B737 - Air Conditioning - Standby Pack average temperautre

[owl-attack]

Deal all

I have been having some trouble understanding the STANDBY PACK AVERAGE TEMPERATURE in the FCOM - AIR CONDITIONING chapter

What i understand about UNBALANCED PACK TEMPERATURE CONTROL MODE is: (please correct me if i am wrong)
1. Any failure which affects the supply of trim will cause the packs to operate independently
2. If the flight deck trim air is lost (failure of both primary and standby flight deck temperature control) the CONT CABIN ZONE TEMP light will illuminate and the left pack will provide conditioned air to the flight deck as per the flight deck temperature selector, and the right pack will satisfy the demand of the passenger zone which requires most cooling.
3. If a passenger cabin zone trim air is lost, or all trim air is lost, the right pack will produce conditioned air which is average temperature of the two cabin zones and the left pack will produce conditioned air at a temperature which meets flight decks temperature requirement.


Here is where my real confusion is:

STANDBY PACK AVERAGE TEMPERATURE
If all zone controls and primary pack controls fail, the standby pack controls command the packs to produce air temperature which will satisfy the average temperature demand of the two cabin zones. The trim air modulating valves will close. The flight deck zone temperature selectors will have no effect on the standby pack controls

Can somebody explain this above para to me in simple terms.

So in this above scenario there is no temperature control for the flight deck, why so? why can't the packs go to Independent mode of operation in this above scenario, Had it been designed to go to independent pack operation (where the left pack takes care of the flight deck temperature and the right pack produces temperature to satisfy the average of two cabin zones.)that would make more sense right, that way even the flight deck will have some temperature control

What am i missing

Thank you guys

[owl-attack]

Will appreciate if some one can provide me an answer

Regards

[Skyjob]

Originally Posted by owl-attack

If all zone controls and primary pack controls fail, the standby pack controls command the packs to produce air temperature which will satisfy the average temperature demand of the two cabin zones. The trim air modulating valves will close. The flight deck zone temperature selectors will have no effect on the standby pack controls

Without reading anything but the above on this matter, I think the reason for this is that the average temperature in the two cabin zones satisfies by definition both cabin zones and most likely will thus flight deck as well. In other words, why make both cabin zones really cold and comfy and warm up front, or in reverse both cabin zones toasting and cold up front...

[Jet II]

Originally Posted by owl-attack

Here is where my real confusion is:

STANDBY PACK AVERAGE TEMPERATURE
If all zone controls and primary pack controls fail, the standby pack controls command the packs to produce air temperature which will satisfy the average temperature demand of the two cabin zones. The trim air modulating valves will close. The flight deck zone temperature selectors will have no effect on the standby pack controls

Can somebody explain this above para to me in simple terms.

So in this above scenario there is no temperature control for the flight deck, why so? why can't the packs go to Independent mode of operation in this above scenario, Had it been designed to go to independent pack operation (where the left pack takes care of the flight deck temperature and the right pack produces temperature to satisfy the average of two cabin zones.)that would make more sense right, that way even the flight deck will have some temperature control

What am i missing

Thank you guys

Think about it - if all controls fail then the pack cannot compute a desired temperature. Lets say that the flt deck temp sensors failed so that they went to max hot (or cold) - the crew have no control, in that scenario is it safer simply to average the temp demands of both zones and therefore provide a working environment that is safe rather than comfortable.