GPS Altitude with WAAS

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.


I don't buy it. On a good weather day in California it is not uncommon
for the alimeter setting itself to account for 500 ft altimeter errors
in the mountains. If you add up non-standard lapse rate, cold air and
old and distant altimeter settings you can eat into the 2000 feet rather
quickly. Then deal with turbulent air and downdrafts in the mountains
on top of this. I don't like it one bit.

Go land on a 2000 foot runway and tell me that's plenty of room between
a little airplane with poor climb performance and a big mountain that
you can't see.

"John Bell" writes:

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

Thanks -- those are good articles. The Nav Canada paper on non-WAAS
GPS VNAV (the Graham paper) is especially interesting.


All the best,


David

Exaclty, the AIP is not the CARS (Canadian Regs)
Just as the AIM is not the FARS

Stan

On Wed, 24 Sep 2003 00:22:02 GMT, David Megginson
wrote:

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

writes:

Exaclty, the AIP is not the CARS (Canadian Regs) Just as the AIM is
not the FARS

But in both cases, they are a fairly normative definition of
airmanship. If you ignore a "must" in the AIM or AIP, the FAA or
Transport Canada is probably more than happy to nab you for poor
airmanship.

It's a very imperfect analogy, but you can think of the AIM or AIP as
a sort-of collection of common law, beside the formal legislation of
the FARs or CARs.


All the best,


David

I would say that we are exceedingly fortunate in having Nav Canada as an
information source to supplement (complement?) the FAA.

Bob Gardner

"David Megginson" wrote in message
...
"John Bell" writes:

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

Thanks -- those are good articles. The Nav Canada paper on non-WAAS
GPS VNAV (the Graham paper) is especially interesting.


All the best,


David

John Bell wrote:
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


This one's for the Canadians on this thread. A notice on the new
Oakland, California (KOAK) "RNAV (GPS) RWY 29" approach
(http://www.myairplane.com/databases/.../OAK_agr29.pdf):

"BARO-VNAV NA below -15 deg C (5 deg F)"

And this is for a decision altitude of only 294 ft AGL. Seems like the
FAA is moving towards taking into account temperature errors in
barometric alitmetry. And, by implication, this supports the premise
that WAAS altitude figures are more accurate than the trusty old
"sensitive altimeter." (In the legend they specifically state that
WAAS-based VNAV can be used when BARO-VNAV is not approved due to
temperature.)

"Fred E. Pate" writes:

And this is for a decision altitude of only 294 ft AGL. Seems like
the FAA is moving towards taking into account temperature errors in
barometric alitmetry.

Thanks -- that's an interesting note.

And, by implication, this supports the premise that WAAS altitude
figures are more accurate than the trusty old "sensitive altimeter."
(In the legend they specifically state that WAAS-based VNAV can be
used when BARO-VNAV is not approved due to temperature.)

The altimeter becomes increasingly accurate near the ground (assuming
you have the correct altimeter setting) and increasingly inaccurate
away from the ground; WAAS, I'll guess, has about the same accuracy
all the way down (or up). I don't know at what point they typically
cross over, but it would vary depending on the temperature gradiant.


All the best,


David

I've never lived in Oakland, but I thought it was exceedingly rare for =
the
temperature there to dip below -15C.
---JRC---

"Fred E. Pate" wrote in message =
...
=20
This one's for the Canadians on this thread. A notice on the new=20
Oakland, California (KOAK) "RNAV (GPS) RWY 29" approach=20
=
(http://www.myairplane.com/databases/.../OAK_agr29.pdf):
=20
"BARO-VNAV NA below -15 deg C (5 deg F)"
=20
And this is for a decision altitude of only 294 ft AGL. Seems like =
the=20
FAA is moving towards taking into account temperature errors in=20
barometric alitmetry. And, by implication, this supports the premise=20
that WAAS altitude figures are more accurate than the trusty old=20
"sensitive altimeter." (In the legend they specifically state that=20
WAAS-based VNAV can be used when BARO-VNAV is not approved due to=20
temperature.)

On Mon, 29 Sep 2003 00:16:00 -0700, "Fred E. Pate"
wrote:

John Bell wrote:
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


This one's for the Canadians on this thread. A notice on the new
Oakland, California (KOAK) "RNAV (GPS) RWY 29" approach
(http://www.myairplane.com/databases/.../OAK_agr29.pdf):

"BARO-VNAV NA below -15 deg C (5 deg F)"

And this is for a decision altitude of only 294 ft AGL. Seems like the
FAA is moving towards taking into account temperature errors in
barometric alitmetry. And, by implication, this supports the premise
that WAAS altitude figures are more accurate than the trusty old
"sensitive altimeter." (In the legend they specifically state that
WAAS-based VNAV can be used when BARO-VNAV is not approved due to
temperature.)

It seems a bit strange...
..
DH on the ILS at MBS is only 200 feet AGL and that is with no
temperature correction. In the winter we regularly see below zero
F and it's not rare to see it at minus 20 at night

OTOH with those temperatures we either have severe clear, or blowing
snow. It's rather uncommon to see clouds near the ground when it's
that cold here in the flat lands.

That and you can be "on top" of a raging blizzard at 4,000.



Roger Halstead (K8RI EN73 & ARRL Life Member)
www.rogerhalstead.com
N833R World's oldest Debonair? (S# CD-2)

Roger Halstead writes:

OTOH with those temperatures we either have severe clear, or blowing
snow. It's rather uncommon to see clouds near the ground when it's
that cold here in the flat lands.

Maybe it's just not cold enough. Once you get past -30 degC or so,
you can get a dense freezing mist (like smoke) rising off any open
water, like a lake or wide river -- the parts that are frozen over
start to make a creepy, moaning sound.

I agree that truly cold temperatures ( -20 degC) tend to mean VMC, at
least where I live -- it's one of the fantastic things about winter
flying (clear skies, excellent visibility, good climb performance,
minimal turbulence, high ground visibility at night, and early sunsets
that make it easy to stay night-current). The downsides are having to
plug in the engine heater overnight and dealing with the @#$%#@ wing
covers.


All the best,


David