For purposes of the FAA instrument written, which system becomes primary for pitch following an AHRS failure?
The AHRS is the Attitude and Heading Reference System. It is synonymous to the Gyroscopic systems of a steam gauge aircraft.
The ADC or Air Data Computer is synonymous to the pitot-static system in a steam gauge aircraft. Just like a steam gauge aircraft, the AHRS and the ADC are two separate systems. And, for good reason. They compliment each other while at the same time providing a redundant backup when needing to fly partial panel.
The PFD is the Primary Flight Display. It is synonymous to your PC computer monitor. If your PC is not working, it does not mean that your monitor won’t work. And if your PFD does not work, a TAA will have an MFD (Multi-Function Display) that can display the same info.
That being said, the AHRS is not as important in VFR flight as the ADC. Ask yourself the question of which parts of the AHRS are required by subsections b &amp; c of Title 14 Code of Federal Regulations Part 91.205. No gyroscopic instrument is required by 91.205 for VFR flight. As long as the magnetometers are working and displaying properly, you are still capable of flying relatively safely. Even if the magnetometers are inoperative, you should still have your magnetic compass to meet the requirements of sub-section b.