An an off lined FPC is brought online automatically.

The off-lined FPC is automatically brought online. This can happen when the FPC is brought off line via the CLI and was not powered off.

If you take the FPC off line via the CLI using the request chassis fpc slot <x> offline command, but have not configured it to be powered off under the chassis level ( set chassis fpc <x>  power off  ), then if the configuration involves chassisd this will automatically bring it online.
 

If you suspect that one or more FPCs may cause some kind of issue, you may need to offline them, prior to any maintenance action. To offline the FPC, you need to do so via the CLI by using the  request chassis fpc slot <x> offline command and then configure it to be powered off under the chassis level ( set chassis fpc <x> power off ).

In the following snapshot, FPC1 is off lined first and then the configuration change of adding the interface description ( set interfaces ge-1/0/0.0 description test-2 ) is made to it. This will automatically bring the previously off lined FPC 1 online.

Feb 16 18:03:25 apple-re0 mgd[24800]: UI_CMDLINE_READ_LINE: User 'lab', command 'show chassis fpc '
Feb 16 18:03:33 apple-re0 mgd[24800]: UI_CMDLINE_READ_LINE: User 'lab', command 'show configuration | display set | match -1/ '
Feb 16 18:03:44 apple-re0 mgd[24800]: UI_CMDLINE_READ_LINE: User 'lab', command 'configure '
Feb 16 18:03:44 apple-re0 mgd[24800]: UI_DBASE_LOGIN_EVENT: User 'lab' entering configuration mode
Feb 16 18:04:00 apple-re0 /usr/sbin/cron[27859]: (root) CMD (newsyslog)
Feb 16 18:04:02 apple-re0 mgd[24800]: UI_CFG_AUDIT_SET: User 'lab' set: [interfaces ge-1/0/0 unit 0 description] <unconfigured>  -> "test-2"   <<- config-changed for ge-1/0/0 with description
Feb 16 18:04:02 apple-re0 mgd[24800]: UI_CMDLINE_READ_LINE: User 'lab', command 'set interfaces ge-1/0/0.0 description test-2 '   
Feb 16 18:04:08 apple-re0 mgd[24800]: UI_CMDLINE_READ_LINE: User 'lab', command 'commit full and-quit '
Feb 16 18:04:08 apple-re0 mgd[24800]: UI_COMMIT: User 'lab' requested 'commit' operation (comment: none)
...
Feb 16 18:04:22 apple-re0 chassisd[1391]: CHASSISD_GBUS_RESET_EVENT: FPC#1 - Assert Board Reset       <<<---FPC1 now online itself @ 18:04:22
Feb 16 18:04:22 apple-re0 chassisd[1391]: CHASSISD_SNMP_TRAP10: SNMP trap generated: FRU power on (jnxFruContentsIndex 7, jnxFruL1Index 2, jnxFruL2Index 0, jnxFruL3Index 0, jnxFruName FPC: E-FPC Type 2 @ 1/*/*, jnxFruType 3, jnxFruSlot 1, jnxFruOfflineReason 7, jnxFruLastPowerOff 318901291, jnxFruLastPowerOn 320093206)

Formerly fpc1 offlined by cli
===================================
lab@apple-re0> show chassis fpc
Feb 16 18:04:17
Temp CPU Utilization (%) Memory Utilization (%)
Slot State (C) Total Interrupt DRAM (MB) Heap Buffer
0 Online 25 2 0 1024 3 23
1 Offline ---Offlined by cli command---
2 Online 25 1 0 1024 2 23
3 Empty
4 Online 43 4 0 2048 3 23
5 Online 26 1 0 1024 2 23
6 Online 43 4 5 1024 4 23
7 Empty

lab@apple-re0>
lab@apple-re0> show chassis fpc
Feb 16 18:04:22
Temp CPU Utilization (%) Memory Utilization (%)
Slot State (C) Total Interrupt DRAM (MB) Heap Buffer
0 Online 25 2 0 1024 3 23
1 Offline ---Offlined by cli command---
2 Online 25 1 0 1024 2 23
3 Empty
4 Online 43 4 0 2048 3 23
5 Online 26 1 0 1024 2 23
6 Online 43 4 5 1024 4 23
7 Empty

lab@apple-re0>
lab@apple-re0> show chassis fpc
Feb 16 18:04:26
Temp CPU Utilization (%) Memory Utilization (%)
Slot State (C) Total Interrupt DRAM (MB) Heap Buffer
0 Online 25 2 0 1024 3 23
1 Offline <<<-- now, in the process of bootup now
2 Online 25 1 0 1024 2 23
3 Empty
4 Online 43 3 0 2048 3 23
5 Online 26 1 0 1024 2 23
6 Online 43 4 5 1024 4 23
7 Empty

lab@apple-re0>
lab@apple-re0> show chassis fpc
Feb 16 18:04:34
Temp CPU Utilization (%) Memory Utilization (%)
Slot State (C) Total Interrupt DRAM (MB) Heap Buffer
0 Online 25 2 0 1024 3 23
1 Offline
2 Online 25 1 0 1024 2 23
3 Empty
4 Online 43 3 0 2048 3 23
5 Online 26 1 0 1024 2 23
6 Online 43 4 5 1024 4 23
7 Empty

lab@apple-re0>

lab@apple-re0> show chassis fpc
Feb 16 18:34:44                              (it onlined long time ago, I checked @ 18:34:44)
Temp CPU Utilization (%) Memory Utilization (%)
Slot State (C) Total Interrupt DRAM (MB) Heap Buffer
0 Online 25 2 0 1024 3 23
1 Online 25 2 0 256 15 46   <<<--status online 
2 Online 24 1 0 1024 2 23
3 Empty
4 Online 42 3 0 2048 3 23
5 Online 26 1 0 1024 2 23
6 Online 43 3 5 1024 4 23
7 Empty

lab@apple-re0> 

lab@apple-re0> show system uptime | match cu
Current time: 2012-02-16 18:35:17 GMT

lab@apple-re0> show chassis fpc detail | match "slot|uptime"
Slot 0 information:
Uptime 37 days, 1 hour, 38 minutes, 20 seconds
Slot 1 information:
Uptime 29 minutes, 35 seconds         <<<--  FPC1 onlined 29 minutes ago.
Slot 2 information:
Uptime 37 days, 1 hour, 38 minutes, 14 seconds
Slot 4 information:
Uptime 37 days, 1 hour, 39 minutes, 29 seconds
Slot 5 information:
Uptime 36 days, 23 hours, 13 minutes, 43 seconds
Slot 6 information:
Uptime 36 days, 23 hours, 35 minutes, 12 seconds

lab@apple-re0>