VSI Limitation Logic or Bug?

[Torbinator]

Hello, there've been a few times on approach when I go to use the VSI mode on autopilot to adjust for the next step down altitude only to find that it will not let me scroll to more than a 650 FPM descent rate. Sometimes this number is varies between 600 and 900 FPM, and other times there is no issue. Is this intentionally modeled in the systems, or is it a bug? The few times it has occurred I was flying with online ATC services such as PilotEdge or Vatsim, so I didn't have time to try to talk troubleshoot it.

I have linked a recent episode this happened to me on my way into SFO. Just before the clip I linked, I changed my target altitude from 4,000 to 3,000, then did a quick drag on the VSI scroll wheel to get it going down. As I went to increase it beyond 850 FPM, it wouldn't increase descent rate. I then tried to scroll to another desired descent after selecting a lower altitude, and it at one point allowed for 900 fpm down, then only 700 fpm down:

https://clips.twitch.tv/RudeGleamingLEDTheTarFu

 

Edited September 29, 2020 by Torbinator

[Litjan]

Hi,

this is a current limitation of the manipulator we use for the V/S wheel. I explained it in another post recently, it basically works like this:

In the real 737, the "wheel" for the V/S can spin without limit. X-Plane does not allow that, it needs a certain "limit" for maximum revolutions. Normally a value is tied to these revolutions - like on a thermostat. 20 degrees is always at X degrees manipulator turns.

Unfortunately the "0" point for the vertical speed wheel "resets" every time you pick another pitch mode...but the wheel does not reset. So now the new "0" is not in the "center" of the revolutions anymore.

So if you keep using the V/S wheel to descend - picking another pitch mode in between frequently - you will eventually run out of V/S downward travel.

To "cure" this (until we find a fix) you can turn the V/S wheel "upward" again while NOT in V/S mode (you could just go to CWS P for a second) - then reengage V/S. Now you have the "full travel range" available again.

Cheers, Jan

 

[Torbinator]

Ahhh, excellent tip for now ... I hope you can find that fix!