PCM/Failsafes

Report from the B.M.F.A. Technical Council – 30th October 1999.

Compiled by Andy Ellison, R/C Power Technical Committee

The Coroner's inquest into the accident has concluded that a contributory cause in this tragic accident was the settings of the computerised transmitter based programmable Failsafe.

In this instance the radio in use was transmitting in Pulse Code Modulation mode (PCM) and the programmable Failsafe was set to HOLD.

On Saturday 30th October, after a long study and information gathering exercise, restricted by thr legalities surrounding a case of this type, the B.M.F.A. Technical Council sat to deliberate proposals put forward by myself relating to the use of PCM radio and its association with programmable radio Failsafe devices. This was in order that the problems associated with its usage could be clarified and passed on to R/C fliers via this publication, the B.M.F.A. members handbook and by the instigation of competition rule changes. The following report is a synopsis of my findings. Please bear in mind that the terminology relating to this topic varies greatly from one radio manufacturer to another, and so for the purpose of this safety notice I have generalised. The term HOLD refers to a scenario whereby the servos HOLD the position they were in just before the interference arrived. The term PRESET refers to the moving of the servos into pre-programmed positions which were defined by the operator before flying, and Failsafe is a generic term to cover both.

Computerised radios are an ever increasing feature of the modern R/C flying club. Regrettably it is also the case that many of the users of such devices do not fully understand their operation nor indeed the content and facilities of the operational software contained within. This is particularly the case when PCM mode is selected with respect to the PRESET/HOLD options. In my research I found that a disturbing number of average club fliers using these radios (around 80%) were unaware that selection of PCM mode automatically brings with it the Failsafe features of the programme. Basically if you select PCM you have a Failsafe set to prevent the model from flying away. The programme has no respect for the weight of the model (Failsafes are mandatory on models over 7kg dry weight) and modellers transmitting on PCM, when questioned on the settings of their Failsafe, frequently answered that they did not have a Failsafe operating on their model as it was under the 7kg limit. The unfortunate fact of the matter is that they did not realise it, they had 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, or have members in your club who do, you should be aware of the following basic findings.

• Almost all sets give an automatic PRESET/HOLD feature within the programme as soon as PCM is selected. This Failsafe becomes operational if interference occurs at the receiver of the model. Control returns to the pilot when the interference ceases. If interference does not cease, the pilot will not regain control.
• The Failsafe software cannot be totally inhibited. It will either hold all (HOLD) or go to positions programmed by the pilor (PRESET). Some sets allow a combination of this feature, i.e. some channels will go to 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 (1.0 seconds, 0.5 seconds or 0.25 seconds) 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 Failsafe does not work in the event of receiver nicad 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 (PPM) is swapped by a change of receiver, the Failsafe 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.

To illustrate the seriousness with which the B.M.F.A. are taking this matter, the Technical Council unanimously agreed a plan of action which has been reported to the coroner's inquest and includes:

1. The forwarding of our findings to all known radio manufacturers highlighting the problem and requesting/advising that they:-
   1. Negate the problem through the use of revised software i.e. inhibit the Failsafe function so that it must be selected to turn it on, and modify the factory setting for the throttle function so that it does not default to "HOLD"

      and

   2. Publish in the operations manual for the Radio, clear and concise instructions highlighting the potential danger of an "un-programmed" transmitter based, radio Failsafe device.

2. The findings are passed to the C.I.A.M./F.A.I. with the express request that they distribute them to all National Aeromodelling Governing Bodies in a suitable publication.

3. The C.I.A.M./F.A.I./B.M.F.A. instigate an emergency general rule change to ensure that any powered models flown under the protection of a Radio Control Failsafe device must ensure that said device brings the motor/engine to run at it's lowest speed, (stopped in the case of electric powered models), and specifically not to HOLD the last position of the motor/engine control.

There are other factors which contributed to the sad death of Adam Kirby and these are covered elsewhere. The purpose of this safety notice is one of education and promotion of awareness. It is the strong recommendation of the B.M.F.A. Technical Council that clubs and individual flyers alike take on board the points made above and assess them against their own operating system and philosophy as responsible users of this equipment and as model flyers to ensure that such an occurrence is not forthcoming again.

We recommend that clubs incorporate a regime that will ensure that all operational Failsafes in use on powered models (including "add on" devices used with PPM) must set the throttle to tick-over (stopped in the case of electric power) regardless of the other control operations governed by the Failsafe. Also that particular care is taken when operating on PCM or when swapping from PPM to PCM to ensure that the PRESET/HOLD feature is correctly programmed.