Now follows the scenario we all face daily...
DIRECTOR: “IMAGINAIR 123, turn right heading 280 degrees, descend now flight level 270, speed 280 when level.”
In the company you work for: the PNF was probably writing down the clearance as it came in, while the PF dialled the numbers into the boxes. Then the PNF looked up and checked that all the boxes had the right numbers in them, and if that was the case said “Heading checked, flight level 270 checked”. This is a robust executor and monitor loop, suited very well to all scheduled and charter type operations.
This is what happens in the flight deck I work on, when it works.
PF starts to turn the heading bug, but he hears the level change request from director so moves his hand away to give priority to the PNF to do his task while he holds the information he needs in his head (280 and 280).
PNF captures the information he needs (270) and dials it into the altitude pre-select. His hands are needed to do this, so he doesn’t write the clearance.
PF looks at the alt pre-select window and with his head full of 280 says 270 checked (because he remembers that there was another number, and he’s pretty sure that is what he sees in the alt pre-select window. With the absence of annotated comparison, is this a form of confirmation bias?).
PF then completes dialing 280 into the heading window while remembering speed 280 for when he levels out.
That was when it works! Can you imagine what happens when PNF dials in 280 into his bit? Exactly, “IMAGINAIR 123 TCAS decent”, or if the numbers were slightly different, “TERRAIN TERRAIN”, or worse, silence.