I was reading the APS-3200 training manual (pg 268) and faced this part:
What's the starting envelope of APU?
What does mean "Normal start throughout the operating envelope: minus 300 m to 11900 m (minus 1000 ft to 39000 ft)"?
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The starting envelope determines the range of altitude (and possibly temperature) at which the APU can be started in flight. In your case, the manufacturer specifies that the APU can be started at altitudes between 1000 ft below MSL and 39,000 ft above MSL, which is also the operating envelope of the APU.
Here is an APU operation and restart envelope of an Airbus A320:
The cyan range is the operation and restart envelope (as a function of altitude and temperature). This APU model can only be used for bleed air and electrical power up to 22,500 ft altitude. Above, it can only provide electrical power. It also says that the battery is capable of starting the APU below 25,000 ft.
At too high an altitude, the starter motor cannot spin the APU fast enough at light off to prevent thermal damage (insufficient airflow due to low air density, due to altitude); there will come a point at which insufficient cooling airflow is available internally during the start process, during bleed air use, or for cooling of the APU generator, each of which may have different altitudes (start, electrical use, bleed use).
During the start process, the APU may experience a hot start, or failure to light off due to insufficient airflow.
The limitations in the manufacturer manual for the APU may be different than the limitations in the aircraft manual for which the APU is installed. Ultimately, the limitations that apply are those of the aircraft manufacturer, which takes into account the APU manufacturer limitations, as well as aircraft limitations with regard to installation location, etc.