Safety Bulletin
BMFA SAFETY NOTICE
PCM / FAIL-SAFES
Report from the BMFA Technical Council -
30th October 1999
Compiled by Andy
Ellison. R/C Power Technical Committee.
On Sunday 14th March 1999 eleven year old Adam
Kirby was struck by a radio controlled powered model aircraft and killed.
The coroners 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 the legalities surrounding
a case of this type, the BMFA 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 BMFA 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 brings with it the failsafe features of the program.
Basically if you select PCM you automatically have a failsafe set to prevent
the model from flying away.
The program has no respect for the weight of the
model (fail-safes 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 on their model as
it was under the 7Kg limit.
The unfortunate fact of the matter is that they
did indeed have a failsafe operating on their model but did not realise
it. Moreover, because 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 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 failsafe 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 ( 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
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
(PPM) is swapped to PCM 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 BMFA
are taking this matter, the Technical Council unanimously agreed a plan
of action which has been reported to the coroners inquest and includes
:
1.The forwarding of our findings to all
known radio manufacturers highlighting the problem and requesting/advising
that they :-
-
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
-
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 CIAM/FAI with the
expressed request that they distribute them to all National Aeromodelling
Governing Bodies in a suitable publication.
3. The CIAM/FAI/BMFA 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 its 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 BMFA Technical
Council that clubs and individual fliers 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 does not happen again.
We recommend that particular care is taken by
all individuals when operating on PCM or when swapping from PPM to PCM
to ensure that the PRESET/HOLD feature is correctly programmed with throttle
to tick-over (stopped in the case of electric power).
We also strongly recommend that all clubs incorporate
a regime into their flying field rules that will ensure that all operational
fail-safes 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
and regardless of the size of the model.
Andy Ellison
R/C Power Technical Committee.
© British Model Flying Association. Updated:
11 November, 1999
|