Strange landing behavior, 4+1, hard landing wiht ranger finder

VTOL 4+1. 2500mm aircraft. 12kg. The first landing was successful - log 00149. The second one did not slow down during landing and fell on its belly. the settings have not been changed, the batteries are new. I can’t find the problem.

we observe this problem regularly. about every 10th landing. log analysis does not help.

good landing - 149 - 00000149.BIN — Яндекс Диск
bad landing - 150 - 00000150.BIN — Яндекс Диск

How was this vehicle tuned? What steps did you take?

I don’t have an answer to your question. another person developed it, there is no connection with him. I’m using it. I don’t understand how an intermittent landing error affects the initial setup.

You are correct:
Intermittent landing errors do not affect the initial setup.

But:
The improper initial setup can cause intermittent landing errors and all sorts of other problems.
Not correctly configured vehicles do tend to fall on their belly.

There’s some way to find them. Are they just randomly shooting like that

Yes, there is a methodic way to do the initial setup:
Methodically configure and tune ArduCopter It can also be partially used for plane.

this advice can be given for any problem with an ardupilot

Yes, that is the basis for the entire software. You need to configure the way you want the software to behave. Without configuring it, the software will not behave as you want/need. And the software can not guess some of the hardware parameters.
Once that is done correctly one can look at other issues.

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and now it is impossible to assume that everything is working correctly?

what should I do if the problem repeats after the setup that you suggest? It’s not just one plane. this is 4-5 planes with an intermittent error.