Need you help - weird altitude loss in 2.78 auto modes

I am new to APM and just started to enjoy it…I habe the 2.5 version with airspeed sensor…generally it flies really well in

  • MAnual
  • Stabilize
  • FBWA

And all of them work nicely. FBWA has a bit of too slow gaining height, goes up very slowly, but is a pretty Relaxing mode, Need to narrow it a bit more towards stabiize for my taste, but it makes sense.

The second group with the Auto-functions nevertheless were trouble maker:

  • Cruise
  • Loiter
  • RTL

On cruise, it immediately started to accelerate and descend drastically. From a mode were altitude hold should be the priority, I would nevere xpected this. Or is target speed the priority no matter of altitude ?

I would have expected something like a Change in throttle as the throttle stick is no longer a throttle stick, but an air Speed stick (I have the air Speed sensor active). But I have NO IDEA why the plane lost height so quickly (or at all, all of these modes should maintain the height)and accelerated at the same time. If I accelerate in stabilize it would not loose height, so has nothing to do with prop- forces.

On Loiter, it did the same: became faster and faster and lost height…

…so after max 5 sec I needed to Switch back to the first three modes or I would have crashed the plane.

It was a windy day today, but that cant be the explaination. I used the Skywalker Profile from the ardupilot Webpage, but not went through the Tuning process of all PIDs myself (yet).

I have no clue yet what was going on…your help is highly appreciated.

tlogs or dataflash logs would be good to post if they are available.

here you go…

Ok, I found one error: Throttle needed to be reversed. This works now with Air Speed off. Tomorrow testing with air Speed ON. Keep you posted.