starts or other operational issues. Understanding this limitation is crucial for safe aircraft operation, especially in mountainous areas or during high-altitude flights where engine starts might be necessary.