Receiver based failsafe is where you, from your transmitter and receiver, configure channels to output desired signals if your receiver detects signal loss.
The idea is that you set throttle and other controls so the aircraft descends in a controlled manner. See your receiver's documentation for this method.
It is possible to use both types at the same time, which may be desirable. Flight controller failsafe can even help if your receiver signal wires come loose, get damaged or your receiver malfunctions in a way the receiver itself cannot detect.
The failsafe system is not activated until 5 seconds after the flight controller boots up. This is to prevent failsafe from activating, as in the case of TX/RX gear with long bind procedures, before the RX sends out valid data. Note that you need to activate the 'FAILSAFE' feature in order to activate failsafe on flight controller.
The failsafe system attempts to detect when your receiver loses signal. It then attempts to prevent your aircraft from flying away uncontrollably by enabling an auto-level mode and setting the throttle that should allow the craft to come to a safer landing.
c) the failsafe guard time specified by `failsafe_delay` has elapsed.
Note that:
d) The failsafe system will be activated regardless of current throttle position.
e) The craft may already be on the ground with motors stopped and that motors and props could spin again - the software does not currently detect if the craft is on the ground. Take care when using MOTOR_STOP feature.
### Testing
Bench test the failsafe system before flying - remove props while doing so.
1. Arm the craft.
1. Turn off transmitter or unplug RX.
1. Observe motors spin at configured throttle setting for configured duration.
1. Observe motors turn off after configured duration.
1. Ensure that when you turn on your TX again or reconnect the RX that you cannot re-arm once the motors have stopped.
1. Power cycle the FC.
1. Arm the craft.
1. Turn off transmitter or unplug RX.
1. Observe motors spin at configured throttle setting for configured duration.
1. Turn on TX or reconnect RX.
1. Ensure that your switch positions don't now cause the craft to disarm (otherwise it would fall out of the sky on regained signal).
1. Descend the craft and observe throttle position and record throttle value from your TX channel monitor. Ideally 1500 should be hover. So your value should be less than 1500.
c) Ensure your receiver does not send out channel data that would cause a disarm by switch or sticks to be registered by the FC. This is especially important for those using a switch to arm.
3. Set 'failsafe_throttle' to a value that allows the aircraft to descend at approximately one meter per second (default is 1000 which should be throttle off).
Guard time for failsafe activation after signal lost. This is the amount of time the flight controller waits to see if it begins receiving a valid signal again before activating failsafe.
Delay after failsafe activates before motors finally turn off. This is the amount of time 'failsafe_throttle' is active. If you fly at higher altitudes you may need more time to descend safely.
Throttle level used for landing. Specify a value that causes the aircraft to descend at about 1M/sec. Default is set to 1000 which should correspond to throttle off.
With a Graupner GR-24 configured for PWM output with failsafe on channels 1-4 set to OFF in the receiver settings then this setting, at its default value, will allow failsafe to be activated.