Futterman From United States of America, joined Sep 2003, 1301 posts, RR: 40 Posted (9 years 3 months 1 week 5 days 17 hours ago) and read 2172 times:
With the 744 ECS and checklists in mind, the three air conditioning packs are usually set to either "OFF" or "NORM". However, there are two other notches on the dial for "A" and "B", and I'm wondering what they are.
My understanding is that the "A" and "B" systems are just another example of redundancy in the cockpit. For example, in "NORM" mode, the pack controller automatically selects either the "A" or "B" system "on alternating flights" (got that from an AOM, which I found to be pretty interesting), but you can also use the other two modes to specify either system as the primary and have the other stand by as a backup.
Is that really all it is, though? Why would one want to (or need to, aside from a known faulty pack) submit their own preference for A vs. B and not have the system make the selection automatically?
JetMech From Australia, joined Mar 2006, 2726 posts, RR: 52
Reply 1, posted (9 years 3 months 1 week 5 days 16 hours ago) and read 2160 times:
G'day Brian ,
I think that the A/B selection is for the pack controller only, not the pack itself. In normal operations, all three packs on a 744 will be operating. Each of these three packs is made up of one set of machinery only, that is, there is not a separate set of machinery for the A and B selection. The redundancy in the pressurisation / air-conditioning system comes from having three separate packs. I don't remember the details, but there are either two controllers per pack or two separate "lanes" in the one pack controller box. I believe that the A/B selection allows the pilots to force the use of a particular pack controller or lane if the system is malfunctioning and does not correct itself.
JetMech split the back of his pants. He can feel the wind in his hair.
Futterman From United States of America, joined Sep 2003, 1301 posts, RR: 40
Reply 2, posted (9 years 3 months 1 week 5 days 6 hours ago) and read 2108 times:
Thanks for the clarification, JetMech.
Quoting JetMech (Reply 1): The redundancy in the pressurisation / air-conditioning system comes from having three separate packs. I don't remember the details, but there are either two controllers per pack or two separate "lanes" in the one pack controller box.
I just came across some more information, and there don't seem to be two controllers per pack. Just two controllers, period, each with three channels for the three packs.
Quoting 2000 Air New Zealand 747-400 ZK-SUI A.O.M. - page 293: ...The packs are controlled by two identical pack controllers, A and B. Each controller has three separate channels, one for each pack. Control of the packs switches automatically to the other controller at touchdown. If a controller detects a fault in a pack channel, control of the respective pack switches to the other controller...
So it looks to me that there is redundancy on both levels. Three packs, yes, but the fact that there are two controllers - as well as the fact that you only need one, hence the automatic selection and the A/B switches - looks to me like another instance of "better safe than sorry".
As far as when one would opt for the A/B switch versus "NORM":
Quoting 2000 Air New Zealand 747-400 ZK-SUI A.O.M. - page 294: ...Pack control, fault detection, and overheat protection are all automatic. When an overheat or controller fault is detected, the respective pack valve closes resulting in a pack shut down....If a pack controller does not switch automatically to the other controller, selecting A or B manually selects the respective controller when the Trim Air switch is ON....
But how does the crew know which controller is faulty for a specific pack? Is there an appropriate EICAS alert?
BoeingFixer From Canada, joined Jul 2005, 578 posts, RR: 0
Reply 3, posted (9 years 3 months 1 week 5 days 2 hours ago) and read 2084 times:
Quoting Futterman (Reply 2): As far as when one would opt for the A/B switch versus "NORM":
Quoting Futterman (Reply 2): But how does the crew know which controller is faulty for a specific pack? Is there an appropriate EICAS alert?
The crew will get an EICAS message of "PACK 1, 2, 3" which is an advisory level message alerting the crew to one of the following: "Pack controller fault, or pack operation fault, or pack overheat, or pack 2 shutdown with either cabin pressure relief valve open." The crew then has to refer to their QRH Non-Normal checklists for the Air-Sytstems and the PACK 1,2,3 EICAS message. This will help them isolate the problem and indicate to the crew which controller they should be using or if the pack needs to be turned off.
Under normal circumstances, the system is left in NORM.