Roy Smith wrote in message ...
(C Kingsbury) wrote:
The conservative choice would be to listen to the first box that cries
"miss." snip


I don't know if I agree with that. Let's say your calculations are
wrong and the timer runs out when you're 1/2 mile from the MAP. Which
has less overall risk:

1) Continuing another 1/2 mile to the real geographic MAP based on your
GPS, finding the runway, and landing uneventfully.

2) Going missed, and being back in the clouds trying to decide if you
should try the approach again or divert.

#1 sure sounds safer to me.

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. 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.

Or perhaps you're used to an approach at your home field where the DME
counts down, instead of up. You get distracted and see 3DME here, and
think, OK, I have 1.5 to go. So you putter on until you're 5 miles
away. Right about where that 606' obstacle is. Downdraft anyone?

My situation is purely hypothetical, but not at all unrealistic. I've
made every one of these mistakes in isolation. Even if I used the GPS
as a primary means of determining MAP, I would back it up with the
timer, which would quickly catch the gross errors described above.

Second, your response assumes that diverting to the alternate actually
increases risk. This is a variable situation. Where I fly in the
Northeast, you usually don't have to go more than 20-30 miles to find
an airport serviced by an ILS, which is usually what you'll put in as
an alternate if you're headed to a field with no precision approaches.
If your alternate is an asphalt patch with an NDB on the field, well,
then maybe you're better off trying to limbo your way in.

Best,
-cwk.