What is the difference between using flight level change (same area as the speed buttons on the MCP) and using the vertical acceleration?Thanks

What is the difference between using flight level change (same area as the speed buttons on the MCP) and using the vertical acceleration?Thanks

Generally FLCH will climb or descent to MCP set altitude maintaining set speed, by altering the attitude of the climb or descent. VNAV will climb or descend according to the flight profile calculated in the FMC. Vert speed will climb or descend at the set descent rate, adjusting the throttle to maintain speed setting if possible. If the assent rate is too high you will lose speed and possibly stall. Conversely if your descent rate is too steep, you will increase speed, and possibly go into an overspeed situation.

What tf said - but to give a worked example:- if you are asked by ATC to reduce altitude from 20000 to 15000, you might use FLCH to make the vertical transition. You will lose 5000' over a period of time. Time is not the main factor.- but if ATC says get down to 12000 asap, within say 30 miles (Radar Contact will do this often), the FLCH command might not get you down quickly enough. So use a specific rate of descent (1800 fpm or whatever) to drop more quickly, to ensure you are where you need to be - by increasing the rate, you will drop more quickly - as you'll need to, when time is a factor.