J

jraymond4321

Member
I purchased a RV-6a and it now has about 100 hours TT.

My altimeter is reporting 200-300 feet below actual altitude for the last 10 hours or so, it was accurate before.

I was going to adjust it, but it is a Falcon guage so I have to take the panel off and I haven't gotten around to it.

I was flying down to Miami Thursday, with flight following, and ATC said my mode C was reporting 7100, when I was actually at @7500 (verified with 2 GPS). On the way home, I checked when i left the tower, and same thing (mode C reported 2200, while I was at @2500).

Does this sound like any particular problem (ie. pitot leaking, or blockage)? I don't fully understand how the mode C works in conjuction with the pitot system, and if or how it is related to the altimeter.

I would like to have a better understanding before I bring it to the shop.

Additionally, would I need to go to an avionics shop for this, or my normal maintenance shop (transponder is avionics, but pitot would be maintenance)?

Thanks in advance

James Raymond
N436JE
Neptune Beach, FL
 
So it's your altitude encoder which is reading low, not your altimeter? It's not too uncommon for them to be out of calibration, but just for kicks, make sure it's not connected to the PITOT line instead of the STATIC by accident.
 
Your Encoder is set for pressure altitude (29.92). Set your Altimeter to 29.92 and see if they agree. They won't agree if you have your alt. set at VFR settings (actual reported pressures). I would get a pitot static check and have a shop check the encoder. It may be the item needing adjustment. But first, verify your altimeter at a known field elevation and pressure. GPS's are not always precise at determining altitude.

Hope these checks help, if you haven't already tried them.

Roberta
 
Checking your altimeter against a controller is a good idea but it is not perfect. Your encoder responds to a pressure setting of 29.92 and only gives altitude reports with 100 ft increments. It changes at the 50 ft point.

The controller doesn't see you response directly. The ATC system adjusts the received mode C altitude for the local pressure in the area you are flying. If you are enroute, there could be some difference between your altimeter setting and the value entered into the ATC System. This shouldn't be much of a problem for a terminal area if you and the ATC System are set to the correct ATIS.

GPS altitudes are not as accurate as Lat Longs. Normally accuracy is about 1.5x the horizontal accuracy. This should be about 75 ft with SA turned off (as it is now) but the accuracy will vary according the current position of the constellation and your GPS receiver's selection of satellites to get the best Geometric Dilution of Precision (GDOP).


Ken
 
You say your altimeter and Mode C were working correctly until 10 hours ago. How do you know? By comparing them to the GPS?

Don't waste your time comparing your altimeter against the GPS. The GPS reports geometric altitude, plus or minus its error. The altimeter reports barometric altitude. Geometric altitude and barometric altitude are only the same thing if the temperature is the same as the standard atmosphere, all the way from the ground to the aircraft altitude. If the temperature is warmer than standard, the altimeter will be reading lower than the geometric altitude. The difference between barometric altitude and geometric altitude is approximately 3.5% for every 10 deg C that the temperature differs from standard (this is a gross simplification, that assumes that the altimeter setting comes from a place at sea level).

So, if 10 hours ago, the temperature was somewhere close to standard temperature, then I would expect your altimeter/transponder and GPS to be reporting roughly the same thing (plus or minus the normal errors associated with each device - perhaps 50 to 75 ft for the altimeter, 100 ft for the transponder, and 100 to 200 ft for the GPS). If the temperature has warmed up, then the GPS will be reporting geometric altitudes that are higher than the barometric altitudes reported by the altimeter and transponder. This is completely normal.

The altitude encoder and the altimeter should be seeing the same static pressure. So, they should be reporting the same altitude. If you have a static system problem, then there is no point asking the controller what the mode C says, as its output will be affected by whatever is going on with the static system.

Does the altimeter read the correct altitude on the ground at an airport that provides an altimeter setting? If not, then you have an altimeter problem.

Check for a static system leak.
 
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94kb said:
GPS altitudes are not as accurate as Lat Longs. Normally accuracy is about 1.5x the horizontal accuracy. This should be about 75 ft with SA turned off (as it is now) but the accuracy will vary according the current position of the constellation and your GPS receiver's selection of satellites to get the best Geometric Dilution of Precision (GDOP).
Ken
Click to expand...

FWIW, I've been taking my Garmin 296 along, for a number of vehicle (land) trips around the Mt. West. Altitudes have ranged from 2000' to over 9000.

I wanted to check the Garmin's altitude readout using WAAS against posted altitude signs at mountain summits, state & federal parks, etc.

Now granted, the posted signs might be a few feet off, although some might read 8915', instead of rounded off at 8900'.

The "big" surprise, is that the Garmin, when using WAAS, was hitting these altitudes almost exactly, or perhaps three to five feet off, most of the time. A few times it would be around 15 feet off, and never worse than 35'. It also is consistant within 35' when walking around my yard on different days. But I think it's more accurate while on the move.

At this point, I'm starting to trust it, at least as much as an altimeter, and perhaps more. The "panel" page also keeps up while driving up and down the hills or making turns. If worse went to worse, I suppose I could now trust the panel page too!

L.Adamson
 
Sure, the GPS should provide a reasonably accurate geometric altitude. But, if you are flying, and ATC gives you an altitude, or you need to fly at a VFR cruising altitude, don't forget that those are barometric altitudes. The barometric altitude may differ by over a thousand feet from the geometric altitude if the temperature is significantly different than the standard atmosphere.
 
Just to reinforce what Kevin is saying:

If you are flying GPS (geometric) altitudes, you may not be at the correct barometric altitude. This could cause a conflict with other traffic flying at the correct (baro) altitude. Since the difference between baro and geometric altitude becomes smaller and smaller the closer you get to the ground, comparing the two altitudes on the ground will not reveal the problem.

Pat
 
Kevin Horton said:
Sure, the GPS should provide a reasonably accurate geometric altitude. But, if you are flying, and ATC gives you an altitude, or you need to fly at a VFR cruising altitude, don't forget that those are barometric altitudes. The barometric altitude may differ by over a thousand feet from the geometric altitude if the temperature is significantly different than the standard atmosphere.
Click to expand...

Not something I've gave a lot of thought too,

But for instance, I live in the Mt. West area with many airports ranging from 4000' to 7000'+ altitudes. Since it's common practice to set the altimeter to the "un-controlled" airport's runway altitude before takeoff, rather than relying on some report of barometric pressure; wouldn't it be very conceivable of having a drastic difference in VFR crusing altitudes, if mine started exact, and someone else was a 1000' off?

Or, am I just getting the wrong idea here?

L.Adamson
 
1000 feet off? That would be an entire inch of mercury...I doubt others would be that far off. As far as temperature, what you need to be more concerned with is a change in temperature from one location to another. If the temperature is relatively constant accross a large area, that shouldn't be a problem..crossing a front or something, or entering a temperature inversion might have an effect, but temperature changes shouldn't make that much of a difference.
 
pbesing said:
1000 feet off? That would be an entire inch of mercury...I doubt others would be that far off. As far as temperature, what you need to be more concerned with is a change in temperature from one location to another. If the temperature is relatively constant accross a large area, that shouldn't be a problem..crossing a front or something, or entering a temperature inversion might have an effect, but temperature changes shouldn't make that much of a difference.
Click to expand...

I guess we had better go 29.92 for ALL altitudes, or scrap the altimeter altogether, and go for WAAS gps's.... :D

At least when flying VFR crusing altitudes in mountain country...........of the northern hemisphere, where WAAS works!
 
pbesing said:
1000 feet off? That would be an entire inch of mercury...I doubt others would be that far off.
Click to expand...

1000 feet off would only happen if the temperature was extremely cold, and you were quite high. For example, if the temperature is 40 deg C colder than ISA (i.e. it is -25 deg C at a sea level airport), the error is about 14% of the altitude difference between your altitude, and the altitude of the airport where the altimeter setting came from. If you are 7,000 ft above the airport where the altimeter setting came from, the error is about 1,000 ft. This assumes that the temperature is 40 deg C colder than ISA all the way up to your altitude, which is not realistic. In the real world, you would have to higher than this to get an error of 1,000 ft.

pbesing said:
As far as temperature, what you need to be more concerned with is a change in temperature from one location to another. If the temperature is relatively constant accross a large area, that shouldn't be a problem..crossing a front or something, or entering a temperature inversion might have an effect, but temperature changes shouldn't make that much of a difference.
Click to expand...

The problem is not temperature changes, but differences in the actual temperature from ISA temperatures.

Barometric altimeters rely on the fact that the air pressure decreases as the altitude increases. The air pressure is created by the weight of the air above compressing the air beneath. As the altitude increases, there is less weight of air above, so the air pressure is lower. The long term average of air pressure vs geometric height was used to create the International Standard Atmosphere, and this relationship is used to calibrate our altimeters. If the air temperature decreases, the air density increases. This changes the relationship between geometric altitude and air pressure.

There is a lot more info available on the web:

Just how high are we then?
AIM 7.2 - scroll down to "7-2-3. Altimeter Errors"
Lies Your Barometric Altimeter Tells You
The Chilling Result of Cold Temperatures on Barometric Altimeters

The above links concentrate on the safety effect of the errors created by cold temperature. The same effect is seen in warm temperatures, but it works in the other direction. I.e. your geometric altitude is higher than the barometric altitude. This isn't a safety problem, but it means we cannot use the geometric altitude reported by the GPS as a way to check the accuracy of the barometric altimeter.

If we want a comprehensive check of the accuracy of our barometric altitude, we need to do several things:
  • check the displayed altitude on the ground, against a known elevation, with a current altimeter setting.
  • Have an instrument shop test it with a pitot-static test set to check the instrument error at other altitudes.
  • Conduct a static system leak check.
  • Determine the static system position error via a flight test. Detailed test procedure on my web site.
 
Far 91.411

jraymond4321 said:
I purchased a RV-6a and it now has about 100 hours TT.

My altimeter is reporting 200-300 feet below actual altitude for the last 10 hours or so, it was accurate before.

I was going to adjust it, but it is a Falcon guage so I have to take the panel off and I haven't gotten around to it.

I was flying down to Miami Thursday, with flight following, and ATC said my mode C was reporting 7100, when I was actually at @7500 (verified with 2 GPS). On the way home, I checked when i left the tower, and same thing (mode C reported 2200, while I was at @2500).

Does this sound like any particular problem (ie. pitot leaking, or blockage)? I don't fully understand how the mode C works in conjuction with the pitot system, and if or how it is related to the altimeter.

I would like to have a better understanding before I bring it to the shop.

Additionally, would I need to go to an avionics shop for this, or my normal maintenance shop (transponder is avionics, but pitot would be maintenance)?

Thanks in advance

James Raymond
N436JE
Neptune Beach, FL
Click to expand...


When does the log book say that the transponder was tested in accordance with FAR 91.413 ? http://www.risingup.com/fars/info/part91-413-FAR.shtml

If you have a transponder in a VFR experimental aircraft, it MUST be tested by an approved repair station every 24 calandar months. If the home built experimental is used for IFR, it MUST be tested in accordance with FAR 91.411 . http://www.risingup.com/fars/info/part91-411-FAR.shtml

If it has not been done, it needs to be done. If it has been done, the approved repair station that did the work should take a look at it.

Gary
 
Kevin Horton said:
Just how high are we then?
Click to expand...

As to GPS accuracy, in this URL, I'm assuming it was published in 2000 (by the title). It doesn't mention WAAS correction, and uses +/- 90' as the average GPS vertical error in the US, and an average 485' error in other parts of the world.

Using WAAS, which is available in a large portion of the northern hemisphere, this horizontal & vertical error is reduced by five times, 95% of the time.
 
L.Adamson said:
Not something I've gave a lot of thought too,

But for instance, I live in the Mt. West area with many airports ranging from 4000' to 7000'+ altitudes. Since it's common practice to set the altimeter to the "un-controlled" airport's runway altitude before takeoff, rather than relying on some report of barometric pressure; wouldn't it be very conceivable of having a drastic difference in VFR crusing altitudes, if mine started exact, and someone else was a 1000' off?

Or, am I just getting the wrong idea here?

L.Adamson
Click to expand...
This is only a problem if some people are mistakenly using GPS altitude to cruise at, instead of using their barometric altimeter. Everyone should be using barometric altitude, with an altimeter setting from a source somewhere close to the actual location. If this is the case, everyone in the same vicinity should have similar altimeter settings, and they should be at similar altitudes. The fact that the barometric altitudes that they are at differs from the geometric altitude is not a problem.

The temperature errors in barometric altitude are only manifested if there is an altitude difference between the place where the altimeter setting came from, and the aircraft altitude. So, if you are on the ground at an airport at 10,000 ft, with an altimeter setting that came from that airport, then there is no error. If you have an altimeter setting from a sea level airport, and you fly next to a 10,000 ft mountain, and the temperatures differ from standard, then your altimeter will not read 10,000 ft when you pass by that 10,000 ft high mountain.
 
L.Adamson said:
As to GPS accuracy, in this URL, I'm assuming it was published in 2000 (by the title). It doesn't mention WAAS correction, and uses +/- 90' as the average GPS vertical error in the US, and an average 485' error in other parts of the world.

Using WAAS, which is available in a large portion of the northern hemisphere, this horizontal & vertical error is reduced by five times, 95% of the time.
Click to expand...
Yes, this document is a bit dated with respect to GPS accuracy. The info on temperature effect on barometric altitude is still relevant though, as the laws of physics are still the same as they were in 2000.

WAAS is available on some handheld GPSs now, but the only approved, panel mount GPS with WAAS is the Garmin GNS480, as far as I can tell. So the non-WAAS info is still relevant to the majority of users.
 
Kevin Horton said:
1000 feet off would only happen if the temperature was extremely cold, and you were quite high. For example, if the temperature is 40 deg C colder than ISA (i.e. it is -25 deg C at a sea level airport), the error is about 14% of the altitude difference between your altitude, and the altitude of the airport where the altimeter setting came from. If you are 7,000 ft above the airport where the altimeter setting came from, the error is about 1,000 ft. This assumes that the temperature is 40 deg C colder than ISA all the way up to your altitude, which is not realistic. In the real world, you would have to higher than this to get an error of 1,000 ft.
Click to expand...


But, if you took the altimeter setting before you took off (not if you took it while flying 7000 feet above), then you would be reasonably accurate, unless you passed over a front or area of low pressure (or high temperature or high pressure, just an error in the opposite direction.) Now if the lapse rate wasn't standard, then you are really monkeying with things.
Correct, it's not likely to have this 1000 foot error. On the other side of things, as long as everyone has the same setting, we all have the same altitude, regardless if true altitude is off or not. Perfect example of this is why all aircraft use the atlimiter settings to 29.92 at FL180. At that point, all aircraft accept pressure altitude for indicated/absolute/true.

Kevin Horton said:
The problem is not temperature changes, but differences in the actual temperature from ISA temperatures.
Click to expand...

I disagree here because any change in current temperature effects true altitude. Remember the saying "From high to low look out below"?. My concern more so would be someone who did not change their altimeter when they crossed a pressure change. I've been flying and seen 1/2" change over 100 miles before which equals 500 feet error. Granted, this is a pressure example, but a drastic change in temperature can have the same effect, regardless of the difference from ISA temperature. If you have a starting temperature, even if equal to the ISA temperature, and you have a drastic change in temperature without changing the altimeter setting, you will have significant change in altitude.
 
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The required 2 yr. transponder check only checks for transponder power output and squawk code accuracy. It does not include the altitude reporting accuracy. This is done at the initial static system check and is not required for VFR aircraft unless the static system has been opened or a component within the system has been changed.
Mel...DAR
 
pbesing said:
But, if you took the altimeter setting before you took off (not if you took it while flying 7000 feet above), then you would be reasonably accurate, unless you passed over a front or area of low pressure (or high temperature or high pressure, just an error in the opposite direction.) Now if the lapse rate wasn't standard, then you are really monkeying with things.
Click to expand...

Read the info at the links I posted earlier. It isn't practical to regurgitate all the details in this thread.

The problem isn't the lapse rate, it is differences between the actual temperature and the ISA temperature. Let's imagine an airport at sea level next to a 7,000 ft high mountain. You put the airport's altimeter setting in the altimeter on the ground, and your altimeter should read the airport elevation, with a small tolerance for altimeter instrument error, etc. If the air temperature at all altitudes is the same as ISA, you can take off and fly by the top of that mountain, and your altimeter should read 7,000 ft, plus or minus the instrument error. So far, so good.

Now, let's imagine the temperature drops 10 deg C at all altitudes, but the air pressure at the ground is the same as before. The altimeter setting will be the same as before, as the air pressure and the elevation haven't changed, so we need the same altimeter setting to make the altimeter read the same as the airport elevation. The lapse rate (i.e. the amount the temperature drops for every 1,000 ft increase in pressure altitude) is still the same as standard (-6.5 deg C per 1,000 m, or about -2 deg C per 1,000 ft). The air density is increased, because the air is colder. The pressure altitudes are closer together. If you climb 1,000 ft barometric altitude, you have actually climbed only about 965 ft geometric altitude. If you climb to 7,000 ft barometric altitude (i.e. your altimeter reading), you will only have climbed about 6,755 ft. You are still 245 ft below the mountain top.
 
Thanks for all the responses.

Roberta, you mentioned to set my Alt to 29.92 and see if it agrees to my encoder. How do I verify my encoder output?

Also, per Kevin's reply, It sounds as if my Alt and Mode C are fairly close, but I need to check the barometric pressure as he was talking about. I am in Fla, and it is quite a bit hotter here now. I purchase my plane back in Oct, and most of the previous flying was quite cooler. Does anyone know of a online calculator to check what the difference between geometric and barometric altitudes?

Thanks again in advance.

BTW: the 2nd GPS I was using was a WAAS enabled handheld.
 
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jraymond4321 said:
Does anyone know of a online calculator to check what the difference between geometric and barometric altitudes?
Click to expand...
It is very difficult to make an accurate correction, as you would need accurate knowledge of the air temperature at every altitude between the aircraft and the ground. Not the forecast temperature, but the actual temperature - they usually differ. And these temperature values would need to be fairly accurate, so you couldn't just use your aircraft's OAT gauge, unless you did quite a bit of work to calibrate it, and determine its recovery factor.

The rule of thumb I reported earlier is probably as accurate as you can do, given the inaccuracies of all the input data. The difference between the GPS altitude and the barometric altitude will be about 3.5% for every 10 deg C that the temperature in the air mass between you and the ground differs from ISA. Note - this error is only present in the altitude band between you and where ever the altimeter setting came from. So, for example, if the altimeter setting came from an airport at 5,000 ft, and you were at 7,000 ft, with an air mass that was 10 deg C warmer than ISA, the error due to temperature would be about 3.5% of 2,000 ft, or about 70 ft.

If your altimeter reads correctly on the ground, and you have no static system leaks, and your transponder reports pressure altitudes that agree with the pressure altitudes on your altimeter, then your system is probably working correctly. The recent change in baro altitude vs GPS altitude is almost certainly due to the changes in air temperature.

Some modern transponders will display their reported pressure altitude on the cockpit control, if you push the correct combination of buttons. Many transponders don't have this feature though. In that case, you need to ask the controller what your transponder is reporting. It can get confusing though, because although the transponder reports pressure altitude, I believe that the controllers have a way to correct the reported altitudes for the local altimeter setting.

Fly safe. Have fun.
 
Correcting from 29.92

"believe that the controllers have a way to correct the reported altitudes for the local altimeter setting."

Kevin... I was always told the the controller's equipment does correct for baro. pressure.

I think it's actually active all the time, not a selection.

This article explaining Mode C transponders does confirm that....

http://www.airsport-corp.com/modec.htm

gil in Tucson
 
Getting back to the original post. If both the ALT and the encoder are reading low, it sounds like you have a problem in your static plumbing, i.e. you have a leak somewhere. If the altitude is reading correctly on the ground, just adjusting the ALT and encoder will only mess things up further. In my plane I have 190 ft difference when I switch to the alternate static source in cruise. So, depending on the pressure difference between your normal static source and where the leak is, a 300 ft difference is conceivable. Check out your plumbing.
 
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