GPS Altitude with WAAS

(Michael) writes:

But think for a second - why do you suppose MEA's and OROCA's provide
1000 ft of obstacle clearace normally, but 2000 in designated
mountainlous areas?

That's exactly why, of course.

If you're IFR, you're not going to be clearing that peak by less
than 2000 ft, and that is going to keep you out of the rocks in even
the worst case scenario. If you're VFR, then you can see the peak
and don't really need the altimeter anyway.

That's what one would hope. Unfortunately, there are many gradations
between CAVU and IMC, and pilots do seem to have an unfortunate
tendency to fly into mountains or get stuck in canyons from time to
time while (legally) VFR.

The most interesting result of WAAS, I think, will be its educational
effect on pilots. Right now, I often see postings from people who
think they can use their altimeters to check the vertical accuracy of
their GPS's. At cruise altitude, the GPS is likely to be by far the
more accurate of the two even without WAAS (though obviously we need
to stick with the altimeter to ensure separation). Near to the
ground, as you mention, the altimeter may become the more accurate of
the two, assuming that you have the latest altimeter setting for the
field.


All the best,


David

David Megginson wrote
That's what one would hope. Unfortunately, there are many gradations
between CAVU and IMC, and pilots do seem to have an unfortunate
tendency to fly into mountains or get stuck in canyons from time to
time while (legally) VFR.

Some people fly to the legal limits of the certificates and ratings
they hold; others do not. I realize this is obvious. What should be
equally obvious (but sometimes is not) is that in order to do so with
a reasonable margin of safety, you need a much higher level of skill
and knowledge than what is required to pass the checkride.

What we're doing here is exploring one tiny corner of the knowledge
envelope in great detail. Most pilots couldn't care less. Most
pilots are also not going to fly in the mountains in anything less
than good day-VMC, so they really don't care - or need to.

The examiner is not able to check every possible knowledge area in
depth - that's not his function. He only performs a rough check to
make sure the instructor didn't leave out anything really major.
Anything else would take much too long.

So really, all you need is a few people who don't know what they don't
know and pay the ultimate penalty to observe what you've described.
I'm not sure if lack of knowledge of temperature errors has ever
caused someone to fly into an obstruction he thought he was clear of,
but I can certainly believe it has happened.

Michael

(Michael) writes:

So really, all you need is a few people who don't know what they don't
know and pay the ultimate penalty to observe what you've described.
I'm not sure if lack of knowledge of temperature errors has ever
caused someone to fly into an obstruction he thought he was clear of,
but I can certainly believe it has happened.

I wonder about that when I read an accident report where it appears
that the pilot tried to dive under MDA or DH on a cold day and ended
up hitting something.

Obviously, you don't get a lot of -50 degC days, even in Alaska or the
Canadian arctic, but still, if you combine all of the other vertical
and lateral errors (within legal tolerances) with a more realistic
cold-temperature error, it could be that a small minority of those
CFIT accidents are not hot-dog pilots ignoring minima after all.


All the best,


David