Setting up the FMC

[Will]

So let's say you get into the aircraft for preflight and the IRS switches are off. You turn them to NAV, and they start the alignment process. Until that's complete, you can't enter the IRS position.

But does that stop you from doing other parts of the FMC preparation, like loading/inspecting your route, and setting up the DEP/ARR pages?

Part of me feels like I should just wait until I have everything set up on POS INIT before doing other elements of the FMC preflight, but does it make a difference?

[Mariano]

Will,

You can enter the IRS position as soon as you see the boxes (after IRUs check DC power supply), and you can do most of the FMC setup while the IRUs are aligning.

Nevertheless, you won't be able to cross-check the route distance in PROG 1/2 against the OFP total distance, for example, but most other operations can be accomplished.

Just think about a fast realignment before taking the runway (rare these days, even in 767s) while the IRU selectors are in the ALIGN detent while we enter the new position; the FMCs keep all the data that was programmed during pre-flight preparation. Moreover, during short turnarounds, we start loading the next flight's data as soon as we set the IRU selectors to OFF and back to NAV. Sometimes we don't have a choice ;-)

Best regards,

Mariano

[Will]

Thank you! That's very helpful.