Boeing FMC question
 

"If the MCP altitude is set to an altitude below the current cruise altitude but above the first descent constraint, and the aircraft is greater than 50NM from the top-of-descent point, the cruise altitude is lowered to the MCP selected altitude. If the aircraft is within 50NM of the top-ofdescent, an early descent is initiated consisting of a 1250 fpm descent until the descent path is intercepted."

What happens if the MCP is placed below the first descent constraint at more than 50 miles back from TOD?

Which Boeing?

And which descent mode?

"VNAV ALT – when a conflict occurs between the VNAV profile and
the MCP altitude, the airplane levels at the MCP altitude and the
pitch flight mode annunciation becomes VNAV ALT. VNAV ALT
maintains altitude"

Whatever the alt selected in the MCP (unless using ALT INTERVENTION) the airplane will start descent at TOD in vnav mode.

If the aircraft is more than 50 nms from the TOD, then it will perform a normal cruise descent and level off at the new cruise altitude and the FMA will read VNAV PTH.

It won't actually do a thing unless you push a button first. ALT intervene or on the cruise page accept the new level as entered into the MCP ALT window......

Boeing 747

Not sure if they are the same but a B787 would enter an ACT ECON CRZ DES to the MCP altitude if more than 50 NM from T/D.