"Mxsmanic" wrote

It changes at a variable rate, and once it approaches a degree, charts and
navaids have to change. It's a tremendous amount of extra work, an
additional
source of error, and an imprecise and fickle basis for navigation.


I came to aviation from boats. In that field of navigation, in my part of
the world we mostly use true north as reference, but I've seen charts using
magnetic north, e.g. in the Caribbean where the magnetic variation is
considerable (around 15 degrees). Also, all boat GPSes can be set to show
either true or magnetic north, so using magnetic north is obviously a viable
method also at sea. So, let me give some arguments why using magnetic north
on aviation charts is not as stupid as it may sound.

1. A GPS does not show a reliable heading unless the aircraft is moving
(unless the GPS is of a very advanced type with dual antennas, not widely
used in aircraft). Before every takeoff the DG needs to be set, and the
magnetic compass enables this also when the aircraft is stationary.

2. The extra work referred to by Mx above is the same as would otherwise
have to be done in-flight in each aircraft, adding to the crew's workload
and introducing many possibilities for errors. (Mariners actually do this
en-route, but they have much more time to do the calculations than pilots
have.) For aviation, it's safer to do the calculations and corresponding
changes to charts and navaid data at a central source where they can be
quality-controlled much more extensively than what the co-pilot can do in an
aircraft in-flight. Charts need to be updated frequently anyway, regardless
of changes in magnetic variation, so it's not a big deal.

3. Even though electronic or inertial systems would allow navigation without
any reference to magnetic north, making the above mentioned calculations
unnecessary, experience and tradition so far points to the conclusion that a
magnetic compass and the corresponding magnetic designations on charts are
still useful. Note that when the magnetic compass is most critically needed,
i.e. in case of a failure of the electronic navigation systems, that's also
the time when the pilots have least time and opportunity to perform the
extra calculations that would be required if the chart data were given in
true north.

4. The magnetic compass system gives a simple foolproof method to ensure the
aircraft is lined up on the correct runway, anywhere in the world. Last
August, 49 real people died at KLEX in an accident that might have been
prevented by this simple check.

5. Omitting the magnetic compass would make the aircraft totally dependent
on external sources for all navigation other than chart-based VFR (unless it
has an inertial navigation system, which is obviously a much more expensive
solution viable only for large aircaft). This is so far not considered
acceptable.