Hi @Glen, @Tridge and I discussed this in our weekly dev call today (which everyone is welcome to attend!) and he said he’s seen a similar (or same?) problem when flying his Parrot Disco in ArduPilot CRUISE mode so there must be a software bug lurking somewhere. Tridge is super busy with some infrastructure upgrades but has bookmarked this particular issue (and one other gremlin we can’t explain) so he’ll put some time in on it when he can.
@MagicRuB, @tridge I’ll have to listen in on the dev calls when I’m not working. Interesting re: the parrot disco and cruise mode. Reproducing the issue will hopefully mean that it’ll be easier to get to the source of it and should exclude the possibility of it being just my airframe / setup.
I completely understand that there’s a lot of work happening right now so thanks for putting it on the list 
Keep up the awesome work. If there’s anything I can do like test flights with extra logging enabled etc. just let me know.
Just wondering if this has started to be looked into? Should I post a bug report over on Github so it’s easier to track the progress of any work towards fixing this cruise bug and maybe encourage others to look into it too? I only say that because I know how busy you guys are! Thanks 
No progress, sorry. I created an issue that I noted above. This may be two issues though so go ahead and create another one and link this thread and the other related issue.
Thanks. I do believe it’s a different issue to the airspeed dropping too low because this can occur at any airspeed in the FBW airspeed min-max range even in straight and level flight. Even so, that was a good find on that other issue though and will make arduplane even better than what it was
I’ve created the CRUISE mode throttle and pitch issue here:
Thanks! We’ll get to it when we can.
Just to update, I’ve replaced the pixhawk with a new one and the issue still exists. Thought it was best to try a new one to eliminate the chance of this being caused by a faulty pixhawk.
I hope you guys can look into it sometime soon. The constant throttle and servo pulsing has me worried that one or the other will fail from being over worked and bring my plane down so I cant really fly it at the moment. I’ve been a big arduplane fan since version 1.x and this is the first time I’ve really struggled with a bug 
Please post any new logs you have.
Issue around the 16 minute mark. It usually occurs after a throttle change (cruise speed adjustment) or when initially entering cruise mode. Moving the throttle around can eventually get it to a point where it stops oscillating again. Or changing flight modes.
The only part of the plane I haven’t changed now are the ESCs and the airframe itself. The ESCs are fast response BLHeli ESCs that I had from another project - would there be any chance the faster throttle response could cause an issue?
I’ve cycled through everything else and redone the hinging, control rods and horns.
It’s just odd that no one else seems to have the same issue except for possibly that one parrot disco.
I wish I could do more to help. I’d be happy to trawl through code but I wouldn’t have a clue what to look for
The fact that it happened on another aircraft means it’s not a hardware problem. It’s probably happening to others too but your astute observation recognized it as a problem. I see the slew is set to the default THR_SLEWRATE=100, I wonder if slowing that down a bit to 127 (slowest) would reduce the effect as a workaround for you until we get to the bottom of this.
I’ve tried lower slew rates like 10 but because the oscilation was less than 10% up and down it didn’t seem to change it much. Sorry for the dumb question but how does 127 make it slower? I thought the slew value was the percentage change allowed per second so lower would be less change per second?
I got a tele radio and recorded a .tlog/.rlog of the issue. Not sure if that adds anything vs the ordinary log but here it is in case it helps diagnosing the issue:
Just a thought, I struggled with a similar problem for a long time but never solved it completely. The plane was a Super Skysurfer (Bixler-type) with a high mounted rear pusher motor which had a nasty tendency to dive (go nose-down) on full throttle. I had guessed that the high thrust-line pushed the nose down anytime throttle was increased and the elevator initially each wasn’t able to cope properly causing a type of phugoid (pitch oscillation).
The MTD has two wing mounted motors so would be less affected but maybe some motor down or up-thrust might help here?
Thanks for sharing. I would consider making changes but the fact is this only occurs in cruise mode. Auto / Loiter / RTL / FBW all work fine. Since cruise mode should be auto with a continuously updating waypoint 1km ahead there must be a bug in there somewhere.
I also seem to have problem with throttle and pitch auto control during CRUISE mode on my quadplane. After a successful transition, the aircraft pitch down and lose its altitude heavily even the pilot pull up the elevator to climb the aircraft.
Kindly find the log file for your review. https://drive.google.com/file/d/1_UJG039dSEZ9neO1ooA7t0R9yU8YjDtk/view?usp=sharing
Appreciate your input.