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Altitude Puzzler: Discrepancy between altimeter and Mode C readout

DavidHarris

Well Known Member
I recently was in IMC in light turbulence. I experienced a brief excursion to 6160'. ATC informed me that my transponder indicated flight level 063 and generated an automated pilot deviation. I'm puzzled why there would be such a discrepancy and looking for ideas how to prevent reoccurrence.

I'm flying a recently completed RV-7A with a Dynon HDX EFIS, IFD540 GPS, and Dynon transponder, ADS-B in/out, ADAHRS and pitot-static system.

As I understand the system operation, the ADAHRS receives a pressure on the static port input and reports it to the EFIS. The EFIS adjusts based on the barometer setting and displays my altitude MSL. It also records the pressure altitude (relative to 29.92) in the EMS log. It sends this pressure altitude over a serial link to the transponder, which broadcasts to ATC, who in turn corrects based on barometric pressure to generate a 3-digit flight level on the controller's display.

In my case, I had been given a barometer reading of 29.73 11 minutes prior to the deviation. My engine logs show a maximum pressure altitude of 6351' at the time of the deviation. This corresponds to 6351 + 1000*(29.73-29.92) = 6161' MSL, in accordance with my recollection of what was displayed on the EFIS.

I expect the same 6351' pressure altitude should have been broadcast by the mode C transponder and ATC should have corrected it to read Flight level 062. However, Center sent me a picture showing me at 063.

Given the digital communication paths, I don't see how there could be a calibration error that would cause the pressure altitude recorded by my EMS to be different than that received by ATC. And the only way the altitude displayed on my EFIS would be different than that displayed by ATC would be if we were referencing different barometer settings. The pitot-static system tested within spec in January, and even if it had an error, I would think it would affect the EFIS and transponder identically. To the best of my understanding, my altitude seen by ATC comes from my static system and transponder, not a geometrical altitude from my IFR GPS and ADS-B. My ADSB also recently passed its PAPR check.

I'm pretty confident that I read back and entered the correct barometer setting shortly before the deviation. But I'm puzzled what else could cause this discrepancy and whether the problem might reoccur in the future. Maybe I misunderstand how my altitude is determined for reporting to ATC? Any other ideas? (I don't know the answer to this puzzler...)

Thank you,

David
 
And the only way the altitude displayed on my EFIS would be different than that displayed by ATC would be if we were referencing different barometer settings...
David
I?d say your understanding is correct.
This used to happen to me with some frequency, before my home field (LVK) obtained a remote-reporting ATIS. Approach control was in SCK, which every so often had noticeably different altimeter settings from LVK (different enough that approaches were not authorized if the tower was closed). If I changed the setting to LVK?s while still on the SCK side of the pass, I might get a query from approach.
I guess I would have asked ATC to confirm 29.73? as the current setting.
 
static port accuracy

There is a true airspeed Excel spreadsheet calculator floating around out in the ether. The value is that dynamic (as opposed to test set) static system errors are identified. While your pitot/static system may test fine on the ground, static ports can be finicky. If something (shape, protrusion from the surface etc) isn't quite right, the error affects not only airspeed indication but also altitude.

I mention this because the shape of the static ports I originally installed on my RV-7 led to an error of almost 300 feet at a true airspeed indication of around 160 KTAS (which turned out to be inaccurate). Going back to the primitive-appearing but per-plans correct static ports solved the problem in my case. The true airspeed calculator is what made me realize I had a problem.
 
The above post is correct but not the OP?s problem. He may be at the wrong altitude, but neither his altimeter nor his mode C output knows that.
My guess is that ATC and the OP are using different altimeter settings, resulting in different indicated to pressure back to indicated altitudes.
 
I found a good way to check your transponder is to go flightaware and review your flights to see if all ATC readings are consistent. In your case for example, if they are showing a consistent 100 feet to high then you may have to correct your output by the same amount. I have a Stratus that reports the pressure altitude which is helpful with reporting and True airspeed calculations.
 
Thank you for the suggestion about FlightAware.

I just reviewed the FlightAware altitudes and they consistently agree with my pressure altitudes to within the 25' resolution of FlightAware. It's amazing that everything is time-stamped well enough to line up my log files and FlighWare's logs with that kind of precision.

That supports the hypothesis that I was using a different altimeter setting than ATC, even though ATC had given me a setting 11 minutes previously and I had read it back. However, our settings would have had to be at least 0.09" different.

David
 
Thank you for the suggestion about FlightAware.

I just reviewed the FlightAware altitudes and they consistently agree with my pressure altitudes to within the 25' resolution of FlightAware. It's amazing that everything is time-stamped well enough to line up my log files and FlighWare's logs with that kind of precision.

That supports the hypothesis that I was using a different altimeter setting than ATC, even though ATC had given me a setting 11 minutes previously and I had read it back. However, our settings would have had to be at least 0.09" different.

David
You may be able to go back on ATCLive and find your communications. I have done that locally as the ATC frequency was broadcast.
 
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