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For engine fire, the reason to close the packs (affected side) is to prevent smoke from filling up the cabin? But why the Isolation Valve and APU Bleed Air need to be closed too?

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    $\begingroup$ Might be to stop the flame propagation to the APU, through the bleed duct. So, closing the bleed would essentially stop the flame propagation to the APU. $\endgroup$ – Salmonbeing00 Jul 26 at 11:35
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Closing the isolation & APU bleed valves keeps hot bleed air away from areas near the engine that may have damaged bleed air ducting. Closing the Pack valve causes the other pack to switch to high flow, making up for some of the lost output of the affected-side's pack. The steps are together in the checklist because it's all on the same overhead panel, even though the reasons differ.

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  • $\begingroup$ Closing the pack valve would also provide some additional containment of smoke, fumes, and flames if the isolation valve fails. $\endgroup$ – Sean Jul 27 at 0:25
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I don't fly B737 aircraft. I would imagine the APU bleed air is used to start the engine. The other way is to probably use a huffer cart.

If there is an engine fire, that smoke can travel the same path back to the APU and then that smoke can be brought into the cabin through the other pack. So the idea, is to prevent any smoke from entering the cabin either directly from the engine or indirectly through the APU.

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  • $\begingroup$ If the APU bleed valve is open, it's so that the running APU can supply bleed air. The bleed air FROM the APU would have far more pressure than smoke merely "in" the duct. In fact the bleed air from the APU would clear smoke out of the bleed duct pretty quickly. No, the issue is avoiding the (smoke-free but very hot) bleed air from the APU getting to places that have been possibly damaged by the engine fire. $\endgroup$ – Ralph J Jul 27 at 2:16

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