ADS-B Altitude reporting error

[Webb]

I looked at FlightAware track and altitude reporting was way off and I’ve never seen this before.

When I was at 7000, it shows me me 500 high. When at 9000, 500 low. Not the entire time but at least half.

I filed for 7000, later climbed to 9000 for cooler air. Obviously on an IFR flight from the altitudes, talking with ATC and resetting the baro on the altimeter when changing controllers, and on autopilot.

If my altitude was that far off, I would have gotten altitude alerts from ATC and probably told to call a number when I land.

It did correct itself a little over halfway through the flight but leaves me wondering if others have seen errors like this.

 

[Cth6]

Source

Your ADSB out is just sending the pressure altitude from your transponder. It doesnt do anything to the data.

I noticed you are showing a pretty long flight.

Your indicated altitude will occasionally vary from the pressure altitude. Variables include how often you update your altimeter setting. How far the barometric reporting station is from your actual location. The pressure gradients for that area….

 

[Webb]

4 hour flight from Jackson MS to Naples FL. Pressure during that time was 30.03 or 30.04 according to my flight notes I write when getting handed off. I remember commenting to my wife that the pressure wasn’t changing after about the 10th handoff.

The return trip 5 days later was back to normal.

If there was a leak, the error would be consistent., Not both high and low. It’s just an oddity that makes you go hmmmm.

Lots of military airspace in the Deep South. Maybe some testing was going on. Date was July 1st in the AM. I wonder if anyone flying near near the gulf on July 1st would see the same on their FlightAware report.

 

[Cth6]

Pressure

Pressures are recorded at the surface and our altimeters and alt encoders are programmed to follow standard rates. Assuming that standard rate is followed in nature, which is rare, our altimeters would correctly report our elevation above MSL regardless of altitude.

As the temperature deviates from standard rates we will see a higher or lower pressure than expected at a given altitude. So when you climb from actual 7000’ to actual 8000’ you may only see a 900’ increase in your altimeter because the pressure did not change as expected.

I pulled the Skew T for Tampa around the time you transitioned the area on the first. Note the red line. That is the temp. It follows a standard rate to about 3500’ then inverts until about 7000’ before returning to a rate less than the standard rate. So in this scenario you would see correct altimeter readings up to about 3500’ then you see a positive or negative error depending on the altitude you were at.

Bottom line, I dont think you were experience some technology anomaly, just flying through some normal summer southern weather.

 

[lr172]

I looked at FlightAware track and altitude reporting was way off and I’ve never seen this before.

When I was at 7000, it shows me me 500 high. When at 9000, 500 low. Not the entire time but at least half.

I filed for 7000, later climbed to 9000 for cooler air. Obviously on an IFR flight from the altitudes, talking with ATC and resetting the baro on the altimeter when changing controllers, and on autopilot.

If my altitude was that far off, I would have gotten altitude alerts from ATC and probably told to call a number when I land.

It did correct itself a little over halfway through the flight but leaves me wondering if others have seen errors like this.

I believe that ads-b out sends both baro altitude and a GPS altitude. Not sure which one ATC uses nor which one flightaware uses. However, if they are using different sources and one was flawed due to a problem and the other not, I can see how FA was off 500 but ATC thought you were doing fine. I have a suspicion that FA uses the GPS altitude and have a strong feeling that ATC uses Baro. Remember that ATC doesn't know your actual altitude, only your reported altitude and that can come with errors when equip starts mis-behaving. So, if you are reporting a steady 7000 even though you are going up and down 500', ATC will not complain, as they don't know it is happening. I would get a performance report and look for inconsistencies in the two altitude data fields. 500' is suspicious as that is about the difference between alt using the ext static port vs altitude using cabin based static on an RV. That said, the difference would always be in the same direction. Also note on your chart that the speed changes when the altitude changes, and that implies a static issue.

 

[Cth6]

Flight Aware

According to FAs website (Mar ‘22) they use. >>> “For ADS-B it will be uncorrected pressure altitude, which is approximately above sea level but needs to be corrected for local weather conditions.”

 

[BobTurner]

Since he gave us his altimeter settings we know pressure altitude was about 100’ lower than indicated altitude. Something else is going on here. Maybe flight aware?

 

[Cth6]

Had some fun with data analysis

Webb,

I wanted to share a bit more around my logic that having a 500' discrepancy is not out of the ordinary. Unfortunately I could not get the raw sounding data for that image I shared earlier. I did find the raw sounding data for a few hours later 2 July 00Z. By this point the temp inversion had pretty much passed, probably due to the flip of the off shore breeze in Tampa.

To orient the data. The green cells are actual data from the balloon launched from a point 8NM from KTPA on 2 July 00Z. I did convert the meters into feet for ease of reading. The blue cells are standards based on the 43 foot data using the standard adiabatic lapse rate for temp and the barometric formula for adjusted standard pressure at altitude. The yellow is a basic computation of the delta between the actual and standard baro then translated into the number of feet using 1 InHg = 1000'

So if you look at the red box, you can see the balloon sensors were at 8602' MSL and they recorded a pressure of 22.09 inHg and an OAT of 12.2*C. If the atmosphere followed constant rates, we should have expected to see 21.74 inHg and an OAT of 8.6*C. So if we were flying over KTPA at 00Z on 2 July and we had our altimeter set at 29.88 inHg the difference between our actual pressure altitude and what the altimeter was reporting would equate to an error of 344 ft.

So if I had the raw data at the time we had at inversion with those aggressive rates you would see a period of time the altitude was reporting high followed by a low period. If you look at the first 1000' you will see a minor inversion.

Thanks for bringing it up. I had fun doing some data analysis.

Chad

 

[Webb]

The altitude reporting was correct by the time I got to Tampa.
I was much closer to Eglin and Tyndall when this was going on.

I’m calling it a fluke and one of those things that make you go hmmmmm.

At least I didn’t get “call this number when you land”.

 

[RV8JD]

If in doubt, just request a PAPR for that flight. Quick and easy. If no red boxes, you're good to go.

Link --> https://adsbperformance.faa.gov/PAPRRequest.aspx