So the GPS system didn't go down, just one sat
providing SBAS and you could have fixed it on the
fly by turning off SBAS in the receiver.
You also still had the GPS is the iPad. We aren't
talking about flying IFR in gliders as it is not allowed in Australia.
Might be an idea to get GPS of a different brand
in the PC-12 so common failures don't knock out the whole system.
This is done in FBW control systems where
microprocessors of different types and
architectures are run in parallel with firmware developed by separate teams.
In any case, the presence of the standby compass did what for you?
Mike
At 12:17 PM 8/18/2016, you wrote:
> If the GPS system goes down, getting lost is
the least of your problems. I can think of only
two scenarios - hostile action (probably
nuclear war) or a Carrington event. In any
case over the next few years we will be talking
around 100 GNSS satellites from several
independent systems. GPS/Glonass receivers are
commonly available and
GPS/Glonass/Galileo/Beidou/QZSS are
increasingly available. > One of those rare
times that I pipe up on something here these
days. Hereâs evidence of such a double
failure in recent times, due to neither of your
scenarios, in a real aircraft with a lot of
internal redundancy:
http://www.bushflyingdiaries.com/2013/10/double-gps-system-failure.html
And I have another GPS failure mode to relate.
It amounts to âhuman error in running the GPS
network, triggering a software driven example
of poor choice in system specification' For a
week or two, a year or two back (Iâd have to
dig up the dates) the entire fleet of Pilatus
PC-12âs in Australia had non-functional GPS
systems, due to what turned out to be a stuffup
in the configuration of a satellite on the
northern edge of the Australian region, that
started presenting invalid SBAS data that the
PC12 Honeywell GPS systems decided was an
attempt to subvert the GPS system. In response,
in accordance with the (then) software specs
(i.e. not a bug, but an intentional feature),
the GPS systems on board shut down on the basis
of not being able to trust the data being
received. It arguably should have just shut
down SBAS reception and reported that and kept
right on going otherwise - but thatâs not how
the system designers had specified the outcome
in the presence of bad SBAS data. It took
Honeywell about a week to identify the cause
and come up with the (obvious) workaround -
disable SBAS at each aircraft start (its on by
default) - until the Satellite itself got fixed
(and it did then get fixed). In that week or so
in the middle, the entire Australian fleet of
PC-12NGâs (including mine) had no onboard GPS
that worked. Amusingly, the less sophisticated
GPS in my iPad kept running with AvPlan just
fine (redundancy, redundancy, redundancy) The
washup here is that another scenario exists in
addition to hostile action or a Carrington
Event - human error in the operation of the GPS
system itself, combined with GPS receivers that
are too smart for their own good (arguably also
human error - in terms of specification of how
theyâre to work in the presence of doubtful
input data). The root cause here appears to
have been a botched configuration change on a
*production* satellite somewhere in the
vicinity of the equator. Simon
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