Altimeter slope adjustment is off or my GPS is messed up

Kiddo's Driver

Kiddo's Driver

Pattern Altitude
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Display name:
Jim
So my GPS and Altimeter agree on the ground (I think...I better go check that...). I noticed that at 4500 feet on the altimeter (set to an airport AWOS within 10 miles of where I was flying.) my GPS was reading 4700 feet. At 6500 feet on the altimeter (set as above) my GPS was reading 6900 feet.
Its like the zero is set correctly, but the slope is set too steep. Yes, I know it is mechanical, but I have an electrical background. There is some mechanical equivalent to the electrical terms I used above. I just don't know them.

Anyone know the details? Have it happen to them? Know what it took to fix the issue?

I'll go verify they match other and the field altitude (1360 feet). I'll also make sure that the pressure compensation is correct (set it to AWOS reported pressure and verify that the altimeter reads about field altitude).

I'll also go check and see when the last time Merlin's Avionics did my tests.

Jim
 
GPS altitude is only accurate when the geometry of the satellites are numerous and in a good position...
 
Pressure altitude does not equal geometric altitude.

The thing that you would have to fix to get them to agree would be the actual (vs. assumed ideal) rate of pressure change with altitude.
 
So, assuming no issue with the equipment, the right answer is to do what I/we were trained to do. Adjust the pressure periodically as you fly along based on info from the airports you are flying past or what the controllers are giving you if you are using flight following. Essentially don't worry about what the GPS is saying.
 
Kiddo's Driver said:
So, assuming no issue with the equipment, the right answer is to do what I/we were trained to do. Adjust the pressure periodically as you fly along based on info from the airports you are flying past or what the controllers are giving you if you are using flight following. Essentially don't worry about what the GPS is saying.
Click to expand...


Correct....:yes:
 
Assuming the temperature is higher than standard, this is normal. If you're comparing your altitude to the surrounding terrain, the altimeter will read higher than "actual" (geometric) by a certain percentage - the higher you go, the greater the altitude error in feet. For terrain purposes, the GPS is much more accurate, however barometric altitude rules for ATC purposes.

If you're worried about ATC clearances, follow your altimeter. If you're worried about terrain, follow the GPS.

ADS-B actually broadcasts _both_ barometric and geometric altitudes.
 
Capt. Geoffrey Thorpe said:
Pressure altitude does not equal geometric altitude.

The thing that you would have to fix to get them to agree would be the actual (vs. assumed ideal) rate of pressure change with altitude.
Click to expand...
That rate of pressure change would be the "slope" adjustment. As long as it is in spec on the 24 month calibration I won't worry about it.

I just checked the plane.
GPS 1362 feet
Altimeter set to AWOS pressure, 1340 feet
(somewhere on the airport) field altitude, 1360 feet
 
dans2992 said:
Assuming the temperature is higher than standard, this is normal. If you're comparing your altitude to the surrounding terrain, the altimeter will read higher than "actual" (geometric) by a certain percentage - the higher you go, the greater the altitude error in feet. For terrain purposes, the GPS is much more accurate, however barometric altitude rules for ATC purposes.

If you're worried about ATC clearances, follow your altimeter. If you're worried about terrain, follow the GPS.

ADS-B actually broadcasts _both_ barometric and geometric altitudes.
Click to expand...
Temperature was higher than standard, but the altitude was lower than geometric.

I was not talking to anyone, but that does not mean I am not part of the system. I still have to fit into the rest of the system so I fly the pressure compensated altitude. It was just weird seeing a 400 for difference. I was going West, VFR at 6500 with the GPS reading 6900. If GPS lined up with the rest of the ATC system and my Altimeter was messed up I "would be flying West VFR at the East IFR altitude." Hench my questions.
 
Kiddo's Driver said:
That rate of pressure change would be the "slope" adjustment.
Click to expand...
There is no "slope" adjustment for the atmosphere (the actual air you are flying in - not your altimeter). The rate of change in pressure with altitude will vary with temperature and humidity. It is what it is. And, thus, the difference between your GPS and your altimeter will vary from day to day. They will match when you are sitting on the ground at the airport because the altimeter setting is the setting that gives you the right altitude on the ground.
 
Capt. Geoffrey Thorpe said:
There is no "slope" adjustment for the atmosphere (the actual air you are flying in - not your altimeter). The rate of change in pressure with altitude will vary with temperature and humidity. It is what it is. And, thus, the difference between your GPS and your altimeter will vary from day to day. They will match when you are sitting on the ground at the airport because the altimeter setting is the setting that gives you the right altitude on the ground.
Click to expand...

There is some mechanical representation for X amount of pressure change equates to Y amount of altitude change on the dial. That is the slope adjustment.
 
Kiddo's Driver said:
There is some mechanical representation for X amount of pressure change equates to Y amount of altitude change on the dial. That is the slope adjustment.
Click to expand...

No, that's a fixed setting of 1000' per 1" Hg, and is not adjustable, afaik.
What you were seeing is perfectly normal and correct. If the temperature is above standard, then your indicated altitude with a correction from a nearby field (lower than your current altitude) should read lower than the true geometric altitude (as indicated by an accurate WAAS/GPS). The amount of difference will be a function of your elevation relative to the field that gave you the setting and the temperature difference from standard. This is not a fixed number, since the vertical temperature profile fluctuates.
Bottom line, your altimeter and GPS are fine.
 
Yes, I have seen the "DIFFERENCE" between the altimeter and GPS altitude.
I have 2 GPS, an old garmin 296 and a Tablet with New ADS-B (in) using the "Dual" ADS-B receiver. There are some minor differences between the two GPS's (25-75ft). but I use the GPS's to validate my Altimeter is in range.

I use ATC a lot on my cross country flights. When ATC gives update on my travels, I update the altimeter. (again, I use the GPS's to make sure i'm within range of what I need to be) Double check, double check, backup system, backup systems.

My Vacuum system just went out on my last flight. So, I pulled my 'backup" vacuum and it gave me just enough to run my "STEAM GAGES" again. but if it didn't, I know with my GPS Altitude i'm in the ballpark, and would (notifying ATC the my loss of normal Altitude gage).

The Nexus 7 Droid Tablet has so many great app's... compass, artificial horizon, GPS, Projected fuel burn, airport landing plates, weather(ads-b), seeing other planes(ads-b), etc...

The New electronics have made flying a lot more safe.

Stephen
N42278 / N985KK
Cessna 182 / 150
:thumbsup:
 
sgcromer said:
Yes, I have seen the "DIFFERENCE" between the altimeter and GPS altitude.
I have 2 GPS, an old garmin 296 and a Tablet with New ADS-B (in) using the "Dual" ADS-B receiver. There are some minor differences between the two GPS's (25-75ft). but I use the GPS's to validate my Altimeter is in range.

I use ATC a lot on my cross country flights. When ATC gives update on my travels, I update the altimeter. (again, I use the GPS's to make sure i'm within range of what I need to be) Double check, double check, backup system, backup systems.

My Vacuum system just went out on my last flight. So, I pulled my 'backup" vacuum and it gave me just enough to run my "STEAM GAGES" again. but if it didn't, I know with my GPS Altitude i'm in the ballpark, and would (notifying ATC the my loss of normal Altitude gage).

The Nexus 7 Droid Tablet has so many great app's... compass, artificial horizon, GPS, Projected fuel burn, airport landing plates, weather(ads-b), seeing other planes(ads-b), etc...

The New electronics have made flying a lot more safe.

Stephen
N42278 / N985KK
Cessna 182 / 150
:thumbsup:
Click to expand...

Vacuum system has absolutely nothing to to do with an altimeter...
 
Jim,

The baro altimeter and the GPS altitude should match fairly closely at field elevation on the airport. Since the pressure setting on the baro altimeter at the airport is based on adjusting the pressure to field elevation, effectively the temperature effect is folded in. As you go above field elevation, the baro altimeter does not correct for temperature, so on a hot day the warm air at a given altitude will be higher pressure (think hot air balloon) than the model that is used for the altimeter. The higher actual pressure will cause the altimeter to read lower than it would on a standard day. The GPS is totally unaffected by temperature, so this does not represent an error source. The GPS error is affected by geometry of the satellites and does not vary with altitude. So if your GPS indicates within 50 feet of your field elevation, it will indicate a similar error at altitude. Below 18000 MSL in the US, we are supposed to adjust our baro altimeters to the same value as other aircraft within a 100 NM, so this provides us with separation from other traffic that are indicating a different altitude. Effectively we are all using the same pressure reference if we all use the same altimeter setting. Above 18000 MSL, we transition to flight levels where we all set our altimeter to 29.92, again the same pressure setting.

The baro altimeter works fairly well at low altitude for terrain separation, but that is because we maintain a 1000 foot buffer above the obstacles and terrain. In mountainous areas, more buffer is needed to be safe, so we go to 2000 feet buffer. Cold days are the real issue here, because the baro altimeter reads higher than we actually are, with the error increasing the higher and further we are from the location where the altimeter setting is determined. Fortunately, the error is smallest when low and close to the airport, for example at the DA or MDA on an approach. But even these low and close altitudes can be unsafe in extreme cold conditions and adjustments need to be made to the altitudes we use on an approach.

Here is a link to a neat model that allows you to see the error for various temperatures at airport elevation verses your height above the airport.

http://www.luizmonteiro.com/Learning_Alt_Errors_Sim.aspx
 
John Collins said:
Jim,

The baro altimeter and the GPS altitude should match fairly closely at field elevation on the airport. Since the pressure setting on the baro altimeter at the airport is based on adjusting the pressure to field elevation, effectively the temperature effect is folded in. As you go above field elevation, the baro altimeter does not correct for temperature, so on a hot day the warm air at a given altitude will be higher pressure (think hot air balloon) than the model that is used for the altimeter. The higher actual pressure will cause the altimeter to read lower than it would on a standard day. The GPS is totally unaffected by temperature, so this does not represent an error source. The GPS error is affected by geometry of the satellites and does not vary with altitude. So if your GPS indicates within 50 feet of your field elevation, it will indicate a similar error at altitude. Below 18000 MSL in the US, we are supposed to adjust our baro altimeters to the same value as other aircraft within a 100 NM, so this provides us with separation from other traffic that are indicating a different altitude. Effectively we are all using the same pressure reference if we all use the same altimeter setting. Above 18000 MSL, we transition to flight levels where we all set our altimeter to 29.92, again the same pressure setting.

The baro altimeter works fairly well at low altitude for terrain separation, but that is because we maintain a 1000 foot buffer above the obstacles and terrain. In mountainous areas, more buffer is needed to be safe, so we go to 2000 feet buffer. Cold days are the real issue here, because the baro altimeter reads higher than we actually are, with the error increasing the higher and further we are from the location where the altimeter setting is determined. Fortunately, the error is smallest when low and close to the airport, for example at the DA or MDA on an approach. But even these low and close altitudes can be unsafe in extreme cold conditions and adjustments need to be made to the altitudes we use on an approach.

Here is a link to a neat model that allows you to see the error for various temperatures at airport elevation verses your height above the airport.

http://www.luizmonteiro.com/Learning_Alt_Errors_Sim.aspx
Click to expand...

A minor nit, which I mentioned above, is that the actual temperature "correction" (as calculated/depicted in that linked website) depends on the specific lapse rate (or vertical temperature profile) and elevation difference between your aircraft and the station supplying the QNH, where in reality the lapse rate normally varies quite a bit, and of course your height above station varies too.
Bottom line is that at higher than standard temps expect GPS to read higher, and vice versa, but by how much exactly may vary quite a bit.
Another bottom line is that, like a stopped clock supplying perfectly accurate times twice a day, a barometric altimeter provides good separation to nearby traffic and correct elevation upon landing.
 
Last edited:
RotorDude said:
No, that's a fixed setting of 1000' per 1" Hg, and is not adjustable, afaik.
Click to expand...
As an Electronics Technician, Reactor Operator, and Instrumentation and Control Technician in previous lives I find it hard to believe that there is not some adjustment on the side or back of the altimeter that can be adjusted during the calibration process. There has to be a way to compensate for aging internal components. Otherwise that guy that I pay to check everything once every two years is simply giving me a pass/fail status. If it passes the test I'm good. If it fails there is nothing he can do and you must replace the altimeter.

The sounds like a good time to give Merlin a call and see what he says.
 
Kiddo's Driver said:
As an Electronics Technician, Reactor Operator, and Instrumentation and Control Technician in previous lives I find it hard to believe that there is not some adjustment on the side or back of the altimeter that can be adjusted during the calibration process. There has to be a way to compensate for aging internal components. Otherwise that guy that I pay to check everything once every two years is simply giving me a pass/fail status. If it passes the test I'm good. If it fails there is nothing he can do and you must replace the altimeter.

The sounds like a good time to give Merlin a call and see what he says.
Click to expand...

There is an adjustment... Look at the little screw in the middle of the knob...
 
Kiddo's Driver said:
As an Electronics Technician, Reactor Operator, and Instrumentation and Control Technician in previous lives I find it hard to believe that there is not some adjustment on the side or back of the altimeter that can be adjusted during the calibration process. There has to be a way to compensate for aging internal components. Otherwise that guy that I pay to check everything once every two years is simply giving me a pass/fail status. If it passes the test I'm good. If it fails there is nothing he can do and you must replace the altimeter.

The sounds like a good time to give Merlin a call and see what he says.
Click to expand...

That's exactly right. AFAIK, assuming your static system is OK (no leaks), the testing system properly (and recently) calibrated, and the technician sufficiently taps the altimeter to "unstick" the needle, once it's out of tolerance during the pitot-static check, the instrument is junk and needs to be replaced.
 
Good point. It looks like the slope is fixed at the factory by the anaroid cabsule/wafers. The only adjustment you can make is to the bias.
http://aviationknowledge.wikidot.com/aviation:sensitive-altimeters

Based on what I saw above, it looks like I might be off by 20 feet or so. Not enough to get worked up about.

I still need to check and see when my last check was done...
 
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