Originally Posted by
LOWI
For anybody curious, a quick update.
Just before reaching the waypoint with an altitude restriction, you can press ALT INTERVENTION button on the MCP and it will allow the aircraft to continue descending. It just places the aircraft into the default 1000fpm descent until it recaptures the new VNAV calculated path.
I am still trying to read through some FCOMs to find out if there are any requirements for this method to work.
On "our" 737s, I'm not quite sure what has happened, but pressing ALT INTV no longer gets the aircraft descending after it has levelled off at an assigned restriction, which is bizarre because when you press it before the initial T/D at cruise level, it does start to descend (at 1000fpm until it intercepts the 3 deg path).
e.g. a restriction of FL150 by KIDLI. Before KIDLI we are further cleared to FL110. Setting FL110 in the MCP causes a new "T/D FL150" to appear a few NM after KIDLI. Pressing ALT INTV when the aircraft is about to, or has already levelled off at FL150 for KIDLI does not keep VNAV coming down. I can bet the house that when I started flying the 737 over 2 yrs ago pressing ALT INTV did the exact same as above, it would again descend at 1000fpm until it intercepted the 3 deg path leading to FL110 - but it doesn't anymore.
Regarding the crew in the OP's scenario, it sounds to me like they just got distracted, forgot, or for whatever reason didn't notice at the time that it was the previous restriction was there. I don't think it offers any insight into their competency on the type whatsoever.
As for the VNAV on the MAX, the less said the better. In VNAV PTH we once had an early descent, the aircraft began descending, and then started a climb! We went V/S to fix it, we never quite got to the bottom of it. From what we could make out, the aircraft had a moment where it was attempting to intercept the VNAV PTH which was above us. Between that and the yoyo'ing of 9999 low up 9999 high and every number in between it's a bit of a wild one at times.