Roy Smith wrote:
In article ,
(C Kingsbury) wrote:


You're begging the question: "If I listen to the GPS and land safely,
isn't it safer to listen to the GPS and not head back up into the
clouds?" Of course it is, but you don't know in advance that following
the GPS will lead you to a safe landing. Check out this approach:

http://204.108.4.16/d-tpp/0409/00654VG23.PDF (VOR-23 @ LWM on Airnav)

Stright-in this will bring you down into a real minefield of stacks
and towers.


All of which are below the MDA. Which is a good reason not to descend
below the MDA unless you have the runway in sight.


Now let's say on the way down you plan to dial the VOR
into the GPS to get a DME reading to use as the MAP. But in the heat
of the moment you put LWM the airport instead of LWM the VOR in. This
means you'll wait until you're past the airport to miss. You're
probably OK so long as you don't go down any further, but you've
unquestionably increased your risk.


You can always screw up. That's why you brief approach procedures and
double-check your setup. What if you calculated the FAF-MAP time as
3:10, but put 4:10 into the clock instead?

I'm still going to believe a handheld GPS is more accurate than a DR
track. It may not be legal, but it's common sense.

Keep in mind that starting the missed too early can be as bad as turning
too late, it the procedure involves a turn predicated on you already
being past an obstacle.

Maybe someone can tell us how a hand-held GPS behaves with lots of
moisture in the air? I've been doing a lot of geocaching recently and
know for sure that trees cause outages. Maybe aircraft-certified GPS
units get around this somehow or at least flag the unreliable situation.
But how do you fly if you can't trust the GPS?