The APU is required to start the engines and increase performance for take-off. Nothing like and extra 900kgs thrust just on the right wing only. If the APU is unavailable after engine start (no etxra thrust for example), I would believe it uses a reduced/regulated take-off weight then.
The
third right thrust lever operates the APU and it thrust for
taxi/take-off. The jet pipe for the APU can be seen at the end of the right engine nacelle.
In this little walk around video, the
pilot explains the APU on this AN24 (passenger version of AN26) ( at time 2:12)
