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Sheppard Air claims to have questions exactly from the FAA test. I'm wondering what folks think of the Q&A for this one. It asks

In the event of an AHRS failure, what becomes your primary pitch instrument?

The given answers are

  1. PFD altimeter
  2. Standby altimeter
  3. Standby attitude indicator

I thought (2) was correct, but they give (1).

What I find puzzling about this choice is that the AHRS is a measurement system that displays its output on the PFD. So if the AHRS has failed, the PFD will too. The section of the FAA handbook in their explanation refers to using the PFD altimeter for partial panel attitude reference, but says nothing about the context being failure of the AHRS. I'm wondering if I misunderstand what's meant by "PFD" and "AHRS", or is this just stupid FAA word games? (Or a Sheppard Air mistake?)

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  • $\begingroup$ AHRS only gives pitch/roll/heading. If that fails, the PFD will still have attitude and airspeed data from the ADC. $\endgroup$
    – StephenS
    Oct 4, 2020 at 19:30
  • $\begingroup$ StephenS beat me to it. $\endgroup$
    – Dean F.
    Oct 4, 2020 at 19:33

1 Answer 1

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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 & 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.

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  • $\begingroup$ Could be clearer that the PFD displays data from multiple sources at the same time (unlike your PC/monitor example), and if one source fails, data from other sources isn’t affected. $\endgroup$
    – StephenS
    Oct 4, 2020 at 20:03
  • $\begingroup$ @StephenS - It’s more like the PFD is the computer and the AHRS and the ADC are just peripherals. $\endgroup$
    – Dean F.
    Oct 4, 2020 at 20:09
  • $\begingroup$ That would be a better example, yes. If your keyboard stops working, you can still use the mouse, at least for the things a mouse can do. $\endgroup$
    – StephenS
    Oct 4, 2020 at 20:10

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