DES NOW in CLB Mode (747-400)

[cf6-80c2b5f]

Thanks for the reply, CCA. I was thinking more along the lines of setting a lower MCP altitude and using VNAV altitude intervention to descend, rather than FLCH or V/S. Would there be no interconnect between the FMC and a lower MCP altitude when you tried to descend?

For example, your cruise altitude is FL340. After departure you encounter a pressurization problem that requires your return. You level off at 10,000' with the MCP. You are in VNAV ALT and still in CLB mode. Without knowing which approach you will be doing (let's say you need to dump fuel first), you are cleared to 6,000'. When you set 6,000' in the MCP and altitude intervene, you won't descend because you are still in VNAV CLB mode. But if you press DES NOW at this point (and execute), will it let you descend in VNAV? Does this give the FMC an altitude to descend to?

If you install an approach for a return while still in CLB mode, when you bring any fix on that approach into 1L (extend from the CF or FF at 6R) it will shift to CRZ mode immediately and then DES mode when you cross the T/D boundary. But if you install the approach without extending at 6R, the first fix of the approach will end up on the last page of the LEGS page and it will not shift to CRZ. Will DES NOW allow a descent in this case?

ADDENDUM:

I think that in order for it to shift into CRZ when you install an approach and extend at 6R, your actual altitude must be at or above the altitude of the fix you are extending from. It's not as much an issue on a return for landing, but on a missed approach, if you level off prior to your missed approach altitude, and you install an approach and extend from the CF, if you are not at or above the CF altitude, it will not shift to CRZ mode.