Nice spot Geoff.

On reading the pdf I found this info....

After the system makes a connection, if loss of signal occurs, the AR500 failsafe drives the throttle servo only to its preset failsafe position (low throttle) that was set
during binding.
€ All other channels receive no output pulses/ commands,and are not active during failsafe.

Seems that the AR500 reciever operates differently to the AR6000 .... rather than outputting a valid signal in failsafe it removes the signal thus... IMO ...the AR500 should be compatible with a std plugin failsafe.