A lost plane device does not always work. If it depends on the receiver battery for power, in a crash this is often disconnected. There are some that contain their own power supply at an added complexity and weight penalty. None will work with the newer 2.4GHz units that have fail safe as, the signal between the fail safe unit appears to be correct even though the RF link has failed as the fail safe consists of continiously sending a pulse train to the device that is exactly like the last valid signal before signal loss. PCM systems might also not work for the same reasons if fail safe is set up.