I've spent some time trying to understand the spurious alerts the Mk-
VIII sometimes produces. I'm hopeful that with a little bit of love,
and some better documentation, more aircraft developers will be able
to use this (rather complex) beast in their aircraft.
My major impression from working the code is that's is a very accurate
modelling of the real unit (given the limits of FG). Every time I've
thought it was doing something wrong, it has generally been the input
data that's wrong.
Case in point - in the Citation Bravo, there's all manner of strange
alerts - I get bank angle alerts on take off, and loads of 'too low,
flaps', 'glideslope' and 'bank angle' warnings on finals. I'm not all
the way there with understand what is happening, but a major part is
that the Bravo's attitude indication system seems to be non-
functional. The key properties here are:
/instrumentation/attitude-indicator/indicated-roll-deg
and
/instrumentation/attitude-indicator/indicated-pitch-deg
In the case of the Bravo, these are always returning bogus values -
roll-deg is 40 degrees (hence the bank alarms!) and pitch-deg is 12
(again, I suspect not helping on landing).
attitude-indicator comes from two possible sources - either the
attitude-indicator.cxx class, which uses a vacuum powered gyro, or the
'master reference gyro' in mrg.cxx, which is electrically powered. The
Bravo's electrical bus is definitely fine, but I'm less sure about the
vacuum system - once the engines are running, it's reporting a value
of (from memory) -14.56 (inHg). I've no idea if negative numbers are
correct, but I can believe that they might be, especially for inches
of mercury. Anyway, the gyro-spin is reported as zero, despite the
attitude indicator being serviceable and the gyro being uncaged.
At this point I could benefit from others (Syd?) about how this is all
meant to be fitting together, so the gyro gets powered and the MkVIII
sees a valid input.
(All of the above is with bleeding edge CVS of data and source)
Other scenarios where the MkVIII behaves badly are welcome - I have a
handle on the code now, so more things to test are good. I'm
collecting any documentation I learn in the following wiki page:
http://wiki.flightgear.org/index.php/Ground_proximity_warning_system
Regards,
James
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