GPS Altitude with WAAS

David Megginson wrote:

On mailing lists,
I've actually had violent reactions from otherwise experienced and
competent pilots when I casually mentioned that pressure altimeters
are routinely off by hundreds of feet at cruise altitude.

It's pretty simple, really. It doesn't matter at all if your altimeter is off
by hundreds of feet at cruise altitude if everybody else at that altitude has
the same error. It would be possibly unsafe for you to set your altimeter
accurately when everyone else is setting it to the broadcast local altimeter
setting.

In short, it doesn't matter. Just do it like everyone else. For noise abatement
reasons.

George Patterson
A man who carries a cat by the tail learns something that cannot
be learned any other way. Samuel Clemens

On Tue, 23 Sep 2003 12:21:09 GMT, David Megginson
wrote:


In Canada, learning about temperature errors in the altimeter is a
standard part of the PPL curriculum, but I've noticed that it's not so
familiar to U.S. pilots (at least not private pilots). We have tables
in our AIP and other publications showing what errors to expect, and
when flying IFR, we are required to add those errors to all instrument
approach altitudes (MDA, DH, etc.) in very cold temperatures.

There is no such "requirement" , either in Canada nor the USA.

Stan

writes:

In Canada, learning about temperature errors in the altimeter is a
standard part of the PPL curriculum, but I've noticed that it's not so
familiar to U.S. pilots (at least not private pilots). We have tables
in our AIP and other publications showing what errors to expect, and
when flying IFR, we are required to add those errors to all instrument
approach altitudes (MDA, DH, etc.) in very cold temperatures.

There is no such "requirement" , either in Canada nor the USA.

From the (Canadian) AIP, RAC 9.17.1:

The calculated minimum safe altitudes must be adjusted when the
ambient on the surface is much lower than that predicted by the
standard atmosphere.

Note the use of the term "must" rather than "should". Granted, the
AIP is not the CARs or the Aeronautics Act, but it is the closest we
have to a definition of what the TATC (formerly Civil Aviation
Tribunal) would use for deciding whether a pilot displayed
incompetence.


All the best,


David

David Megginson wrote
Not at all -- I've just been surprised at how many U.S. pilots don't
seem to know about altimeter temperature errors. On mailing lists,
I've actually had violent reactions from otherwise experienced and
competent pilots when I casually mentioned that pressure altimeters
are routinely off by hundreds of feet at cruise altitude.

Yep. They forgot it, since it wasn't really relevant. As for the
violent reaction, it's not a pilot thing but a people thing. There
are people who are often wrong but never uncertain. It's kind of sad
when an experienced pilot gets that way, but it's really terrible when
an old experienced instructor falls into that mode, since at that
point he's largely worthless.

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? 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.

Let's not forget that the worst case temperature error at 200 ft and
-50C is only 60 ft, while altimeters can be up to 75 ft off in some
cases and still be legal for IFR use.

What if the errors compounded? I agree that it's unlikely (and would
require a very cold day), but using your numbers someone with a 75 ft
altimeter error and a 60 ft temperature error could end up at only 65
ft AGL when the altimeter read 200 ft AGL.

Which is still not the end of the world. In a light airplane, you can
easily go missed from 65 AGL (or land, if you break out). Anything
heavy and fast enought that this isn't true is probably going to have
a RADAR altimeter and Cat II certification anyway.

Michael

(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

"Michael" wrote in message
om...
Let's not forget that the
worst case temperature error at 200 ft and -50C is only 60 ft, while
altimeters can be up to 75 ft off in some cases and still be legal for
IFR use. There used to be a DH penalty for an inop middle marker
(either at the transmitter or receiver end) but this penalty no longer
applies. All this ignores the possibility that the pilost has a RADAR
altimeter available.

In the US, it is up to the pilot to decide whether in his particular
situation, given the available equipment and his skills, he should
adjust the minima as appropriate based on the expected temperature
error.

Michael

I've never seen, nor heard of a temperatures of -68F (at low airport-type
altitudes) that was not associated with an inversion. I suppose that it
might happen in Anarctica in the winter but there aren't any airports there.
In Alaska, when its -40F on the surface it is usually at least 0F at 1000'
AGL.

Mike
MU-2

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

Let me add two links to the discussion:

This is on problems with cold weather altimetry:

http://www.aircraftbuyer.com/learn/train06.htm

This is about the accuracy of unaided GPS altitude in the context of
vertical guidance, but it bears some relavence to the discussion of the
accuracy of GPS altitude:

http://www.bluecoat.org/reports/Graham_2001_RawGPS.pdf

John Bell
www.cockpitgps.com

I'm sure that in this case you are referring to the ICAO Cold
Temperature Error Table, which is part of the AIM. Check it out
online:
http://www1.faa.gov/ATPubs/AIM/Chap7/aim0702.html#7-2-3

I don't know about you, but I have yet to meet a US flight instructor
who does not require his students to have a copy of the AIM and be
conversant with it.

Uh. That table only showed up a couple of years ago. I bet most flight
instructors have no idea its even there. Other than "hot to cold look
out below" there ain't much on temperature errors in the FAA private
pilot knowledge requirements. The reason that table finally showed up
is because pilots familiar with Canadian and USAF procedures have been
pushing the FAA to improve this area of pilot knowledge.

We are not REQUIRED to do anything about those tables.

If you're flying over mountains in Alaska on a cold night with an
alitimeter setting from sea level? You can tell that to the granite.