I've had a couple of crashes where for different reasons I have accidentally ended up in Stabilise mode, with the throttle stick at minimum. This resulted in motors stopping and my not being able to get them started again in time to stabilise and recover; it turns out to take a surprisingly long time during which the copter can start tumbling.
So If I'm flying in manual throttle (Stabilise) mode then I want there to be a delay if I accidentally place the throttle to zero, to prevent the motors stopping and the copter falling out of the sky. This shouldn't apply if I'm in an automatic throttle mode (altitude hold) because there the stick controls the descent rate and the autopilot (APM) won't stop the motors. But, if I swap into manual mode from auto mode, then this needs to be accounted for.
I'm using a Turnigy 9XR transmitter (i.e. er9x firmware), and I have now managed to set this up with the following mix which I thought I'd share in case anyone else agrees that this is a good idea.
Set up four custom switches
SW1 = v<ofs THR -80
SW2 = AND THR SW1 (Specify here the physical switch you want to switch this throttle management on / off with)
SW3 = v<ofs CH5 -50
SW4 = AND SW2 SW3
Then in the channel mixer for the throttle channel
-100%THR Switch(SW4) Offset(80%)
+100%THR Switch(SW4) Offset(80%) Delay(u5:d5)
This means that if I have the THR switch off then the throttle output behaves normally. If I put it on, then when I raise the throttle and fly around in manual throttle mode, if I should mistakenly put the stick to the bottom the output will "bottom out" at 10% for five seconds before cutting out, hopefully keeping the motors spinning so they recover quicker.
If I'm in altitude hold mode this doesn't apply and the stick maps to the output as normal so you can descend at full permitted rate.
However if I'm in altitude hold mode with the stick at the bottom, so output is zero (but the autopilot has control of throttle) and then I switch in to stabilise mode - the throttle will be brought up to 10% for 5 seconds which should hopefully keep the motors spinning whilst I realise my mistake and bring the stick back up.
You can set a beep on SW4 so that you're warned that you've messed up and need to pay attention fast, although at 10% throttle you're going to see the copter falling fast anyway - this should just hopefully make it more recoverable.
This assumes that you have stabilise / acro mode on the lowest Ch5 setting and auto throttle modes above that. The value of -50 will need tweaking to suit the threshold between those modes. The 80% offset, giving a 10% min throttle, may also need changing to keep the motors spinning.
Any comments welcome!