SLOPING OFF...

...the Newsletter of Christchurch and District Model Flying Club for March 2021

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INTERFERENCE?

This article is from the newsletter of County Model Flying Club, and is used with the permission of the author, Chris Montague. It would explain a lot!

If you have concerns over incidents from the past, perhaps leaving your phone in the car would be a good idea.

However, even a Faraday cage won’t stop interference from your brain!

The number of flyable days during January was unprecedented, to use that much hackneyed word.  The winds were calm and the sky was often clear.  It was on one such day towards the end of the month when the conditions were too good to miss.  As has been my wont this winter, I turned up at the patch with my Arrow Hobbies T33 amongst other aircraft.  It didn’t take long to fit the Lipo and check that all was working as it should before getting airborne.  All seemed to be well in the world and the aircraft flew superbly without any problem for about three minutes.  What could possibly go wrong?

 Enjoying the spectacle of the aircraft in its intended environment, I was flying in a benign manner with gentle banked turns against the clear sky.  As I came in on yet another pass, I heard a beep, which I initially thought was my transmitter telling me something.  It was immediately followed by a complete loss of control.  I was fortunate in that it happened when the aircraft was in a gentle turn away from the flight line, and the aircraft carried on this trajectory until the inevitable happened.  It contacted the ground on the far side of the patch, damaging the airframe in the process.  At least the shock of the arrival caused the Lipo to disconnect from the ESC, but it otherwise remained protected by the foam fuselage.

 Having retrieved the bits, I decided to reconnect the Lipo to see if the radio was working correctly.  It wasn’t!  The airborne system was unresponsive with the LED on the aircraft’s receiver showing red rather than green.  It indicated that the signal from the transmitter was not being received.  To try and isolate the problem, I checked the transmitter with another aircraft and that indicated that all was well.  Wanting to take full advantage of the good flying conditions, I carried out a range check and went on to have a number of successful flights with this second aircraft.

Back in the workshop I looked more closely at the damaged T33.  The radio system was still unresponsive with the red LED continuing to show.  Wondering whether the receiver had lost its binding with the transmitter, I decided to go through the binding process to see if that would restore the signal.  As I delved down through the menus on my Futaba transmitter to carry out this process, the problem suddenly became clear.  Somehow my transmitter had changed from the FHSS operating system of the receiver to the FASST operating mode.  When I changed it back to the FHSS mode of the receiver, all worked as it should.

It should be impossible for the transmitter to change its operating mode part way through a flight and yet this is what appeared to have happened.  It caused me to scratch my head.  The only conclusion that I have been able to come to is that the transmitter’s memory for the model was corrupted by some external signal.  Could the presence of my mobile phone in my breast pocket and therefore within a couple of feet of my transmitter have been the cause?  It may just be a coincidence, but I received a text message on my phone at almost exactly the same time that I lost control of the aircraft.  It accounted for the beep which I had heard and initially attributed to the transmitter.

 ‘The jury has been out’ for a good many years on the topic of whether mobile phones can interfere with programmable transmitters.  Even the BMFA acknowledged the lack of evidence when it issued a safety bulletin on the topic back in 2013.  They nevertheless recommended that mobile phones should not be switched on within 10 feet of any programmable transmitter until more information was available. There is a tendency in all of this to think solely about the frequencies on which mobile phones and our transmitters are operating.  However, when assessing the electromagnetic compatibility of two devices or systems, the accepted aim is to identify and limit the unintentional generation, propagation, and reception of any electromagnetic energy that may cause unwanted effects.

What have I have learned from my recent experience with my T33?  First, I should have either switched off my mobile phone before flying, or ensured as a minimum that I kept it well away from my transmitter.  It is what I usually do, but I had omitted to do so on this occasion.  Secondly, and in hindsight, I shouldn’t have flown again with the same transmitter until I had identified the cause of the crash.  I didn’t know at that time that the model’s data had been corrupted, or have any knowledge of the degree to which such corruption might have affected the settings of other models.  As a final observation, I still had my mobile phone in my pocket – the likely source of the interference – and was lucky not to have had a repeat of the problem on subsequent flights that day.  It was bad enough to damage one aircraft.  To damage two would have been unthinkable!

And later...

Hello Mike,

You pondered whether the BMFA will do any actual testing.  They have said in response to my own queries that, during extensive testing back in 2013 when the problem was first highlighted, it was never conclusively proven that there was an issue as it couldn’t be consistently replicated.  Since then, the Joint Radio Control Users Community, now the Joint Radio Council, has become defunct as there has been no requirement for it to consider the issue any further!  Given this state of affairs, the BMFA maintains that mobile phones should be kept away from transmitters with the recommended pre-flight checks generally mitigating the potential issues.  Although this is their stated line, it is interesting to note that apart from their 2013 Safety Bulletin on the subject, they do not make reference to it in their members’ handbook.  It seems in all of this that I may have stirred the proverbial pot.  Whether anything comes of it remains to be seen but, as a minimum, Manny Williamson is likely to report on it in the next BMFA News and that, in turn, should be a useful reminder to others.

All the best

Chris