On 06/14/06 15:00, Peter wrote:
Mark Hansen wrote

This assumes you've switched the GPS/VOR button to VOR (or whatever
it is that switches the CDI on the external VOR head from indicating
the GPS to indicating the VOR).

That I can see; if the GPS/NAV switch is set to NAV then the GPS won't
be driving the HSI anyway.

This is a nice gotcha if you are flying an ILS, and the last leg of
the GPS track just happens to more or less line up with the runway
centreline, and you forget to change the switch to NAV In some odd
installations, if you set an ILS frequency in the radio, that switch
gets overriden by a relay, into the NAV position; not sure whether
that's a good solution.

I was taught to check the position of this switch as part of my
approach briefing. If I'm flying a GPS approach, make sure the
switch is in GPS, otherwise, make sure it is in NAV (or whatever).


By "fly the approach with the GPS", I mean using the GPS as the
primary navigational source - where you could have your VOR/ILS/
NDB receivers switched off (not that you would do that).

Indeed, but who knows what you have switched on or not.

The point is what is needed to fly the approach. If the GPS is
being used as primary (and you're ignoring the other nav receivers,
etc.) then the GPS must switch into Approach (Active) mode. If it
does not, you're not supposed to fly the approach.


If you still use the GPS to drive the CDI when it is not in Approach
(Active) mode, then you're not getting the sensitivity that is
required for this purpose. That is a difference, IMHO.

Yes, and that appears to be the only difference. However, you can
still set the 0.3nm sensitivity manually, as far as I can see.

The way I read the Pilot's Guide, I can't use the GPS for primary
navigation on the approach (with GPS selected on the GPS/Nav switch,
etc.) unless the GPS as switched into Approach (Active) mode.


Some of this stuff is wise, some isn't, but I still like to understand
how exactly it is supposed to work.

Always good to understand how it is supposed to work ;-)
The Pilot's Guide did a good job for me. Plus, I did an IPC using
this box, and so got lots of dual time using it for IR flight as
well.



--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA