Route Autopilot ascends/descends too quickly

1st Official Post

    Hi,

    On commercial jets like 737 Max, A350 787-10 etc, I noticed the ascent and descent happening too fast during route autopilot.

    I select a simple route no additional waypoints just direct runway to runway from say KSFO to KLAX and started route autopilot shortly after takeoff from the UI (not cockpit, no additional cockpit settings touched).

    After a while it would ascend too quickly way above the green boxes in the sky. In descent it would descend way below the green boxes quickly (-2000 to 3000fps) and would be 5000-6000ft altitude with 60+ NM left on the route.

    Can the auto nav autopilot be tuned or should we be tweaking some stuff in the cockpit as well and not just rely on the UI?

    I can supply some videos or more data if needed. But it should be easy to reproduce.

    Thanks!

    • Official Post

    The green boxes do not account for the actual physical performance of the aircraft. So during climb the aircraft, due to the mass being close to maximum landing weight and no where near max takeoff weight, the aircraft can climb quite well. This is expected behavior.

    During the descent the engine thrust is at idle and the descent is mostly defined by the glide ratio of the aircraft. 1500-2500 FPM are typical in this case. If the descent started too early this explains the low altitude. But some arrivals or approaches also have altitude constraints, which means the aircraft has to be at a certain altitude along the route. This could also be the case here if you selected an approach, which is now set by default in the new navigation menu.

    Regards,

    Jan