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How High am I?

bjdecker

Well Known Member
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[Data source - G3X Touch flight log, GTN650xi data log]

I have been chasing an apparent static system "problem" which results in erroneous readings in IAS/TAS.

I flew a number of GPS tracks following the test procedure detailed in Kit Planes, NTPS, etc. and come to the realization that my TAS as reported by Garmin G3X Touch is in error by >5kts.

I then spent time looking for pitot/static leaks (none found), but something interesting did show up in the data wrt Baro/Altimeter/Altitude data.

If WAAS GPS (C146), xDOP, VFOM are to be believed - my aircraft was 500ft Higher than what is indicated on the G3X (7000ft vs 6500ft).

This difference in altitude became larger as the aircraft IAS increased & altitude increased.

This data isn't 100% yet, more to be done - but I thought I should post this if you too are looking for discrepancies in your pitot/static system.
 
If WAAS GPS (C146), xDOP, VFOM are to be believed - my aircraft was 500ft Higher than what is indicated on the G3X (7000ft vs 6500ft).

This difference in altitude became larger as the aircraft IAS increased & altitude increased.

Suggests flow across the static ports which drops pressure slightly.

Ports have a dome head? Mine required filing to a more flat profile. They are billet aluminum, but approximated the shape of a pop rivet. File a little, go fly an NTSP triangle, file again, etc.

Just cover the skin around the port with masking tape and slide a fine file. Didn't require a lot of removal.
 
IFR certified?

Have you done an IFR certification on plane? An IFR certification check will identify any discrepancies between actual altitude and indicated altitude.

It would be worthwhile to do an IFR cert by a shop the knows RVs and the G3X system, even if you only fly VFR. If you do the test connecting directly to the P/S lines (i.e down stream of the static ports) and your error disappears, it would confirm that the static ports themselves (as Dan suggested) maybe the problem.
 
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Rick is right. Gotta get check anyway, and it tells if the system is good without airflow. if so, then the flow field at the ports is the issue.

I'd connect static externally, over the ports.
 
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Please remember that gps altitude is NOT the same as barometric altitude. GPS should show true MSL altitude, while barometric depends on the temperature lapse rate. Temperature higher than standard? Your barometric altitude will be higher than true altitude. Now having said that, 500’ difference is a lot.
There is an easy check to see if there is a static port problem. Set your altimeter to field elevation while on the ground. Do a high speed low pass at a known elevation (e.g., height of the control tower, its height is published). Compare to your altimeter.
 
Please remember that gps altitude is NOT the same as barometric altitude. GPS should show true MSL altitude, while barometric depends on the temperature lapse rate. Temperature higher than standard? Your barometric altitude will be higher than true altitude. Now having said that, 500’ difference is a lot.
There is an easy check to see if there is a static port problem. Set your altimeter to field elevation while on the ground. Do a high speed low pass at a known elevation (e.g., height of the control tower, its height is published). Compare to your altimeter.

Here's a pretty interesting article about why you sometimes get discrepancies between MSL and GPS altitude, if you're interested.

https://www.esri.com/news/arcuser/0703/geoid1of3.html
 
First question would be what static ports do you have?
Vans pop rivet? sounds unlikely.
Which aftermarket one?

I've chased this down on a few planes and the only two that I believe are accurate out of the box are the factory pop rivet and the Cleveland machined one that matches it.
All the others will be wrong, some by a lot.
As you point out the altitude discrepancy can be quite large. Certainly big enough
to make it impossible to be in tolerance on a precision approach.

Cheers
 
If you read the article on the link posted by bill.hutchison just now, you won't even try to compare GPS altitude with pressure altitude. They are totally different beasts.
 
Bill's post is an interesting discussion on what is "mean sea level", and can account for some of the errors in comparing barometric altitude to GPS altitude. But most of the time, it's just the error of barometric altimetry which isn't compensated for non-standard temperatures. e.g., if the temperature at 7500 was 20 C (instead of 0 C, standard), this would result in a 500' error in the barometric altimetry, compared to the true altitude.
 
[Data source - G3X Touch flight log, GTN650xi data log]

I have been chasing an apparent static system "problem" which results in erroneous readings in IAS/TAS.

I flew a number of GPS tracks following the test procedure detailed in Kit Planes, NTPS, etc. and come to the realization that my TAS as reported by Garmin G3X Touch is in error by >5kts.

I then spent time looking for pitot/static leaks (none found), but something interesting did show up in the data wrt Baro/Altimeter/Altitude data.

If WAAS GPS (C146), xDOP, VFOM are to be believed - my aircraft was 500ft Higher than what is indicated on the G3X (7000ft vs 6500ft).

This difference in altitude became larger as the aircraft IAS increased & altitude increased.

This data isn't 100% yet, more to be done - but I thought I should post this if you too are looking for discrepancies in your pitot/static system.

This may be an outlier, but I was flying chase for another 10 one time, on his first flight, and we had a fairly large (can't remember the exact amount), difference between our airspeeds in formation flight. On his first approach, I was flying formation on him and warned him a few times "no slower", because he was getting too slow on approach. Come to find out later he had the alternate static source switch on the panel upside down(and open), which makes his (and my) airspeed read way faster.

Just a thought of something to check, if you have that setup........
 
Here is a test you can try

Once had an RV14 that seemed to always have a headwind when flying. Turned out to be the machined aftermarket static ports.

Go out to the runway and adjust your altimeter to a nice round number. Then take off and fly a SAFE low level fly pass and observe the altimeter reading. Try to subtract your height above the runway as accurate as possible.

If over reading you probably have a low pressure at the ports - read sucking out.
If under reading you probably have a high pressure at the ports - read blowing into.

Depending on what you have it can easily be fixed by adding a small “strip” in front or behind the ports. An AN970-4 washer cut in half works well. Use double sided tape for testing.

Over reading - washer at the back and under reading washer at the front. I hope this is helpful.
 
"Everything is relative"....... Einstein :rolleyes:

Airspeed: Except for takeoff and landing, I am primarily interested in ground speed. Especially when I'm going from A to B. So I look at the winds at altitude, and select an altitude that will minimize my time and fuel burn.

In this phase of flight, I really couldn't care less what the airspeed indicator is reading. Could be Zero, wouldn't bother me.

OTOH, when I'm flying slowly, especially close to the ground, I don't want to stall, so I stay a few knots above whatever my individual airspeed indicator reads at stall, as determined by my phase 1 flight tests. In my particular case, my indicated airspeed is a few knots high for all stalls. (So what?, it's just a number...)

If I had a good Angle-Of-Attack (AOA) indicator, I wouldn't even need an airspeed indicator! (Well, ....... maybe for backup :rolleyes:)

Altitude: Not true for the altimeter, however! Throughout the world, aviation relies on pressure altitude, as adjusted for local barometric pressure. (At least below the Flight Level altitudes.....)

So reasonably accurate pressure altitude is pretty important. Again, pressure altitude and GPS altitude are NOT RELATED. They are usually close, but not always.

So for altimetry purposes, having an accurate static port pressure is very important.

One easy way to get a rough check is to fly along with a few of your friends in close proximity, and compare altimeter readings. If you are all within a few tens of feet, then no worries.

Another way is described earlier in this thread, and involves a low pass along a runway. Either should be sufficient. If you are flying IFR, your required altitude tolerance is +/- 300 feet!

Went looking for my Flame Suit, but couldn't find it! :p
 
I have been chasing an apparent static system "problem" which results in erroneous readings in IAS/TAS.

I flew a number of GPS tracks following the test procedure detailed in Kit Planes, NTPS, etc. and come to the realization that my TAS as reported by Garmin G3X Touch is in error by >5kts.

Brian, was the TAS on the GX3 reporting too high?
 
Brian, was the TAS on the GX3 reporting too high?

Sorry for the delay in responding -- we had to go somewhere in the other aluminum toy for a few days :)

The TAS in this airplane has always "seemed" low (compared to last RV-7 with similar equipment). The NTPS GPS runs seem to have proven this to be the case, GPS & spreadsheet math show 170KTAS while G3X shows 164KTAS.

The ports in question are machined aluminum, with a profile that matches the Van's pop rivet.

And, just overloading this response a bit more, recent pitot/static system test showed no leaks in either system.
 
Bill's post is an interesting discussion on what is "mean sea level", and can account for some of the errors in comparing barometric altitude to GPS altitude. But most of the time, it's just the error of barometric altimetry which isn't compensated for non-standard temperatures. e.g., if the temperature at 7500 was 20 C (instead of 0 C, standard), this would result in a 500' error in the barometric altimetry, compared to the true altitude.

This ^^^ is interesting; down here, the OAT at test altitude is 15°C to 20°C instead of 0°C...
 
First question would be what static ports do you have?
Vans pop rivet? sounds unlikely.
Which aftermarket one?

Richard -- thanks for the response.

I have a set that was machined by a gentleman here in CenTex. The outside-the-skin profile matches that of the Van's rivet, with a .1875" threaded shank into a .170" tail.
 
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