Q: Top of descent (T/D)

  • Hi all,

    For Boeing series - Just want to verify when prior to Top of Descent you need to manually initiate it by setting a lower altitude in MCP and then pushing the ALT button when T/D is reached ? I thought in VNAV PATH if T/D is reached and the altitude of next fix is input the A/P will automatically follow VNAV profile all the way down but I found this isn’t the case I had to manually initiate it- having said that I did modify the cruise altitude — so I am wondering if that causes an issue with the FMS and it won’t descend because it messes up the computations and this only works if cruise altitude is not modified?

  • Before reaching the T/D and if you are flying with VNAV on, all you need to do is reduce the altitude. Do not push anything.

    Then the aircraft should automatically start descending at the top of descent. If the vertical mode changes to VNAV ALT it will not descent and you have to manually activate it by pushing in the altitude knob (not the ALT button underneath the knob!). The aircraft will start a shallow descent until it intercepts the calculated VNAV descent path or if you are already above the profile it will try to catch up with it. You may need the airbrake to be able to do that.

    If you push ALT at any time the autopilot will take the current altitude and maintain that until you tell it to do otherwise.

    So if you lower the altitude and then press ALT it just ignores your selected altitude and does a level off and keeps the altitude at the time you pressed the button.

    Modifying the cruise altitude does not yet change the actual flight plan cruise altitude. VNAV PATH only engages at that predetermined cruise altitude. This will change with future updates to the navigation.

  • No the Airbusses don't descent on their own unless you push something. But similarly to the Boeings you push in the altitude knob to start a descent immediately and it will intercept the VNAV profile. But in the Airbusses also change the speed target on their own to be able to catch up with the vnav path.