Operating the APU

[safeflight_]

I saw some B733 CBT to get more detailed in the aircaft operation.

The APU can feed the packs of each side in AUTO or HIGH.

In which situations are the packs running in HIGH ?

The underpowered APU of the B732 could only feed the starting engine.

Is it allowed to start the engines of the B733 with one pack on AUTO ?

The IXEG can perform it with reduced pressure, but this does not mean it is allowed.

[Litjan]

You run the packs on HIGH when you need maximum cooling. On the ground with the APU feeding them, the HIGH setting gets an extra boost (ca. 20% more flow).

The APU can´t be "overtaxed" - it will reduce the bleed automatically to stay within EGT limits.

To start engines you will always turn both packs OFF - it is possible to run one pack and start, but it is not allowed. You need a minimum of 30psi pressure to start an engine.

You can (we called it Sommerschaltung) close the Isolation Valve once the right engine is running and use that engine to power pack 2 - while starting engine one. The 737 is notoriously bad at cooling in the summer, so every extra second cooling flow helps.

Cheers, Jan

 

[safeflight_]

Thanks for your first hand information.

Will operate the APU then like in real :)

Cheers, Cord