Hello, I was testing Smart RTL along a relatively straight flight path by flying out about 2 miles in auto mode and returning in SRTL. Shortly after switching Smart RTL at the far point, the aircraft abruptly stopped and began descending. No flight mode change or error was shown in MP at this point. I switched into loiter mode and brought it back manually.
Upon reviewing the log, I noticed that SRTL.Action switched to state 5 at the exact moment the descent started. I have a dozen other identical flights that worked fine and never went to state 5. From what I see in the source, it looks like 5 refers to “SRTL_POP_FAILED_NO_SEMAPHORE”, and landing is the desired action in this case, but the comment also says “this can only happen if we fail to get the semaphore which should never happen but just in case, land”
Anyone know what could cause this case, and hopefully how to prevent it in the future? Landing two miles away in this case would have been dangerous, so fortunately we were closely monitoring it and recovered the aircraft.
This post from January shows the same behavior with no resolution: