[Bug] VTOL Fails to Switch to MC Mode During RTL After Losing Manual Control Signal

[liu12344321]

Describe the bug

I have set NAV_FORCE_VT to enable and RTL_DESCEND_ALT to 30m. My understanding is that in the landing mode, the VTOL should descend in fixed-wing mode while circling until reaching this altitude (30m), and then switch to multirotor mode for a vertical descent as it approaches the landing point. However, when the VTOL loses the RC signal, it initiates RTL and begins descending, but instead of switching to multirotor mode as expected, it continues to land in fixed-wing mode. This results in an improper landing behavior.
altitude diagram and motors diagram as below, which may show drone didnt switch to mc in landing mode , after the drone touch the ground, failsafe actived, Quad-chute triggered.

To Reproduce

1. Drone switched on
2. Uploaded mission and execute it.( drone took off)
3. transition to fw mode.
4. pause the mission and let drone loiter
5. lose manual control
6. cause of failsafe drone enters RTL mode
7. drone performs a hard landing in fixed-wing mode

Expected behavior

Drone should always land as a multicopter.

Screenshot / Media

No response

Flight Log

https://review.px4.io/plot_app?log=950f40bb-68d9-4c6c-a634-98e6b1f511b9

Software Version

v1.15.1

Flight controller

Holybro 6c mini

Vehicle type

Hybrid VTOL

How are the different components wired up (including port information)

No response

Additional context

No response

[DominicMarti]

Hi @liu12344321

I think there are two parts to your issue here.

• As far as I can tell from a quick code search, I don't think NAV_FORCE_VT is used in RTL behaviours. Therefore, it probably has no effect on what you want to achieve. There was an issue ages ago that indicated that no VTOL user would want to flar land in an RTL situation anyways, so it didn't make sense to include it: find it here
• Now for the flare landing you saw, this might be an issue of the parameter selection and the estimated altitude. From your log, you can see that the estimated altitude at the time of the quadchute trigger was still well above the 30m you set through the parameter:
  

So this might explain why you didn't see the backtransition triggered as you expected. Please beware that the rally point altitudes in the mission are set relative to the home position. Thus, if your rally point, in this case mission landing, is of higher altitude than home position, and you didn't adjust for it in the mission file, you will hit the ground before the altitude estimation gets low enough to actually trigger land. You can actually create very nasty crashes by setting the rally point altitude improperly, speaking out of experience ^^.

To test, you can either set RTL_DESCEND_ALT much higher, adjust the relative altitude of the rally point in the mission file, use a flat terrain with home and mission landing on equal altitude, or set rally points hardcoded to home.

Let me know if that helps!

Cheers!