Autopilot suddenly kicks off and rolls left

[KungFu]

So, it's been a minute since I've flown this thing.  Several times during my last few flights the autopilot would suddenly cut off and the plane would roll left pretty quickly.  No stall, no input from me just cruising.    At first I though maybe the pitot tubes are icing over so I turn on the heat....still does it.  

I usually trim out level flight before engaging the alt or alt/heading mode.

What could be causing this?

[Lemur_Olo]

Hi KungFu,

I had a similar issue. In my case, it was a rudder pedal that goes to "sleep" after some time of inactivity during the flight. I mean, I was not touching them during the flight,  so Widows set them to power-saving mode. Then, the USB reading of the rudder position went immediately to the max left.

I you use rudder, check USB port setting and turnoff power saving, or something like that (I do not remember exact name).

Regards!

[ASAP]

1) Per the TO Pitot heat should always be on while in flight. Even if there is no expected conditions that would cause it to ice over.

2) It sounds like a random control input. I'd check your control bindings and make sure there isn't something in there that is unintentionally creating a yaw or roll command.

Real world that could happen if there was something like turbulence or you passing through another aircrafts jet wash. Also if your speed changes significantly after setting the autopilot. The roll moment makes me think that its probably an unintentional control though...

[KungFu]

54 minutes ago, ASAP said:

1) Per the TO Pitot heat should always be on while in flight. Even if there is no expected conditions that would cause it to ice over.

2) It sounds like a random control input. I'd check your control bindings and make sure there isn't something in there that is unintentionally creating a yaw or roll command.

Real world that could happen if there was something like turbulence or you passing through another aircrafts jet wash. Also if your speed changes significantly after setting the autopilot. The roll moment makes me think that its probably an unintentional control though...

I would tend to agree, but it happens on no other aircraft and I fly many in DCS (same MP mission/server) Doesn't happen in any other sim/game 

[jaylw314]

1 minute ago, KungFu said:

I would tend to agree, but it happens on no other aircraft and I fly many in DCS (same MP mission/server) Doesn't happen in any other sim/game 

It could be control problem + inadvertent axis binding if it's only in this module

[razo+r]

I have been flying quite a while with AP on and never experienced anything like this. 

Would be great if you could reproduce it and then attach a track of it for us to check. 

[KungFu]

I only fly multiplayer and I've got plenty of tracks for that, but since they are multiplayer I'd rather not upload for others privacy, so here is an instant action trk

I think I may know what is happening, I usually use path to climb out and that's when it happens the most during climb out.  

I think it possibly be wind as you can see it ever so slowly yaw to the left, very slow at first but as I climb it slowly progresses.   It did not roll as much to the left as this is a different loadout than I normally carry, I think that exacerbated the roll.

All controls are centered and I touch nothing other than to get it on path.

a10c.trk

[razo+r]

You made me test it myself and I noticed the PATH AP is not able to maintain the path even without wind (and no payload). Sooner or later it just slowly goes into a bank which will lead to the disconnection of the autopilot as per design.

 

@BIGNEWYMaybe something worth looking into?

[Yurgon]

17 hours ago, KungFu said:

All controls are centered and I touch nothing other than to get it on path.

For me, the track starts with a Master Caution and the landing gear coming down, which is then oversped at circa 220 KIAS, plus the radar altimeter being switched off, and the Master Caution flashing for the entire duration of the track. That looks a bit odd to me, but if that's how you like to fly around, I'm not judging...

Regarding the topic at hand, the autopilot has limited authority; it can only do so much to keep the aircraft attitude within desired parameters.

From the manual:

Quote

PATH (top position). This mode will attempt to keep the aircraft on its current flight path, represented by the total velocity vector symbol on the HUD. This mode will not engage with a bank angle greater than 10°.

You can see in the track that the AP disconnects the instant the bank angle reaches 10°.

With the asymmetric loads that we almost always have on the A-10, it's usually a good idea to help the aircraft by trimming it for hands-free flight before engaging the AP. Even so, every now and then you may need to disconnect the AP, re-trim, and reconnect the AP to help the aircraft.

What I find pretty odd in the track is the fact that the aircraft happily starts a left roll and the AP doesn't provide any kind of counter pedal to maintain heading; for me, it usually does that and the flight can become very uncoordinated while the aircraft desperately tries to maintain heading.

Worth noting, I don't know if the DCS A-10C behaves in accordance with its real life counterpart. It's just that I don't see anything unusual in the track. When the jet starts to roll while on AP, just disconnect AP, trim, reconnect AP. That's what I've always done.

17 hours ago, KungFu said:

I think it possibly be wind as you can see it ever so slowly yaw to the left

What would wind have to do with that?

Besides, I edited the Free Flight mission to have no wind whatsoever and saw the same type of roll that would slowly develop.

In either case, I'm not aware of the aircraft providing any kind of feedback regarding its autopilot authority, or when it reaches the limits of AP authority, so as far as I'm aware we just have to monitor the attitude and eventually help the jet by disconnecting AP and trimming, which we would have done manually if we hadn't been on AP in the first place.

Edited April 16, 2023 by Yurgon

[ASAP]

The autopilot is trying to fly the jet with rudder and elevator trim only so it is very susceptible to disconnects when things happen that require a lot of trim changes like speed changes and asymmetric drag. 
 

with path hold in particular, if you takeoff and set path hold to 10 degrees nose high for instance your airspeed will decay as the jet holds that attitude. At some point the the speed will have changed enough that the AP trim will run out of authority and can cause disconnects.  Normally while climbing a pilot would trim the aircraft up so that a constant speed would be maintained and the climb angle would slowly decrease to maintain that speed as you climb into less dense air. Path hold is good for short durations when you need to go heads down for a couple of seconds, but it’s probably going to present issues if you engage it from takeoff all the way up to 20K feet. Although you could disengage, reset climb angle, trim and reengage.

Asymetric stores will cause problems as well especially with aim-9s or a massive jamming pod. The shape of the A-10 is prone to yaw buildup where the more the jet slips the more air is hitting the sides of long objects hanging under the wing which means the more it wants to slip requiring greater rudder inputs to counteract it. The YSAS fights a lot of this for you but it’s possible if all the autopilot has is rudder to try and hold heading and a side slip develops it could lead to the AP trimming all the way to the APs authority limit and kicking off. At which point the rudder trim could induce a roll. 
 

I’ve heard the autopilot normally flys a bit uncoordinated. It holds heading and attitude pretty well but it’s generally not super comfortable because of the lack of coordination. Also sometimes weird disconnects happen and it’s not always clear why. So theres apparently a couple of possible causes if it’s not random control input issues. 

Edited April 16, 2023 by ASAP

[KungFu]

Quote

For me, the track starts with a Master Caution and the landing gear coming down, which is then oversped at circa 220 KIAS, plus the radar altimeter being switched off, and the Master Caution flashing for the entire duration of the track. That looks a bit odd to me, but if that's how you like to fly around, I'm not judging...

Yeah, it lagged when I entered the mission, I have an Orion 2 throttle that must have had the gear switch down and I didn't notice the gear down due to the studders on mission entry.  Didn't even notice the Master Caution either, it was off when I was able to take descent control

Anyway, from what I'm hearing these seems to be normal behavior and is repeatable.  This is fine, just wanted to know why I was seeing it.