Autopilot stabilizer trim too slow, transitions from IAS to Mach too early

[zbrainlezz]

Flying the L1011 in the climb, with the autopilot in VNAV mode and holding 320 knots indicated, the transition to Mach happens too early - the AP switches to a target of Mach 0.82 at around 25,000ft, when the indicated Mach is still 0.75 or so, which causes the plane to speed up and come very close to exceeding the "never-exceed" airspeed in trying to capture the Mach target. Had this happen twice now, and I've double-checked that everything seems to be configured correctly in the PMS. The correct behavior should be that the AP holds 320 knots until the indicated Mach is 0.82, then transitions to Mach at that point and maintains 0.82 until reaching cruise altitude.

On a semi-related note, the autopilot seems to change the stabilizer trim far too slowly, causing the plane to overshoot speed targets in the climb. I know this is a vintage plane, but I'd think it should still be able to capture and hold a target speed far better than it does.

Edited 5 hours ago by zbrainlezz

[RyanAlmeida95]

I have a similar issue with the trim speed -  During climb or just after takeoff using either VNAV or IAS, the aircraft will shoot straight past the targeted speed...
I also have instances where the aircraft will miss the ALTSEL then slowly trim to recapture - only to miss the ALTSEL again and porpoise around the ALTSEL (+/- 1000ft!).
The times where the aircraft really does this is when there is a higher rate of VS when approaching the ALTSEL generally due to the aircraft high performance, which can be most departure climbs where a level off may be required for a SID.
In general though, if the rate of VS is calm approaching the ALTSEL the aircraft captures the ALT pretty well... so at the moment a more manual approach is needed to attack the issue rather than allowing the AP to handle vertical navigation when approaching ATLSEL with a high rate of VS whilst the trim speed is slow... but then again that might just be the normal to manage that type of scenario with the older aircraft.  

Was messing around with the AP yesterday trying a bunch of departures and ILS approaches (and other wild manouvres) - definatley needs fine tuning in some areas, but I can see in other posts that they seem to be working on these sorts of things in future updates 🙂