FBWB/Cruise Change Altitude Intermittent Bug

Hello everybody,

This is the first time I use this forum for bug reporting, so if I do something wrong, let me know.

I was tuning today a friend’s APM 2.5, running version 3.0.3, from scratch and I noticed something weird. I’m not sure it’s a bug, but hey, what do I have to lose?

My friend is interested in FPV and he asked me to include Cruise mode in his Flight Modes repertoir. After tuning up to the TECS system, I told him to switch to CRUISE to test how the autopilot handles altitude hold and changes in this mode.

You can find the relevant log attachment named “Faulty Flight”.
The plane takes off at 9.8% and enters CRUISE at 53%. At 56.6% I asked my friend to do a full throw on the elevator. Notice that after 18seconds of pushing hard on the stick the commanded altitude changes only by 14m. This corresponds to roughly 0.77m/s climb rate and it was apparent that something went wrong.

I told him to land the plane and then did the following changes.
Set FBWB_ELEV_REV to default (0), which was requested.
Increase FBWB_CLIMB_RATE to 5.
You can see the ensuing flight on the “Fault-free Flight”.
Take off is at 21%.
The plane enters FBWB at 28%.
An elevator deflection occurs at 29.3% for 4seconds and the commanded altitude changes by 20m. A 5m/s change, which is correct.
Then I reduced FBWB_CLIMB_RATE to 4.
An elevator deflection occurs at 34.1% for 7seconds and the commanded altitude changes by 2m. A 3m/s change. Very acceptable.
At 36% the plane enters CRUISE mode.
An elevator deflection occurs at 37.5% for 8 seconds and the commanded altitude changes by 25m. A 3m/s change, also acceptable.

TLDR: In the first flight FBWB altitude handling doesn’t work, in the second it does.

Thanks in advance!

I just saw that the dataflash is required for debugging, instead of the telemetry log, so here is them.