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
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