Originally Posted by
TheBusFlyer
Hello everyone,
Another question on the 777! Trying to understand THR mode coupled with FLCH.
If I get it correctly, the aircraft will try to compute a given value of thrust (leading to a target VS) in order to get to the target MCP altitude in 120 seconds. If it can’t, it will apply Thrust climb during climb (THR REF) or Idle thrust during descent.
1/ I am having trouble understanding how the speed target can still be on the elevator (FLCH SPD) if the given amount of thrust is commanding a vertical speed in order to reach the target MCP ALT in 2min.
Let’s suppose we are climbing from 5000 to 6000ft in FLCH. FMA will display:
THR FLCH SPD.
However are we really pitching for speed here? Isn’t the amount of computed thrust calculated to provide us a target VS ?
To climb or descend at a particular forward speed and vertical speed requires a fixed amount of thrust and a fixed pitch angle - whatever 'modes' you are using. With V/S you set the path with the elevator and thrust controls your speed. With FLCH you set the speed with elevator and thrust controls the rate of climb or descent. In both cases the plane follows the same flight path. The difference between modes is all about priority.
FLCH is considered a 'safe' mode because it prioritises speed. If you're descending in FLCH with THR HOLD and add power you'll reduce the rate of descent. Add enough power you will climb. For small climbs or descents achievable within 125 seconds it calculates and sets an appropriate power setting, but speed is always controlled by elevator.
V/S prioritises V/S which means with insufficient power you can underspeed and with insufficient drag you can overspeed.
You can often see the difference in flight - FLCH tends to hunt the speed by gently pitching up and down - one minute you're at 1500fpm then you're at 600fpm. V/S tends to be smoother as the autothrottles do all the work.