when I decend in line with the VNAV profile and set any MCP ALT, it is correct that VNAV PTH is changed to VNAV ALT but the speed starts to gradually reduce to min. flaps 0 speed. Do you have any idea what I do incorrect or where is the problem?Thanks.Martin

VNAV ALT will hold the plane at the selected MCP altitude.You need to press Alt Intervention to allow it to descend.(VNAV ALT is a Boeing option to stop VNAV from disengaging at an MCP altitude that is NOT in the FMC.)Fred.

Thank you Fred, but the problem is, that when I need to maintain the MCP altitude, the speed suddenly starts go down (the purple speed marker jump step by step down) and stabilize at min. zero flaps speed still with VNAV ALT active.

VNAV is slowing the plane, trying to lose energy. When you continue your descent, the speed will pick back up.It's normal behavior, but if you want to keep the speed up, use speed intervention or change to another mode like alt hold.

This is normal behavior -- Boeing calls it "energy compensation". Idea is that, by leveling off, you're going to be above the VNAV path -- so essentially you will have excessive potential energy. To compensate, the FMC reduces the airspeed (i.e. your kinetic energy) -- as long as you don't get too high above the path, you'll be able to convert the excessive altitude back to energy later on. (See also the FCOM volume 2, page 11.31.30).Edit: Posted at the same time as Matt there...--Martin Boehme

Since the guys already commented on VNAV, it's a much easier practice to use v/s mode on descent and approach and just as effective.. That's probably why about only 30 percent of pilots use VNAV on approach..

Thank you very much for your explanation guys.Martin

Since the guys already commented on VNAV, it's a much easier practice to use v/s mode on descent and approach and just as effective.. That's probably why about only 30 percent of pilots use VNAV on approach..

I wouldn't say V/S is just as effective, just that it has different uses. Screwdriver and wrench.

I wouldn't say V/S is just as effective, just that it has different uses. Screwdriver and wrench.

True.. I think it's a matter of the equipment you started on also depending on what your carrier provides in terms of avionics.. It's tough to change when you've been flying a certain way for so long.. In terms of techology and automation, still a little behind I guess and still have things to learn about vertical navigation.. Still doing a lot of manual flying.