GPS Altitude with WAAS

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

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

(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

"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 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

"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

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

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