A very important point found by Jack, which has been raised a few times before in other threads, is that the minimum operating voltage of the Rx is sometimes now lower than that of the servos. Due to the brown-out problem of early Spektrums, people have become fixated on the Rx minimum voltage and keeping above that, but just above the Rx minimum could be below the servo's minimum and would explain some unexplained crashes where the model seems to have locked out and not gone into failsafe as if the battery had failed, yet those items which would shut down on loss of rx signal such as an ESC or ECU continue working which means the rx is working. Data on servos is not so easy to get as it is for Rx, for example Multiplex only has the instructions for one of its rarely used servos on its website and it states the operating voltage is 4V to 7V. The Multiplex M-link Rx minimum voltage is 3.5V, (whereas the previous PCM rx minimum was 4V) so in that instance if the voltage drops progressively the servo would stop working well before the Rx stopped working and we could see a bizzare loss of control where the servos freeze in last position but the ECU or ESC is still going, the telemetry is still working and reporting 100% link quality, yet all control seems to have been lost. Perhaps servo brown-out will become the new popular cause for a crash!