[Flightgear-devel] Possible cause of JSBSim airport bug

[David M. <da...@me...>]

After a few gdb sessions, I think I've found the reason that
FlightGear isn't passing JSBSim the proper on-runway altitude at
startup time.  

In src/Scenery/tilemgr.cxx, FGTileMgr::update tries to ensure that at
least the tile immediately under the aircraft is loaded right away, so
that it can provide the proper ground elevation to the other
subsystems (fgInitSubsystems invokes FGTileMgr::update right at the
start, before the FDM is initialized).

Once it has loaded the tile, FGTileMgr::update invokes
FGTileMgr::current_elev_ssg to calculate the ground elevation.
Unfortunately, for reasons that I do not yet understand, this method
always fails and returns an elevation of 0 (that's what Tony was
seeing: 0 + 3.28 feet).  As a result, JSBSim is initialized with the
incorrect altitude, and all hell breaks loose.  In fact, gdb always
shows the vector passed to this method as a null pointer, but that
must be a gdb bug (since the method itself doesn't segfault).

It would be a good idea to make JSBSim robust enough to deal with this
kind of problem, but for now, we need to fix current_elev_ssg,
preferably for 7.7; when that's done, on-ground starts with JSBSim
might start working again.


All the best,


David

p.s. I haven't put this info in the bug-tracking system.

-- 
David Megginson
da...@me...