Hi, I've seen a couple of changes in VNAV descend behaviour compared to older versions (Managed descend) I'd like to point out.
VNAV Speed Constraints:
When reaching a speed constraint while in descend the VNAV path seems to no longer calculate those speed reductions. The green dot keeps going down as the plane reduces speed as if ignoring the plane needed to reduce speed according to FPLN. In older versions the descend path took this into account for calculations as the green dot also reduced its descend speed according to the speed reduction.
VNAV speed range:
While in managed descend there's the speed range. Let's say 310 top range, 290 magenta triangle, 270 low range.
The Airplane seems to no longer use the lower range and avoid going below the magenta triangle. If (for whatever reasons) the VNAV path requires the plane to reduce the descent rate, the plane doesn't use the lower speed range, instead it adds power to avoid it and keeps the speed in the magenta triangle. (the top range is properly used when needed).
#VNAV Descend behaviour in the last versions
gleaming grail
somber brambleBOT
gleaming grail
Got some extra info:
Flight Plan was -> SPZO10 GAXUN1B GAXUN ITARA UDENI PEROV5 SPJC16L
I've also seen that the "VNAV Speed Constraints" issue didn't happen in a different STAR of that airport (ATATU3) , something in PEROV5 makes VNAV not 100% accurate
stable perch
Hi, this is something I'm not sure about about, but my understanding is that - for a STAR like PEROV5 - the AC will use a constant path angle segment to meet altitude constraints rather than just an idle descent. However, when the AC raises the nose to reduce speed and meet a speed constraint, it will lose the path because it is no longer maintaining a constant path angle. That's what it should do according to my docs, but I don't know what it does IRL.
Your second issue is related. On a constant path angle segment like the one mentioned above, the AC will always add power to maintain the speed target since going to idle will mean it drops to the lower margin quite quickly. The lower part of the margin is only used for an idle segment