Hello.
During the climb on the autopilot, i usually set the speed of 250kt and keep it to 10000ft. When I hide flaps, I switching to FLCH mode, but then the speed on the MCP is changed automatically to the one that plane currently fly eg: 213kt (i need to set manually 250kt again). I would like to know if this is a normal behavior, or there should be set all time 250 on MCP even if i switch to FLCH ?

musiol1 wrote on Jun 6^th, 2018 at 5:57pm:

Hello. During the climb on the autopilot, i usually set the speed of 250kt and keep it to 10000ft. When I hide flaps, I switching to FLCH mode, but then the speed on the MCP is changed automatically to the one that plane currently fly eg: 213kt (i need to set manually 250kt again). I would like to know if this is a normal behavior, or there should be set all time 250 on MCP even if i switch to FLCH ?

+1 I would like to know as well

From the ‘757 Captain III’ Manual Part II – Aircraft Systems (CS757CAPTAINIII_MANUAL_PART2.pdf), page 86:

3. Flight Level Change (FL CH) Switch
Push -

   - selects FLCH mode and sets IAS/MACH window and command airspeed
     bugs to current airspeed

   - FLCH displays on each FMA

   - AFDS pitch holds existing airspeed and A/T sets required thrust, limited by
     the thrust limit for climb and idle for descent. When selected altitude is
     reached, pitch mode changes to ALT HOLD and A/T changes to SPD
     mode

   - with FLCH mode displayed, pushing switch resets IAS/MACH window and
     commands airspeed bugs to current airspeed

   - changes thrust reference from TO to CLB if above 400 feet radio altitude.

So going by the underlined information above, if the 757 is doing 213KIAS when you press the FLCH button, then YES, that is what it should do. For it to climb at 250KIAS, you need to wait for the 757 to reach that speed before pressing the FLCH button.

Marko, thanks for the explanation but I want to clarify an issue I see in the case the aircraft is climbing in FLCH mode but has not YET reached the FLCH speed - if I set the FLCH to a certain speed it is indeed maintained by the aircraft pitch change HOWEVER once reaching the target altitude AND IF the aircraft has not reached the FLCH speed yet the auto-throttle changes to SPD mode and holds the current speed and not the target speed set initially (since the aircraft did not manage to accelerate to the target speed yet)
In the 737 for example in this case the auto-pilot will capture the altitude and the auto-throttle will change to SPD mode however the aircraft will speed up in ALT hold until reaching the target speed set initially and will maintain it.
I can post a video if needed.

Thanks

Bump anyone?