RTL not activated when PWM falls below FS_THR_VALUE

My Pixhawk -equipped quad has a PPM receiver which was binded at -135% throttle. After binding, the throttle was set back to 100%. The PWM was shown to be 915 at the radio calibration page, which was below the 975 PWM necessary to activate RTL. “Enable always RTL” is also selected under the “RADIO” box in “Failsafe” page. I then redo Radio Calibration.

To test RTL activation, 2 scenarios happened : –

1st scenario
After powering up Pixhawk on the bench using USB, I then arm the motors in Stabilize mode, with GPS 3D fix, then increase the throttle above minimum, then switch off the Tx, RTL is activated at this point. I then repeat this procedure for 3-4 times and everytime it works.

2nd scenario
After 10-15 minutes, without changing any setting, I repeated the above procedure, RTL was not activated, instead, “Land” mode was activated after switching off Tx and shortly followed by motor disarm. Same Stabilize mode, GPS 3D fix, motor armed and then followed by increase throttle above minimum, but with different result.

Appreciate any help.

Looking at the docs, you will see that it state that the vehicle will trigger Land (instead of RTL) if it less than 2m away from home (horizontally). Perhaps on the first attempt your GPS position was more than 2m away from home, but not on the second?

Actually I think that the close to home part doesn’t apply anymore as the RTL now won’t climb much. So the only possibility I see is that the change to RTL was rejected because you didn’t have good GPS signal and so it landed.
Post a log if you want someone to do a better analysis.

7-8 satellites were locked and HDOP was way below 2 at the time of arming.
So may not be the GPS signal.

After the posting, I referred to the Failsafe section of wiki and found
that if loss of signal (simulated by switching off the Tx) is within 2
meters from home, it would switch to LAND. And in fact, I did not move the
quad after arming the motors, until the moment I switched off the Tx. So it
is within 2 meters ( zero meter), that may explain why it switched to LAND