I had the experience today where i was making a fairly short flight using the 787, so I set an altitude into the autopilot of 20,000 feet, and then activated the VS so it would climb to that.  Partway up, I decided that maybe that altitude would be too high for the distance I was covering, so I changed it to 15,000.  But the plane kept climbing right past that and leveled off at 20K.  I can't imagine that a real world pilot cannot easily change what is set into the system, especially to deal with instructions given by the controllers.

Is there an item I'm overlooking in getting the plane to hold a different altitude than the one originally set?

Andy are you using the Heavy Division mod?  You might look at your cruise altitude and make sure that was reset to 15K, also you might first choose ALT hold, then change to 15K, then reselect either VS of FLC.  IOW, it should be working fine if you're operating it correctly.

A real aircraft will never, in any automation mode except glideslope, violate the altitude in the MCP window. If you're using VNAV or "managed" for those that speak airbus, you will need to change the altitude in the FMC as well for an updated top of descent; but no matter what, if you've dialed the altitude into the window on top of the panel in the middle of the cockpit, the autpilot WILL honor it.  

So this sounds like a significant bug, if you set 15,000 in the MCP and it blew through. I've never tried an airliner in the sim so don't really know what state they're in... but this sounds like about what I was expecting lol. 

C'mon PMDG.

This was using the default 787.

EDIT - Did another test, with the default 787 going out of KSFO to SEA.  I set the initial altitude to 30,000 and activated the AP with that set, then reduced it to 10,000.  It blew right past 10K feet and kept climbing to 30K.

Edited January 25, 20224 yr by andyjohnston.net

So I started my descent, and somehow I was going to be down way ahead of where I expected.  So I changed my vertical speed to slow my descent, but it kept going at the same rate as before.  I changed my target altitude to a higher level so I would still be above the ground, but the aircraft just went down until I had to turn the autopilot off and try to take control myself.  Still wasn't enough, so somewhere west of Mount Rainier there is the remains of a 787.

Hi Andy,  Are you using a controller button to activate the AP in lieu of the mouse on the MCP?  Unlike previous sims, that seems to be a no-no in MSFS 2020 and can cause erratic AP behavior.  Just spitballin' here.

No, I don't have a controller.  I just click the AP button on the panel.

The old FS default AP,  one had to engage the ALT button in order to capture. I don't know how this is modeled.

Well, that behavior is entirely wrong. C'mon PMDG.  😉

Andy do yourself a favor and install the Heavy Division mod for the 787X--it's great, it's free, it's easy and VNAV works fine.  Use the Dev Version here:  https://github.com/Heavy-Division/B78XH/archive/refs/heads/main.zip