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Altimeter vs GPS altitude

Jeff R

Well Known Member
The field elevation at my home airport (Merritt Island) is about 6 feet. My Skyview GPS, along with a back up Garmin Aera GPS, will correctly report the altitude. On my Skyview altimeter, the correct elevation appears with the altimeter adjusted to within .01 to .02" of what the AWOS station provides.

In the air, however, I am seeing significant differences. For example, when I went flying at about 4,000 ft per the altimeter, the GPS units were reporting about 4,300 feet. That is a pretty significant difference!

I witnessed the original airspeed/altimeter/transponder calibration, and the Skyview altimeter was right on all the way up to 18,000 feet, so I don't think there is an error in the Skyview static pressure sensor.

Wondering if my GPS antennas were blocked by the fuselage, I flew N, W, S and E and the altitudes didn't vary at all.

I use the Safe Air static ports, one on each side of the fuselage, connected together and to dual ADHRs sensors in the tail.

I tend to believe the GPS altitudes are being reported correctly and since I know the altimeter has been tested and found accurate, the problem points to the static ports.

Anyone else experience big differences between altimeter and GPS altitudes, and has anyone else with the Safe Air static ports had altimeter errors?
 
Trust your instruments

Disclaimer - I am not an expert - I don't even play one on TV. GPS altitudes are calculated, and, unless you have an approach certified GPS, not dependable. That is why we have approach certified GPS devices. They use the WAAS signal to derive altitude.
 
Over the last few years, I've found that the GPS altitude is the correct one, and the altimeter is varying with barometric pressure. If I get a very current altimeter setting, my Garmin 696's panel page altimeter, and the aircraft's will exactly follow each other for a few minutes. It happens every time I get a current setting for my location. After a few minutes, they drift apart, and can be hundreds of feet difference sometimes. Of course, it's the barometer setting that's important for aircraft separation, and I'll take the GPS measurement to miss a mountain.

L.Adamson
 
I think if you check, you will find that there is a difference between GPS altitude, pressure altitude, density altitude, and our barometric altimeter altitude. The one that is accepted by for flight is the standard altimeter. Remember, it is the only altitude readout that requires biannual certification for contiued flight operations. Dan
 
Barometric altitude and GPS altitude are very different and would only correspond occasionally in flight. The GPS measures altitude geometrically and is generally more accurate for showing your actual elevation above sea level. The altimeter shows altitude by sensing pressure and relating this to a model of the atmosphere. If there is a standard atmosphere the two different methods will show close to the same figure. Rarely is there a standard atmosphere. This is why altimeters can not be relied on closely for terrain clearance and is why certain flight levels are not available when conditions are certain way. The two different instruments should be close on the ground if the altimeter has a recent and accurate altimeter setting.

It is necessary to use your altimeter for a reference in flight because everyone else is doing it, but that doesn't mean it is giving the best information for all purposes all the time.
 
Disclaimer - I am not an expert - I don't even play one on TV. GPS altitudes are calculated, and, unless you have an approach certified GPS, not dependable. That is why we have approach certified GPS devices. They use the WAAS signal to derive altitude.

See my last reply. It's a 696 with WAAS, and nails altitude every time, as long as I'm moving. It really IS that good...............as it will exactly correlate with a very current altimeter setting. It's almost weird to see a GPS derived animated altimeter, and the aircraft's follow each other. BTW-- since I do have quite an interest in GPS/WAAS reliability, I've often compared readings to known elevations in numerous mountain areas such as national parks & highway summits. The readings are very close.......and who's to say that the posted elevation is exact.
 
When I fly, it is a very rare case for my 696 to agree with my altimeter even when I have current updates for the pressure. Today's cross country was typical. I was making regular updates to the barometric setting (30.10 thru 30.06) on my altimeter as I was flying at 10'500 and the 696 was reporting 500' higher throughout the "at altitude" portion of the flight. My outbound flight a few days ago had a very different barometric pressure (29.89 thru 29.90) and the delta results were the same.

As a complementary reference, my APRS tracker also has a WAAS GPS and it agreed with the 696.

Since ATC traffic reports agree with my altimeter, I defer to it "for the record".
 
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When I fly, it is a very rare case for my 696 to agree with my altimeter even when I have current updates for the pressure. Today's cross country was typical. I was making regular updates to the barometric setting (30.10 thru 30.06) on my altimeter as I was flying at 10'500 and the 696 was reporting 500' higher throughout the "at altitude" portion of the flight. My outbound flight a few days ago had a very different barometric pressure (29.89 thru 29.90) and the delta results were the same.

As a complementary reference, my APRS tracker also has a WAAS GPS and it agreed with the 696.

Since ATC traffic reports agree with my altimeter, I defer to it "for the record".

Then, we'll battle it out. :) Perhaps your current altitmeter setting just isn't close enough for the exact area. ATC traffic reports are using barametric pressure. I make comparisons to known elevations. No way, is the 696 500' off a known elevation. It's usually not even 10' off.
 
Going back through some in flight pics, some more observations.

Before using XM weather's altimeter setting function...........I see that my altimeter & GPS reading could easily be 300' difference. Once I had the tendency to update my altimeter, as "shown", as the XM setting changed..........then my altimeter & GPS are very close.

L.Adamson
 
Disclaimer - I am not an expert - I don't even play one on TV. GPS altitudes are calculated, and, unless you have an approach certified GPS, not dependable. That is why we have approach certified GPS devices. They use the WAAS signal to derive altitude.

A simple Garmin 496 portable also has a WAAS receiver and provides just as accurate a position as an LPV certified unit. Note that I'm not advocating or suggesting that anyone attempt to shoot an approach with one.
 
Barometric altitude and GPS altitude are very different and would only correspond occasionally in flight. The GPS measures altitude geometrically and is generally more accurate for showing your actual elevation above sea level. The altimeter shows altitude by sensing pressure and relating this to a model of the atmosphere. If there is a standard atmosphere the two different methods will show close to the same figure. Rarely is there a standard atmosphere. This is why altimeters can not be relied on closely for terrain clearance and is why certain flight levels are not available when conditions are certain way. The two different instruments should be close on the ground if the altimeter has a recent and accurate altimeter setting.

It is necessary to use your altimeter for a reference in flight because everyone else is doing it, but that doesn't mean it is giving the best information for all purposes all the time.

++1 Very well explained Larry. As an added note the pressure gradient at altitude change becomes significantly smaller at altitude. Thus a minute pressure change represents a larger altitude change at the flight levels than at the lower altitudes. With GPS any altitude error is constant at any altitude This is why altitude separation is increased above FL300

Jos
 
Remember that the altimeter is nothing more than a barometer calibrated in feet rather than inHg/mb - 1mb is about 30' at sea level. It displays a reduction of pressure as an increase in altitude. The airfield altimeter setting (known as QNH in the UK) is the pressure measured at the airfield corrected to sea level. It is used to calibrate the zero position on a given day. Set on the ground, the altimeter should read approximately the airfield elevation. In a standard temperature atmosphere in flight, it will read AMSL and will therefore closely agree with the GPS altitude.

In a non-standard atmosphere, the pressure gradient is different. In a warm atmosphere, the air is less dense so a given change in pressure represents a larger altitude. Altimeters will therefore under-read. In terms of terrain clearance, this is the safe case. In a colder-than-standard atmosphere, the air is relatively dense and therefore a given pressure change will represent a smaller actual change of altitude. The altimeter will therefore OVER-read. For terrain clearance, this is the unsafe situation. The normal calculations for saftey altutude and minimas have built in factors to account for this at "normal" temperatures. However at more extreme temperatures (normally defined as ISA -15 or less) increments should be added to account for this.

Above Transition Altitude, the altimeter is set to standard (29.92/1013). It now bears no relationship to altitude and reference is made to Flight Levels. This is why national transition levels are always above the highest Safety Altitude.

With all this in mind, the GPS altitude (which is a true, geometrically calculated number) will rarely agree with the altimeter. Since everybody should be using calibrated altimeters with the same setting, this does not matter. Whilst GPS altutude could be used as a get-you-home in the event of an altimeter failure, it should not normally be used or reported.
 
My home airport is under Class C airspace and I sometimes get warnings from the GPS that I'm in the Class C even though my correctly-set altimeter says I'm 300' below it. Really gets your attention the first couple of times it happens.

As has already been well explained, there's no reason to expect the GPS and altimeter to read exactly the same. GPS is probably more accurate but altimeter is the basis for ATC.
 
My home airport is under Class C airspace and I sometimes get warnings from the GPS that I'm in the Class C even though my correctly-set altimeter says I'm 300' below it. Really gets your attention the first couple of times it happens.

As has already been well explained, there's no reason to expect the GPS and altimeter to read exactly the same. GPS is probably more accurate but altimeter is the basis for ATC.

Same here, except under Class B. It put me in a oh **** dive mode, the first time.
 
As has already been well explained, there's no reason to expect the GPS and altimeter to read exactly the same. GPS is probably more accurate but altimeter is the basis for ATC.

To a certain extent. The principle that needs to be appreciated is that an altimeter measures pressure, not altitude. We therefore fly at pressure levels, not heights. The fact that it is displayed in feet gives us a warm, fuzzy feeling and is an approximation to our true altitude given the multitude of errors that the instrument is prone to (when I was a QFI in the RAF I used to know them all - there's something like a dozen :D). The instrument could just as well be calibrated in "whaloons" and we could report our level as "1500 whaloons".

Hence, to say that the GPS altitude is "more accurate" is really meaningless. You simply are not measuring the same thing. Hence, the bottom of controlled airspace is not really at 3000'. It is at an atmospheric pressure equivalent to a reading of 3000' on our calibrated barometer.....
 
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And the datum is different.......

And the world is not a perfect sphere...........

And what Paul said............. ;)
 
Hence, to say that the GPS altitude is "more accurate" is really meaningless.

All depends. If you're using a GPS approach, with WAAS or equivalent, then it's certainly the accuracy you want. I myself, have used it many times to compare altitudes and CFIT "crash sites". In these cases, the GPS/WAAS combination, provides a great source of advanced terrain warning......not to mention the backbone of synthetic vision.

Yes..............it's a LOT more accurate, when you really need it!

L.Adamson
 
Whilst the GPS dictates your track and (accurately) calculates the vertical approach path, any procedural altitudes and minima are STILL based on the baro ......... Hence for stand-alone GPS approaches, you will find a minimum temperature specified.

Don't get me wrong - I think that the GPS is a brilliant piece of kit and adds to safety and awareness. You just have to recognize that the vertical information (whilst "accurate") is not really applicable in the current regime. Just as GPS navigation has replaced radio aids, I guess that the vertical information may one day replace altimeters but it's not going to happen any time soon. In the meantime, we really don't want people trying to give the impression that we should in any way doubt the accuracy of the altimeter because the "more accurate" GPS shows a different altitude nor use the GPS in preference because it is "more accurate". Remember, we are measuring different things despite them both being called "altitude" :rolleyes:
 
Terrain vs Airplanes

Based on my experience using a GPS hiking and running in the local Utah mountains, my GPS is a good indicator of where the terrain is. It matches the charted altitudes quite accurately, usually within about 10 feet or so. So I assume that it will also match terrain altitudes when flying.

However, I assume that other airplanes are using altitudes based on a pressure alitimeter using the current nearest altimeter setting. Therefore, if I'm following ATC directions or worried about avoiding planes using VFR cruising altitudes, I use the pressure altimeter.

One issue that I haven't seen addressed is what to do when approaching an airport for which there is no local altimeter setting available on the radio, and it's been some time since being able to set the altimeter. In this case, I often compare the GPS altitude to the altimeter so I get an idea of how much error there MIGHT be on the altimeter. That's just so I know whether I'm flying the right pattern altitude or not. Of course, when landing, I let my eyes tell me where the ground is. But after landing, I generally find that the GPS altitude is accurate (matches the charted field elevation). In this case, using the GPS altitude to set the altimeter seems like it corresponds to using the field elevation to set the altimeter.
 
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Whilst the GPS dictates your track and (accurately) calculates the vertical approach path, any procedural altitudes and minima are STILL based on the baro ......... Hence for stand-alone GPS approaches, you will find a minimum temperature specified.

Don't get me wrong - I think that the GPS is a brilliant piece of kit and adds to safety and awareness. You just have to recognize that the vertical information (whilst "accurate") is not really applicable in the current regime. Just as GPS navigation has replaced radio aids, I guess that the vertical information may one day replace altimeters but it's not going to happen any time soon. In the meantime, we really don't want people trying to give the impression that we should in any way doubt the accuracy of the altimeter because the "more accurate" GPS shows a different altitude nor use the GPS in preference because it is "more accurate". Remember, we are measuring different things despite them both being called "altitude" :rolleyes:

Unless Galelio & a WAAS equivilent are on line, then there is little point in arguing the differences between GPS in the UK, and here in the USA. That's because the difference is quite dramatic. Vertical GPS guidance is used here for GPS based approaches. That makes it very applicable. And since I'm one who has a very high interest in the "decline" of CFIT........it makes the difference between GPS & barometric altimeters ....even more important.

In the meantime, as I mentioned yesterday............I took a look at a series of in-flight photos taken in my RV. Before using the XM weather option that shows current "altimeter"settings on my GPS screen, it was normal to see variations between my GPS altitude, and the altimeter.... of several hundred feet. Perhaps more. In every photo, in which the GPS is showing the current altimeter setting.........the difference is around 50' to right on. What that means, is that I've been actively adjusting the altimeter in flight, thanks to constant updates from XM. In some cases, the setting had changed a small amount, and I hadn't re-adjusted. That just goes to show the value of XM (or another inflight weather source) & WAAS.

L.Adamson
 
If your gps and barometric altitude are always within 50' then either:
1. You are aways flying low, or
2. You never fly in significantly non-standard conditions, or
3. Something is wrong.
 
If your gps and barometric altitude are always within 50' then either:
1. You are aways flying low, or
2. You never fly in significantly non-standard conditions, or
3. Something is wrong.

Or, 4. You're still sitting in the work shop making airplane noises!
 
If your gps and barometric altitude are always within 50' then either:
1. You are aways flying low, or
2. You never fly in significantly non-standard conditions, or
3. Something is wrong.

Let me have the pleasure of....

Telling you (and the next reply), that you only think, that you know what you're talking about....

I'm usually flying at 7500-11500' msl. On many occasions, I have shown other pilots as well as non-pilot passengers...........just how well the GPS altimeter and the aircraft's follow each other --- exactly, for a few minutes, when I've updated my altimeter setting.....usually from XM weather, or a current area report. As expected, after a few minutes, they will begin to drift apart. I have several comparison pics at 9,500' msl.

Keep the updates fresh............and you won't see that 200'+ deviation, that I've often seen myself. NOTHING is wrong. It's actually RIGHT!

Now..............go out, and do it yourself. If it doesn't work..............then something is definately wrong..:eek:

I've done this for several years...........and all times of the year.

L.Adamson
 
If your gps and barometric altitude are always within 50' then either:
1. You are aways flying low, or
2. You never fly in significantly non-standard conditions, or
3. Something is wrong.

Some more...........

First............read all of my postings concerning 50'

Second...........are you trying to tell me, that all GPS approach airports are low elevation.......because they don't have a chance in **** of any accuracy, if they're higher elevations?

Wouldn't something be wrong, if your GPS & aircraft's altimeter would be way off..........as you taxi to the runway after setting the altimeter to field elevation?

So please tell me..........at what altitude for normal GA flying, does the aircraft's altimeter not have a chance of being close to GPS altitude, if the area barometric pressure setting is current.

And, do you use XM, or any other current weather aids, with altimeter setting updates?

L.Adamson -- Garmin 696
 
To a certain extent. The principle that needs to be appreciated is that an altimeter measures pressure, not altitude. We therefore fly at pressure levels, not heights. The fact that it is displayed in feet gives us a warm, fuzzy feeling and is an approximation to our true altitude given the multitude of errors that the instrument is prone to (when I was a QFI in the RAF I used to know them all - there's something like a dozen :D). The instrument could just as well be calibrated in "whaloons" and we could report our level as "1500 whaloons".

Hence, to say that the GPS altitude is "more accurate" is really meaningless. You simply are not measuring the same thing. Hence, the bottom of controlled airspace is not really at 3000'. It is at an atmospheric pressure equivalent to a reading of 3000' on our calibrated barometer.....

I agree with most of this. However, I'd argue that "accuracy" does in fact have a specific meaning. It is a measure of how closely the airplane indication matches its actual elevation above a datum (MSL). Its true the altimeter only measures pressure, but it reads out in feet (or meters) so it seems fair to judge it by that standard.

If you're flying an instrument approach the accuracy of the elevation indicated by the altimeter is certainly not meaningless!
 
It's just math guys.

If you are at sea level, standard pressure but at 40c... What does your alt read? Do the same at 10,000. I bet your gps is off (doesnt match the alt) by about 500' at each end of that spectrum.
 
Hot topic! I enjoy reading it all!

When it comes down to it, I trust my gps altitude more during x-country. (certified or not).

Christopher.
 
The Anywhere Map people have a good explanation:

http://docs.controlvision.com/pages/gps_altimetry.php

Barometric Altimetry Errors

At higher altitudes, the GPS altitude and Indicated (barometric) altitude often diverge. This is caused by the limitations of barometric altimetry, not by any error in the GPS calculations. All pilots know that temperature affects air density, and that on a hot day, the air is less dense – reducing aircraft performance.

This also affects the accuracy of the altimeter. Setting the Altimeter setting makes the indicated altitude agree with field elevation when the aircraft is on the ground at that airport. At ISA Standard temperature, the altimeter will be accurate as the aircraft climbs away from the airport elevation as well. However, when the air is very cold (more dense) or very hot (less dense) the altimeter will accumulate errors as we climb away from the field elevation where the altimeter setting was taken.

On a day that is 30 degrees Celsius colder the altimeter will show the aircraft about 10% higher than it’s AGL altitude above the station altitude where the altimeter setting was recorded. 30 higher results in the indicated altitude being 10% lower than the actual TRUE (GPS) altitude would indicate. Thus, on hot summer days, the GPS will show a higher cruise altitude than the altimeter, typically by about 500 feet at 10,000 feet. This has an impact on using the VNAV target altitudes and should be compensated for by the pilot. Air traffic control is based on Indicated altitude, not on GPS altimetry.

Rob Hickman
N402RH RV-10
 
Mr. A.,

I really don't like flashing credentials, but as a CFII and a Ph.D in physics yes, I do think I know what I'm talking about.
A standard barometric altimeter uses a model of the atmosphere to change pressure, which it measures, to altitude. There is a single adjustment to the model, the Kollsman window. But the atmospheric model has at least two variables, the sea level pressure and the rate at which pressure decreases with altitude. The AIM says that you set the equivalent sea level pressure into the Kollsman window, but that is not correct. The altimeter setting is whatever it takes to make the altimeter read the true MSL altitude at the reporting station.
A good GPS will read true MSL altitude with good accuracy. So as you approach an airport GPS altitude and barometric altitude will agree, if you have a current altimeter setting from the airport. As you climb, the barometric and GPS altitudes will agree only if the rate of pressure decrease with altitude follows the model. On a non-standard day, that won't be the case. I can be flying right over the airport, with a current altimeter setting, it doesn't matter. Remember your private training, when you had to use an E6B to convert your barometric altitude to true altitude? This is exactly what is going on here. Check GPS altitude for clearing the hills; use barometric altitude to stay at the proper altitudes for flying.
 
I tend to believe the GPS altitudes are being reported correctly and since I know the altimeter has been tested and found accurate, the problem points to the static ports.

Please don't take this the wrong way Jeff, but your question shows to me a lack of understanding on how things work with respect to your altitude when flying. Folks here are beating around the bush and only alluding to the fact that you should NEVER be flying off your GPS altitude. VFR or IFR, everyone else is flying off of their pressure altimeter and whether you're trying to fly correct VFR cruising altitudes, pattern altitudes, or flying in the IFR system, you will be at the wrong altitudes compared to everyone else. This is a DANGEROUS notion. I'm sure you're not the only one that may believe flying off the altimeter is more accurate (I hope you don't think this and that I'm completely wrong about the intention of your post). Flying at absolute altitudes is not the goal when in the air. Everyone using the same REFERENCE is the goal.


When it comes down to it, I trust my gps altitude more during x-country.
Chris, as I said to Jeff. You trust your gps more than what? Hopefully not more than your pressure altimeter. Again, the GPS is more accurate, but that is not what everyone else is referencing.
 
Mr. A.,

I really don't like flashing credentials, but as a CFII and a Ph.D in physics yes, I do think I know what I'm talking about.
A standard barometric altimeter uses a model of the atmosphere to change pressure, which it measures, to altitude. There is a single adjustment to the model, the Kollsman window. But the atmospheric model has at least two variables, the sea level pressure and the rate at which pressure decreases with altitude. The AIM says that you set the equivalent sea level pressure into the Kollsman window, but that is not correct. The altimeter setting is whatever it takes to make the altimeter read the true MSL altitude at the reporting station.
A good GPS will read true MSL altitude with good accuracy. So as you approach an airport GPS altitude and barometric altitude will agree, if you have a current altimeter setting from the airport. As you climb, the barometric and GPS altitudes will agree only if the rate of pressure decrease with altitude follows the model. On a non-standard day, that won't be the case. I can be flying right over the airport, with a current altimeter setting, it doesn't matter. Remember your private training, when you had to use an E6B to convert your barometric altitude to true altitude? This is exactly what is going on here. Check GPS altitude for clearing the hills; use barometric altitude to stay at the proper altitudes for flying.

This about sums it up!
 
Please don't take this the wrong way Jeff, but your question shows to me a lack of understanding on how things work with respect to your altitude when flying. Folks here are beating around the bush and only alluding to the fact that you should NEVER be flying off your GPS altitude. VFR or IFR, everyone else is flying off of their pressure altimeter and whether you're trying to fly correct VFR cruising altitudes, pattern altitudes, or flying in the IFR system, you will be at the wrong altitudes compared to everyone else. This is a DANGEROUS notion. I'm sure you're not the only one that may believe flying off the altimeter is more accurate (I hope you don't think this and that I'm completely wrong about the intention of your post). Flying at absolute altitudes is not the goal when in the air. Everyone using the same REFERENCE is the goal.



Chris, as I said to Jeff. You trust your gps more than what? Hopefully not more than your pressure altimeter. Again, the GPS is more accurate, but that is not what everyone else is referencing.

Tobin, it is evident that there was more than 1 poster confused by this concept. Many others have probably NOT posted but have learned something by this post. I would prefer to see helpful, educational explanations rather than pointing out someone's misconception because I think it would prevent someone from posting questions/concerns in the future in an effort to avoid embarrassment. We should encourage this type of post because it educates not only the original poster but many others who read it after the fact.
 
?I would prefer to see helpful, educational explanations rather than pointing out someone's misconception because I think it would prevent someone from posting questions/concerns in the future in an effort to avoid embarrassment.?

Now that is an intelligent statement! And yes I learned a lot from this thread.
 
Wouldn't something be wrong, if your GPS & aircraft's altimeter would be way off..........as you taxi to the runway after setting the altimeter to field elevation?

So please tell me..........at what altitude for normal GA flying, does the aircraft's altimeter not have a chance of being close to GPS altitude, if the area barometric pressure setting is current.

GPS and altimeter should be very, very close on the ground because the altimeter setting is based on the pressure at the field. Pressure gradient in the air is almost never, ever standard, so you go up and introduce some altitude error.
 
I'm not so sure Jeff has any great lack of understanding. He asked about a particular static port. There is a little thing called static error, introduced by airflow over the port, and responsible for a difference in baro altitude between a moving and stationary aircraft. The conversation leaped to GPS vs baro (which is interesting and certainly another factor), but that tack is not specific to the original question (accuracy with a SafeAir port).
 
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I would prefer to see helpful, educational explanations

Good, then we're on the same sheet. I was trying to be helpful. The responses I was reading from other posters were doing a good job of explaining why there is a difference between GPS and barometric altimeters. What I wasn't seeing was anyone correcting any misconception that one can simply fly off of their GPS alt vice baro alt. I was simply pointing out factual information and did not mean for it to be taken in any other way.
 
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Unless Galelio & a WAAS equivilent are on line, then there is little point in arguing the differences between GPS in the UK, and here in the USA. That's because the difference is quite dramatic. Vertical GPS guidance is used here for GPS based approaches. That makes it very applicable. And since I'm one who has a very high interest in the "decline" of CFIT........it makes the difference between GPS & barometric altimeters ....even more important.

I think you misunderstand.

GPS vertical guidance on an approach is used to give you an equivalent 3 degree slope. In that respect, it replaces the ILS, not the altimeter. To intercept that profile, you fly at a barometric altitude. The minimum also is based on the baro. I am a 747-400 Captain flying worldwide and that is how it is done in every country I know! In fact, there is no way on the 747 to display or use GPS altitude.

As I have previously said, there is nothing wrong with using GPS as an aid to awareness of airspace and terrain but it should never be used as the vertical reference on the basis that it is "more accurate".

As an example: You are under controlled airspace with a promulgated base of 4000'. You are in a significantly colder than standard atmosphere - hence the altimeter over-reads. Your "inaccurate" altimeter reads 4200' whilst you "accurate" GPS altitude is 3800'. Sorry, you are nicked! You are in CAS ......

What is it about this that is not understood?
 
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I am having a bit of trouble understanding the CFIT aspect of this discussion. I think the part on which altitude must be used while flying is obvious. The entire world and all the airspace systems are based on pressure altitude indications. If your using anything else your at the wrong altitude.
The CFIT part is where I get confused. If your VFR your primary CFIT reference is to look outside. They IFR system is set up to provide terrain clearance using pressure altitude. If your flying in conditions or areas where you can't see to visually clear obstacles then you should be on a IFR flight plan. I suppose that you might be legal at night to fly VFR where visually seeing terrain or obstacles might be a issue but If you are flying low enough where a baro altitude is not sufficient to insure terrain clearance at night with limited visibility I would suggest you are doing some that is not smart.
I use my GPS as a backup for CFIT. If I make a major mistake its going to yell at me to let me know. I certainly would not use it to fly low in poor visibility.

George
 
Folks here are beating around the bush and only alluding to the fact that you should NEVER be flying off your GPS altitude.

This is not always correct, we use GPS altitude to draw the Synthetic Vision on the EFIS displays. If you are trying to avoid terrain you should be using GPS altitude.



Rob Hickman
Advanced Flight Systems Inc.
 
The IFR system is set up to provide terrain clearance using pressure altitude. If your flying in conditions or areas where you can't see to visually clear obstacles then you should be on a IFR flight plan. I suppose that you might be legal at night to fly VFR where visually seeing terrain or obstacles might be a issue but If you are flying low enough where a baro altitude is not sufficient to insure terrain clearance at night with limited visibility I would suggest you are doing some that is not smart.
I use my GPS as a backup for CFIT. If I make a major mistake its going to yell at me to let me know. I certainly would not use it to fly low in poor visibility.

George

If you ever fly IFR over terrain or VFR on top of clouds you are one engine failure away (or encounter with ice) from discovering the value of Synthetic Vision and GPS terrain clearance.

Rob Hickman
N402 RH RV-10
 
"The IFR system is set up to provide terrain clearance using pressure altitude."

Yes, that's correct. And there's a buffer (1000' or 2000' in the mountains). But as this thread has indicated this doesn't always work, since the true altitude will be lower than indicated in colder than standard conditions. In extreme cold (Alaska) ATC will issue a NOTAM banning IFR flight at certain lower altitudes, and some approaches may be NOTAMed unusable.
 
What a good thread this has turned out to be. Since this thread has drifted several times, I feel like I can ask my next question.

How does the altitude hold work on my Tru-Trak alt hold? Theoretically I could be flying along at 5500 and enter a different pressure system requiring me to adjust the altimeter setting and then adjusting my altitude. My altitude hold would not make this adjustment and i would be flying at an incorrect altitude. My alt hold is vented to my cabin so it senses cabin pressure....

I suspect that the alt hold senses the pressure and just tries to maintain that pressure so I would have to disable the altitude hold, adjust altitude, then engage it again to conform to ATC altitudes when entering different pressure zones.

Is this how all autopilot altitude hold works? Even certified?
 
"I suspect that the alt hold senses the pressure and just tries to maintain that pressure so I would have to disable the altitude hold, adjust altitude, then engage it again to conform to ATC altitudes when entering different pressure zones."

Basically correct, depending on autopilot. I have the Trio Pro and it allows me to adjust the altitude without disconnecting, but your method would work, too.
 
This is not always correct, we use GPS altitude to draw the synthetic vision

OK Rob, good point, never say never. My point is that when we fly around in the US Air Space system, we do it referencing our baro altimeters, not GPS alt. There are always exceptions, especially during emergencies. If Jeff and Chris don't actually fly referencing their GPS alt vice baro alt, then good for them and my apologies for concluding wrongly. I was not trying to squelch good questions or discussions or discourage anyone from asking questions, nor was I trying to berate anyone. I have, however, spoken directly with enough folks to know that there are pilots flying in the US that actually believed that it was ok and acceptable to fly off of GPS alt because, hey, its more accurate, isn't it.
 
Remember safety first.

Over the last few years, I've found that the GPS altitude is the correct one, and the altimeter is varying with barometric pressure. If I get a very current altimeter setting, my Garmin 696's panel page altimeter, and the aircraft's will exactly follow each other for a few minutes. It happens every time I get a current setting for my location. After a few minutes, they drift apart, and can be hundreds of feet difference sometimes. Of course, it's the barometer setting that's important for aircraft separation, and I'll take the GPS measurement to miss a mountain.

L.Adamson

Remember safety first.
This explains the issue of, "it's the barometer setting that's important for aircraft separation."
 
"I suspect that the alt hold senses the pressure and just tries to maintain that pressure so I would have to disable the altitude hold, adjust altitude, then engage it again to conform to ATC altitudes when entering different pressure zones."

The Dynon Autopilots fly the commanded indicated altitude and when you change the baro setting, the AP will climb or descend to fly that new indicated altitude.
 
AIUI, a regional pressure setting is provided by ATC such that for its period of validity and for the highest terrain in the region and the lowest SL pressure in the region the altimeter will not over-read.

So where there's high terrain, low temperature and/or rapid pressure change; then the ATC advised altimeter setting may give you a significant under-read error unless you are at the combination of time, place and altitude that gives you a correct reading. But this means that your error will always work to give you extra terrain clearance and never place you too low.

If you are using an airfield pressure setting, then it should read correctly at threshold elevation, but may be progressively in error at other times and places and altitudes.
 
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