GPS Altitude with WAAS

[Phil Verghese]

I posted earlier about seeing differences of up to 500' between the
altitude shown on my Garmin GPS 196 (even while receiving WAAS) and my
altimeter. The GPS would consistently indicate higher than the
altimeter. Dan Luke suggested the cause might be non-standard
temperatures. After some more experimenting, I think Dan was correct.

On a flight last week, the GPS altitude and altimeter matched within 20
feet. The temperature was close to standard, which made me think that
temperature might be in fact be the culprit as Dan suspected.

I did another trip over the weekend and found the GPS reading high for
altitude again, and the temperatures were above standard. Here's what I
got....

SATURDAY SUNDAY
Indicated Altitude 7500 8500
GPS Altitude 7700 8677
Pressure Altitude 7350 8310
True Altitude 7700 8700
Density Altitude 8200 9100
Altimeter setting 30.07 30.11
Temp (C) +8 +5
Std Temp (C) 0 -2

So it looks like the GPS is showing True Altitude (which makes a lot of
sense). True altitude is the actual height above MSL, and that will
differ from indicated altitude when the temperature is not standard, or
you forget to change your altimeter setting. The true altitude numbers
above were calculated using my good old-fashioned E6B.

So the GPS is showing the right value. However, that value is different
than what you altimeter shows when the temperature is not standard. When
temperatures are above standard, the GPS altitude will be higher than
indicated and when termperatures are below standard the GPS altitude
will show lower than indicated. Does this make sense?

I wonder how this will affect the upcoming WAAS LPV approaches. They are
going to have a decision altitude based on indicated altitude, not true
altitude. How will the difference between true altitude and indicated
altitude affect the approach, since the GPS will be giving vertical
guidance based on true altitude?

Phil
www.pfactor.com

[Ben Jackson]

In article >,
Phil Verghese > wrote:
>
>So it looks like the GPS is showing True Altitude (which makes a lot of
>sense). True altitude is the actual height above MSL,

More precisely it's showing the height above the datum, which is the
mathematical model of the earth ellipsoid the GPS uses to approximate
the actual Earth.

--
Ben Jackson
>
http://www.ben.com/

[Neil Gould]

Recently, Ben Jackson > posted:

> In article >,
> Phil Verghese > wrote:
>>
>> So it looks like the GPS is showing True Altitude (which makes a lot
>> of sense). True altitude is the actual height above MSL,
>
> More precisely it's showing the height above the datum, which is the
> mathematical model of the earth ellipsoid the GPS uses to approximate
> the actual Earth.
>
General question: is there are practical difference between the datum and
MSL? Neither have any local relevance, AFAICT.

Neil

[David Megginson]

Phil Verghese > writes:

> I posted earlier about seeing differences of up to 500' between the
> altitude shown on my Garmin GPS 196 (even while receiving WAAS) and my
> altimeter. The GPS would consistently indicate higher than the
> altimeter. Dan Luke suggested the cause might be non-standard
> temperatures. After some more experimenting, I think Dan was
> correct.

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.

> So the GPS is showing the right value. However, that value is different
> than what you altimeter shows when the temperature is not standard. When
> temperatures are above standard, the GPS altitude will be higher than
> indicated and when termperatures are below standard the GPS altitude
> will show lower than indicated. Does this make sense?

Yes. A rough rule of thumb is that your altimeter will be off by 4
feet, per degree Celsius difference from ISA, per thousand feet above
the station reporting the altimeter setting. This works, of course,
only with the standard lapse rate -- if there's an inversion or any
other non-standard lapse rate between you and the field reporting the
altimeter setting, this formula won't work.

Let's assume that your field is at 1000 ft MSL with 20 degC above
standard temperature and a standard lapse rate all the way up. When
you're sitting on the field, your altimeter should show your actual
elevation:

4 * 20 * 0 = 0 ft error

So you'll see 1000 ft on your altimeter when you dial in the field's
altimeter setting.

At 200 ft AGL (typical ILS DH), your error will will be

4 * 20 * 0.2 = 16

so that when your altimeter reads 1200 ft on short final, you're
really around 1216 ft MSL, or 216 ft AGL.

At 9000 ft MSL (8000 ft AGL), the error will be much larger:

4 * 20 * 8 = 640 ft

So when your altimeter says 9000 ft, you'll really be up at 9640 ft.
That's no big deal, but it could be dangerous in cold temperatures, if
you were trying to clear (say) an 8000 ft ridge with 1000 ft
clearance. If the temperature was 20 degC below standard, and your
altimeter setting was from a field at 1000 ft MSL, you'd actually be
flying at 8360 ft when your altimeter read 9000 ft, just barely
clearing the ridge.

> I wonder how this will affect the upcoming WAAS LPV approaches. They are
> going to have a decision altitude based on indicated altitude, not true
> altitude. How will the difference between true altitude and indicated
> altitude affect the approach, since the GPS will be giving vertical
> guidance based on true altitude?

The approach will be a safer than approaches based on a barometric
altimeter, all things considered. Barometric altimeters are fine for
cruise, since everyone sees the same error and flies at the same
altitude anyway (just not the one they think they're flying at).


All the best,


David

[Julian Scarfe]

Recently, Ben Jackson > posted:
>
> > More precisely it's showing the height above the datum, which is the
> > mathematical model of the earth ellipsoid the GPS uses to approximate
> > the actual Earth.


"Neil Gould" > wrote in message
k.net...

> General question: is there are practical difference between the datum and
> MSL? Neither have any local relevance, AFAICT.

The geoid height is typically -100 ft over the US, meaning that the zero of
the NAD 83 ellipsoid and mean sea level differ by about 100 ft.

Julian Scarfe

[Peter]

> Recently, Ben Jackson > posted:
>
>>In article >,
>>Phil Verghese > wrote:
>>
>>>So it looks like the GPS is showing True Altitude (which makes a lot
>>>of sense). True altitude is the actual height above MSL,
>>
>>More precisely it's showing the height above the datum, which is the
>>mathematical model of the earth ellipsoid the GPS uses to approximate
>>the actual Earth.

The Garmin GPS receivers that I've used all report elevation above the
geoid which models Mean Sea Level (MSL), not the ellipsoid. Internally
they initially calculate elevation above the ellipsoid, but then they apply
values from a lookup table that has corrections for the geoid - ellipsoid
difference based on the geographic position. I believe Magellan units and
some newer Lowrance units also correct for the geoid-ellipsoid difference
so the reported elevations represent height above MSL rather than the
ellipsoid.

[Michael]

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).

I'm sure that Canada is the promised land as far as aviation education
goes, and that no instructor there ever sends a student to a checkride
when the student isn't REALLY ready to exercise ALL the privileges of
the certificate but is likely to be able to pass the flight test.
Unfortunately, this is not the case in the US. Nonetheless, altimeter
temperature errors ARE a part of the standard US private curriculum.

> We have tables
> in our AIP and other publications showing what errors to expect,

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. Thus I have to assume that every US student pilot
has seen this table at one time or another. It doesn't particularly
surprise me that many don't recall this. I'm sure you learned how to
extract a square root manually in primary school, as did everyone
else. Can you still do it? Maybe. But I assure you that not only
are most adults incapable of doing it, but that many will claim never
to have learned. Use it or lose it. For most private flying in the
continental US, altimeter errors are not particularly relevant.

> and
> when flying IFR, we are required to add those errors to all instrument
> approach altitudes (MDA, DH, etc.) in very cold temperatures.

See, that's the basic difference. We are not REQUIRED to do anything
about those tables. The information is made available to us, along
with a recommendation to adjust minima as appropriate. It is part of
the AIM, which every private pilot candidate is expected to become
familiar with. How we choose to use the information is up to us.

The altimeter errors are very real - but a correction to MDA or DH is
not always necessary. For example, many ILS approaches are equipped
with a middle marker. The marker is not affected by altimeter errors.
It's not really the MAP, but in most cases it's close enough that it
can be used to signal a missed approach in situations where the
altimeter is suspected of reading high. 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

[David Megginson]

(Michael) writes:

> I'm sure that Canada is the promised land as far as aviation
> education goes,

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.

I'm sorry if this came across as a "we're better than you" issue --
there are other areas where the U.S. aviation eductation system does
better than Canada's. For example, we do not have to demonstrate any
partial-panel work for our IFR flight test in Canada (good instructors
still teach it, of course, but it's scary that they don't have to).

> 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.


All the best,


David

[Tom S.]

"David Megginson" > wrote in message
...
> (Michael) writes:
>
> > I'm sure that Canada is the promised land as far as aviation
> > education goes,
>
> 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.

I think what they teach us is density altitude differences, but that's for
performance during takeoff. I wonder if the new RVSM rules/equipment might
refocus some discussion in that direction?

Tom

[David Megginson]

"G.R. Patterson III" > writes:

> 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.

Unless you're trying to clear mountains using an altimeter setting
from a low-elevation field.

> It would be possibly unsafe for you to set your altimeter accurately
> when everyone else is setting it to the broadcast local altimeter
> setting.

Absolutely -- no one is suggesting changing the altimeter setting.
You just have to be aware of how inaccurate the altimeter is when
obstacle clearance might be an issue. For example, if you are
planning to clear a ridge by only 1000 ft in the winter, you might
want to think again.


All the best,


David

[G.R. Patterson III]

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

[Michael]

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

[David Megginson]

(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

[Mike Rapoport]

"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

[Michael]

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

[David Megginson]

(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

[John Bell]

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

[Fred E. Pate]

> 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.

[Fred E. Pate]

> 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.

[David Megginson]

"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

[Bob Gardner]

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

[Fred E. Pate]

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/approach/SouthWest/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.)

[David Megginson]

"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

[John R. Copeland]

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/approach/SouthWest/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.)
>

[Roger Halstead]

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/approach/SouthWest/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)

[David Megginson]

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

[Roger Halstead]

On Mon, 29 Sep 2003 17:17:49 GMT, David Megginson
> wrote:

>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

I have a couple layers of the new blankets sewn together to fit around
the entire front of the plane from mid windshield forward around the
engine compartment and underneath where it completely covers the nose
gear doors, and around the front to wrap the first quarter of the
prop. The back of the spinner is even warm.

Problem is...the Deb doesn't warm up till I'm at cruise, or duri8ng a
long climb to altitude.

You can actually stay comfortable in heavy slacks and a long sleeve
shirt, but that is *after* climb out. <:-))

Getting the Cub Cadet with snow blower out, cleaning the ramp and
maybe even part of the taxiway does not make the inside of the plane
feel any warmer<sigh> It's kinda like working in an unfinished house
in the winter. Man, but it gets cold in there.

I've flown from Michigan to Gainsville Ga (LGM) without seeing a
cloud. Then again, I've flown from the middle of Tennessee and not
seen the ground till some where around Jackson Michigan.

Unlike the summer storms, you can fly over a blizzard at some where
between 3000 and 5000 AGL. The winter storms seldom get very high.
But I have been sitting "up there" and after hearing some of the
conversations on UNICOM thinking it'd be real nice to have one more
fan. That was when I realized it's a good idea to not only have a way
out, but have a way down when single engine. If it's really cold you
don't have to worry about ice in the clouds either, but since then I
make it a point to try to stay near the edges of those big storms when
there is no safe way down.
>covers.
>
Roger Halstead (K8RI EN73 & ARRL Life Member)
www.rogerhalstead.com
N833R World's oldest Debonair? (S# CD-2)
>
>All the best,
>
>
>David

[David Megginson]

Roger Halstead > writes:

> Problem is...the Deb doesn't warm up till I'm at cruise, or duri8ng a
> long climb to altitude.
>
> You can actually stay comfortable in heavy slacks and a long sleeve
> shirt, but that is *after* climb out. <:-))

The heat in my Warrior is more than sufficient to keep the cockpit
warm enough for no coat or gloves at -30 degC OAT once I'm flying (I
have to turn it down quite a bit), but then, in the winter I'm always
wearing an extra layer of socks and thermal underwear just in case.
One of my favourite ways to fly in the winter is to open the overhead
vent a bit *and* pump the heater up to full, but that's probably just
strange.

> Getting the Cub Cadet with snow blower out, cleaning the ramp and
> maybe even part of the taxiway does not make the inside of the plane
> feel any warmer<sigh> It's kinda like working in an unfinished house
> in the winter. Man, but it gets cold in there.

I pay more than some people (CAD 65/month =~ USD 47/month) for an
outside tiedown on the grass, but since my tiedown happens to be at a
major airport (CYOW), I have the benefit of its services, like two ILS
approaches and good snow removal.

> Unlike the summer storms, you can fly over a blizzard at some where
> between 3000 and 5000 AGL. The winter storms seldom get very high.
> But I have been sitting "up there" and after hearing some of the
> conversations on UNICOM thinking it'd be real nice to have one more
> fan. That was when I realized it's a good idea to not only have a way
> out, but have a way down when single engine.

This will be my first winter flying with my instrument rating, so I'll
have a lot of learning to do. Last winter, when I was doing the IFR
training, we couldn't find IMC no matter how hard we looked.


All the best,


David

[[email protected]]

: The heat in my Warrior is more than sufficient to keep the cockpit
: warm enough for no coat or gloves at -30 degC OAT once I'm flying (I
: have to turn it down quite a bit), but then, in the winter I'm always
: wearing an extra layer of socks and thermal underwear just in case.
: One of my favourite ways to fly in the winter is to open the overhead
: vent a bit *and* pump the heater up to full, but that's probably just
: strange.

My Cherokee probably has the potential of keeping things warm, but
the air leaks elsewhere make it chilly. Seems like while your shoulders,
neck, and arms are freezing from a draft, you simultaneously smell burning
rubber from down by the rudder pedals.... :)

-Cory

--
************************************************** ***********************
* The prime directive of Linux: *
* - learn what you don't know, *
* - teach what you do. *
* (Just my 20 USm$) *
************************************************** ***********************

[John R. Copeland]

Probably poetic license about the "blizzard" part?.
I wish you'd merely said "heavy snow".
Blizzards really are major storms, with winds alone that'll keep me on =
the ground.
---JRC---

"Roger Halstead" > wrote in message =
...
>=20
>=20
>=20
>=20
>=20
> Unlike the summer storms, you can fly over a blizzard at some where
> between 3000 and 5000 AGL. The winter storms seldom get very high.
>=20
> .
>=20

[David Megginson]

writes:

> My Cherokee probably has the potential of keeping things warm, but
> the air leaks elsewhere make it chilly. Seems like while your shoulders,
> neck, and arms are freezing from a draft, you simultaneously smell burning
> rubber from down by the rudder pedals.... :)

I *feel* the burning down in my feet, even through boots and two pair
of socks -- that's why I eventually have to turn the heat down.


All the best,


David

[Ray Andraka]

WIth my Six, I've found that opening the floor vent about half way and turning
the cup to the front avoids the hot foot while at the same time keeping from
freezing out the back seat passengers. Perhaps that would work in the smaller
CHerokees as well?

David Megginson wrote:

> writes:
>
> > My Cherokee probably has the potential of keeping things warm, but
> > the air leaks elsewhere make it chilly. Seems like while your shoulders,
> > neck, and arms are freezing from a draft, you simultaneously smell burning
> > rubber from down by the rudder pedals.... :)
>
> I *feel* the burning down in my feet, even through boots and two pair
> of socks -- that's why I eventually have to turn the heat down.
>
> All the best,
>
> David

--
--Ray Andraka, P.E.
President, the Andraka Consulting Group, Inc.
401/884-7930 Fax 401/884-7950
email
http://www.andraka.com

"They that give up essential liberty to obtain a little
temporary safety deserve neither liberty nor safety."
-Benjamin Franklin, 1759

[David Megginson]

Ray Andraka > writes:

> WIth my Six, I've found that opening the floor vent about half way
> and turning the cup to the front avoids the hot foot while at the
> same time keeping from freezing out the back seat passengers.
> Perhaps that would work in the smaller CHerokees as well?

It sounds like a good idea -- I'll give it a try.


Thanks,


David

[Mike Rapoport]

The pigs will be flying long before there is a 5F day at OAK.

Mike
MU-2


"Fred E. Pate" > wrote in message
...
> 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/approach/SouthWest/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.)
>

[[email protected]]

Sure, but that's not tne point. These restrictions will be standard on all of these types of approaches nation wide. Its the first time I am aware of that the FAA has included temperature altimetry errors in instrument procedures.

Mike Rapoport wrote:

>
>The pigs will be flying long before there is a 5F day at OAK.
>
>Mike
>MU-2
>
>
>"Fred E. Pate" > wrote in message
...
>> 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/approach/SouthWest/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.)
>>
>
>

[Mike Rapoport]

I agree in principle but in actuality it never gets cold enough in the US to
make a difference. The only times that we have really low temperatures (-40
and below) there is an inversion.

Mike
MU-2


> wrote in message ...
> Sure, but that's not tne point. These restrictions will be standard on
all of these types of approaches nation wide. Its the first time I am aware
of that the FAA has included temperature altimetry errors in instrument
procedures.
>
> Mike Rapoport wrote:
>
> >
> >The pigs will be flying long before there is a 5F day at OAK.
> >
> >Mike
> >MU-2
> >
> >
> >"Fred E. Pate" > wrote in message
> ...
> >> 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/approach/SouthWest/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.)
> >>
> >
> >
>
>
>

[[email protected]]

Okay. But if you're on a precision approach you'll be below the inversion, no?

Mike Rapoport wrote:

>
>I agree in principle but in actuality it never gets cold enough in the US to
>make a difference. The only times that we have really low temperatures (-40
>and below) there is an inversion.
>
>Mike
>MU-2

[Fred E. Pate]

Mike Rapoport wrote:
> The pigs will be flying long before there is a 5F day at OAK.
>
> Mike
> MU-2

Oh yeah, I forgot to add that those pigs will be using WAAS and LAAS
equipment to make their approaches into Oakland when it happens :)

[Fred E. Pate]

Roger Halstead wrote:

> 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
>

Yup, but temperature doesn't effect the glideslope, just where you put
the MAP along that glideslope. On the so-called "Baro-VNAV" approaches,
the glideslope is defined using barometric altimeter data, so the entire
glideslope will be shallower and closer to the ground on a cold day.
And if there is a low-level temperature inversion the glideslope won't
even be a straight line.