737 Autopilot question

[crisk73]

Hi, I'm lately struggling to understand how the 737 (NG and Classic) Autopilot behaves just after takeoff and what's the proper procedure to follow to get a smooth transition to flaps UP speed.

The scenery is this: MCP SPD set to V2, HDG mode selected to follow rwy heading, A/T armed, both FDs on, TO/GA engaged. 80knts, V1, rotate...

For the IXEG I've observed that prior to TRA (throtte reduction altitude) after FMA announcing throttle ARM, if you push the CMD button then FLCH + N1 modes are automatically engaged. Ok, easy, but is that correct? I read that if no vertical or lateral modes are selected, the A/P should maintain attitude and bank angle.

PPRuNe source: "When the A/P is engaged and proper mode selection is not made, AP will switch CWS ROLL,CWS PITCH mode and flashing amber CWS PITCH,CWS ROLL announciaitons on FMA will come on and ask you to give proper roll and pitch modes".  Link:http://www.pprune.org/tech-log/115253-737-autopilot-when-engaging.html

So the correct sequence would be for example: at TRA push N1 (or it gets automatically engaged), then select one lateral mode (say LNAV), one vertical mode (say FL CH) and last hit CMD. Correct?

Maybe @Litjan can help me here. 

Thanks!!

Cris

[mfor]

 

44 minutes ago, crisk73 said:

PPRuNe source: "When the A/P is engaged and proper mode selection is not made, AP will switch CWS ROLL,CWS PITCH mode and flashing amber CWS PITCH,CWS ROLL announciaitons on FMA will come on and ask you to give proper roll and pitch modes".  Link:http://www.pprune.org/tech-log/115253-737-autopilot-when-engaging.html

Well you probably pushed the TOGA button and thus putting the flight computer into TOGA mode, activating several ap modes as shown at the top of the EADI. As far as I know it will

• activate HEADING HOLD mode
• engage TO thrust (later reduced to N1)
• provide FD pitch cue to maintain MCP speed+15
• provide FD lateral cue to maintain heading (or LNAV once engaged)

When you drop out of TOGA mode by activating AP CMD your MCP speed dial will be set to you current speed and you'll be put into LVL CHANGE mode (or ALT HOLD if you've already reached the MCP altitude) along with either HDG HOLD or LNAV if you had engaged that.

So you don't really need to activate any AP modes if you're happy with the defaults and activate LNAV and VNAV whenever you think is suitable.

I usually arm LNAV shortly after takeoff, engage the AP after reaching 1500,  and once above 3000 I speed up via the MCP speed bug while cleaning up the aircraft before engaging VNAV.

FWIIW: If you perform a manual takeoff (i.e. not use the toga button) pushing the AP CMD button will indeed put you into CWS mode as you expected.

Edited July 14, 2017 by mfor

[crisk73]

Ok now I understand. It's the TOGA mode engaged to trigger the subsequent ap modes.@mfor thanks for your thorough clarification!