Descent Autopilot question

[shortspecialbus 13]

I'm guessing this is expected behavior, but it seems a little odd.I was following the CYY5 star into KMIA, and I had put 4000 into the FCP for a descent altitude. I was then cleared down to 1600 which was the only altitude constraint (actually part of the ILS for the runway.) As soon as I lowered the altitude in the FCP, the aircraft pitched down rather violently, causing the passengers in fspassengers to freak out as well as a "sink rate" audible warning from the plane.I was at approximately FL220 at the time and a ways out still.It seems to me that the autopilot redid a calculation of where its optimal flight path was, and tried to quickly catch up by dropping at about 5500 fpm.The autopilot was in FMS SPEED and PROF modes. I forget exactly what was annunciated on the PFD, but I'm pretty sure it was all purple aside from the 4000 in the "PROF TO" spot.Now, should I have put the autopilot into a VS mode or something of the sort? Is this expected behavior, or is something wonky here?Thanks!-stefan

[Check_Airman 3]

You got a "sink rate" aural at FL220 over Florida? The pitch down seems strange indeed. Try it again, and post a few screenshots if it happens.Paul

[FLEX1978 373]

Sorry but you cant get a 'Sink rate' at FL220 if you did then you need a complete reinstall.Rob

[downscc 1,327]

You must be using the default ATC, which is terribly unrealistic (as real as it gets - ho, ho ho). You would never be cleared to FAF from center airspace (unless it is an uncontrolled airport).Possibly you pushed the altitude dial? Getting the FMA indications is the only way to understand what happened.

[aentwis 61]

Without actually being in fornt of the MD11 panel and looking at specifics, I am surprised that any change to the altitude selected on the FCP would have any effect on current descent rate in PROF.Changing the target altitude does not have any effect on the current PROF. Only if you introduce an additional constraint somewhere along the flight path does the vertical profile need to be re-assessed. The descent can increase to rather high rates, but I have noticed only smooth transitions...As Dan and Paul already mentioned, we would need to see some of the indications to get an idea of what might be going on here...Andrew

[shortspecialbus 13]

I also thought it was strange. And I was using default ATC for that flight, I was having a minor issue with RCv4.3 so I had turned it off.I'll play with things to see if I can reproduce it. It was really a rather violent pitch change.Thanks,-stefan

[Turbine777 13]

Without actually being in fornt of the MD11 panel and looking at specifics, I am surprised that any change to the altitude selected on the FCP would have any effect on current descent rate in PROF.Changing the target altitude does not have any effect on the current PROF. Only if you introduce an additional constraint somewhere along the flight path does the vertical profile need to be re-assessed. The descent can increase to rather high rates, but I have noticed only smooth transitions...As Dan and Paul already mentioned, we would need to see some of the indications to get an idea of what might be going on here...Andrew

If he just recently crossed TD it most definitely would happen (or can happen), well at least in the MD-11, I have witnessed it also. My AP acts like it's on crack sometime. Of course I am still a beginner in the Douglas, but have followed the tutorial a ton of times and am still reading the manuals. It does very steep banks, above the 25 limit selected, and I don't think I have done that many landings with full AP that I still don't hear "Don't Sink" being yelled all the way to Touch Down. I have scratched my head a million times why even though I am doing everything exactly as the flight before, sometimes I don't need to "Rehit" perf after departure, or why sometimes it just decides it doesn't like the FL I have entered in the FMC as well as on the MCP and goes 1000 feet above then back down to the deignated FL. The one that drives me nuts the most is how it seems hit or miss on whether or not the AP will actually desend at TD or if it will still just stay locked on HOLD, or why I always have to add drag. I know I have a lot to learn, but I am very consistant and meticulous about doing everything by the book (in this case, the tutorial). But as far as what he's saying I too have witnessed it when trying to figure out the issue with the AP not desending after TD. I tried an experiment where some flights I lowered my Alt within 10 miles of TD, and others soon as I crossed TD. Not always, but a couple times the AP does indeed drop like a stone (Just ask my FSPassengers). This happens to me also a ton on final, where the AP descends well below the glideslope, trimming trees then just before touchdown he gets back on it. Perhaps that issue is an Afcad one, who knows... I still loves it, but wish some of these abnormalities made more sense.