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Uncommanded Altitude Gains


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I've seen a few topics that are similar to this but with a few differences.  I'm having random issues with the aircraft pitching up while flying with the AP engaged, typically during the approach phase when I am in a slow, low altitude state.   Typically, I'm in NAV or HDG plus ALT.  This issue typically occurs, when switching to VS to descend further, the aircraft instead pitches up and I'm seeing vertical speeds of plus 1-2,000 FPM.    No matter how much I turn the "down" knob the aircraft keeps climbing.  If I turn off VS and go back to ALT, the behaviour continues, even though I'm now a few thousand feet over the selected altitude.    As speed decays, I end up going to TOGA or MAX power to keep from stalling, at which point the vertical speed slowly starts to drop and starts to finally decrease.   It's a case of too little, too late as by then the approach is completely ruined.

 

I don't have any screen shots to share, usually I'm much too busy fighting the aircraft.   I will note that on occasion when this happens, I've let the speed decay below the "bug" setting.   Would this have any impact on the pitch up?   I haven't read anything to indicate that either ALT or VS has any ability to overide a vertical speed or altitude setting.    If anything, I'd expect the aircraft to pitch down if I got too slow, not to pitch up which would get me even closer to stalling.

 

Thoughts?   

 

 

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Yes, I've seen this when I've got too slow - I usually hand-fly out of it ( well these days I'm better at not getting into it in the first place, kindof like staying out of VRS in DCS helicopters... ) until I'm at a better speed & put the AP back in. I've no idea what the real thing would do, there's some pilots here who actually fly them.

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50 minutes ago, Richard Dastardly said:

Yes, I've seen this when I've got too slow - I usually hand-fly out of it ( well these days I'm better at not getting into it in the first place, kindof like staying out of VRS in DCS helicopters... ) until I'm at a better speed & put the AP back in. I've no idea what the real thing would do, there's some pilots here who actually fly them.

The problem with cancelling the AP and hand flying is that often, you are out of trim and it just makes things worse until you can wrestle the jet back under control.     All that being said, it reinforces what I thought, in that being below speed contributes to this.  Just wondering if it's being below "bug" speed or actually being at stall speed?   

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Quote

 

this is the same bug discussed in much detail here: 

It happens when the trim position is mismatched with the AP's own trim value at the time you engage. 

 

The most common result is a hard pitch up maneuver, which frequently happens by engaging AP at a near cruise situation, where the plane is fast and the trim is far nose-down.  In such a case, the AP will "snap back" the trim to the middle and slowly move it towards stability from there, easily causing a few thousands of feet bump in the process.

 

If you land and takeoff again without restarting the flight from the main menu, this same issue will not repeat if you re-engage the AP in a similar near-cruise setting; This is because the last trim position used by the AP while it was on would've been further nose-down from when you disengaged it at the end of the previous flight.

Conversely, if you engaged the AP early on after takeoff, while low and slow in this second flight, you'd get a very alarming pitch down pull that might even result in the eating of dirt if not quickly corrected.

   

 

In short, this complicated stuff all happens because of the simple fact that the AP doesn't follow the trim position while it's disconnected.  When you engage, it'll always return (with a snap) to where the trim was when it was last disengaged (or to neutral, if it's your first flight)

 

Simple fix (for devs):   Ensure that the AP's internal variable tracking the trim position gets reset to the actual trim setting of the aircraft when the AP is turned on.  Or, just have it keep constant watch on it, even with AP disabled, so that it all matches up properly whenever it's engaged.

 

 

Until then, all we can do is anticipate the bug and do our best to minimize its effects.

 

 

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23 hours ago, jay jay said:

The problem with cancelling the AP and hand flying is that often, you are out of trim and it just makes things worse until you can wrestle the jet back under control.     All that being said, it reinforces what I thought, in that being below speed contributes to this.  Just wondering if it's being below "bug" speed or actually being at stall speed?   

You have a certain amount of control travel until the AP disengages, if you notice yourself getting slow you can at least stop it climbing ( too much ) without the AP dropping out if you're not all the way down at near stall. It's irritating but a bit of workload reduction.

 

Having the HUD working again might help matters too...

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I seem to be able to duplicate altitude deviations consistently. Too high Power settings seem to cause most above commanded altitude issues when on AP. The one I cannot explain is descending improperly in my case. When I am in level flight on autopilot and enter the STAR, (I use pilot2atc and it commands to descend via the STAR to altitude xyz then I set ALT to xyz and then enable VNAV) I notice that the plane starts descending before the TOD is reached on the MFD. I do not increase or decrease the airspeed, the plane just starts descending. After that, it takes manual intervention to get it back on the proper VNAV descent path and thereafter seems to behave properly. This behavior is sometimes exhibited when trying to capture glideslope. It will descend prematurely right through the GS capture and never capture the GS. I notice over and over that altitude is not held properly when in cruise with high wind speeds at altitude. IMHO, some Aerosoft investigation is needed for these problems.

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