One of the problems that came to light when the tragic death of eleven year old Adam Kirby was investigated was that many model flyers fail to set the fail-safe of their PCM radios correctly. Adam was struck by an R/C powered model aircraft and killed in England on 14th March 1999. The coroners inquest into the accident concluded that a contributory cause in this accident was the settings of the computer transmitter based programmable fail-safe. The radio in use was transmitting in pulse code modulation mode (PCM) and the programmable fail-safe was set to HOLD. Since the terminology varies from one radio manufacturer to another, for this article the term HOLD refers to a scenario where the servos HOLD the position they were in just before the interference. The term PRESET refers to the moving of the servos into pre-programmed positions which were defined by the operator before flying, and FAIL-SAFE is a term to cover both.

Computer radios are an ever increasing feature of modern R/C flying, regrettably many of the users of such devices do not fully understand their operation nor the content and facilities of the operational software used. This is particularly the case when PCM mode is selected with respect to the PRESET/HOLD options.

A number of fliers using these radios may be unaware that selection of PCM mode brings with it the fail-safe features of the program. Basically if you select PCM you automatically have a fail-safe set to prevent the model from flying away. So the unfortunatly you may have a fail-safe operating on your model without realising it. Moreover, because you did not realise it, you have not programmed it, and in the event of signal loss, it would defer to the factory default settings, usually HOLD. This of course includes the throttle servo. If this interference occurs at take-off for example and the throttle holds at an opened position with minimal deflection on the other control surfaces, the results can be disastrous. If you use a typical Computerised Transmitter, you should be aware of the following basic findings.

Almost all sets give an automatic PRESET/HOLD feature within the program as soon as PCM is selected. This failsafe becomes operational if interference occurs at the receiver of the model or the transmitter signal is lost. Control returns to the pilot when the interference ceases. If interference does not cease, the pilot will not regain control.The fail-safe software cannot be totally inhibited. It will either hold all (HOLD) or go to positions pre-programmed by the pilot (PRESET). Some sets allow a combination of this feature. i.e. some channels will go to a PRESET position and some controls will HOLD at their last position.
The default setting (if nothing is programmed) is to HOLD at last input position, including throttle. This will become operational after a pre-determined interval set by the pilot or after a default interval if nothing is programmed. The interval is the length of time it takes, starting the moment interference occurs, until the servos assume their pre-set positions. The fail-safe does not work in the event of receiver ni-cad loss.

Programming of the PRESET feature is done by opening the memory of the computer, putting the sticks, switches and potentiometers in a selected position and then closing the computer memory. This can be quite difficult to follow even from the radio instruction manual.

If a model normally flown on normal modulation (FM/PPM) is swapped to PCM by a change of receiver, the fail-safe feature is again automatically selected even though none of the other settings in the transmitter are altered. There is no warning given to the pilot to remind him that he is now flying under the protection of PRESET/HOLD software.

One recommendation that has been made is to ensure that when any powered model is flown under the protection of a Fail-safe device, particular care should be taken by all pilots of PCM (or when swapping from FM/PPM to PCM) to ensure that the PRESET/HOLD feature is correctly programmed with throttle to idle (stopped in the case of electric power), and especifically not to HOLD the last position of the motor/engine control.