DES mode not engaging

[RobW 0]

Hi Folks, I've noticed on the last couple of flights that the FMC doesn't switch over to DES mode after crossing TOD, it remains in CRZ. The aircraft descends and shows the profile but N1 only decreases to 42% and the mode still reads CRZ. Any ideas whats going on? ThanksRob Wilkinson

[Spin737 102]

Are you in VNAV? Otherwise you could use the ALT INT or the DESC NOW prompt on the Cruise page.

[scandinavian13 5,189]

Furthermore, have you reset the MCP ALT?

[RobW 0]

Yep - I've reset the MCP ALT ahead of TOD and A/P is in VNAV mode. The Aircraft starts its descent but the flight display still shows CRZ rather than DES.

[Spin737 102]

Are you talking about the Thrust Limit?

[davidz 26]

Hi Folks, I've noticed on the last couple of flights that the FMC doesn't switch over to DES mode after crossing TOD, it remains in CRZ. The aircraft descends and shows the profile but N1 only decreases to 42% and the mode still reads CRZ. Any ideas whats going on? ThanksRob Wilkinson

42% is flight idle. CRZ is the thrust limit - this does not indicate your stage of flight. Have a look at the N1 Limit page in the FMC - this shows you all of the different thrust limits.

[RobW 0]

Thanks David, appreciate the help Rob Wilkinson