QRH procedure for "PACK" of B737-800

[heart1]

In the even of both packs tripped off, according to QRH, after we descent at or below 10,000' , we're instructed to put Pressurization mode Selector to MAN, and fully open the outflow valve to increase airplane ventilation.

Anyone knows why then we turn R recirculation fan switch to AUTO, and L recirculation fan to OFF? If we are to open the overboard exhaust valve, aren't we supposed to turn the R recirculation fan switch to OFF instead of the L one?

[DongJoon Choi]

I don't know the answer to your question, but the following statement (extracted from AMM Part I Chapter 21) suggests that the issue is smoke control:

"The air conditioning overboard exhaust valve reconfig cont circuit breaker is for unpressurized dispatch. If the circuit breaker is open, the bypass circuit enables the recirculation fan to operate if the smoke control relay R648 energizes. The smoke control relay is energized during these conditions:

* Left or right pack switch is in HIGH

* Airplane is in the air

* R RECIRC FAN switch is off.

See the overboard exhaust valve system for more information about the smoke control relay. (SECTION 21-27)"

As I mentioned above, R/H recirculation fan can operate even though R/H RECIRC Fan switch is turned off

Refer to DDG21-01-01-06(M.O)


OEV automatically fully open during unpressurized flight(differential press less than 1psid) when R/H fan switch is in AUTO

Regards,

PS
I have been also researching and digging the question like you.

[heart1]

Thank you for your reply. Think the OEV is working under normal mode instead of smoke removal mode. The abnormal procedure seems to make more sense to me now. Many thanks.