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(Student pilot, ~ 22hrs training)

On a box-standard AI in a C172 there is a knob that allows you to move the symbolic airplane up and down. I've never managed to ask what it is for, but have now been intrigued enough to google it. An RC Allen instrument manual points out that it "adjusts the Symbolic Airplane to compensate for viewing parallax. Adjust Symbolic Airplane so that it is visually aligned with the horizon from pilot’s normal sitting position."

Artificial horizon with a symbolic airplane adjust knob

Two questions:

  1. My best guess is that I could/should adjust it during the run-up along with the "instruments check" item on the checklist. However, is the Cessna always sitting correctly for that to be exact (or exact enough)? If there is a slightly different pressure in the nose wheel oleo the nose wouldn't necessarily point to the horizon, or when there is a slight slope of the taxiway/run-up area. Would it be better to (re-)adjust it in level flight?

  2. Why do more modern AIs no longer appear to need that? The viewing parallax is still there, why isn't the button?

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Instrument parallax has to do with viewing position of the pilot compared to the instrument. You adjust the symbolic airplane to the horizon bar on the outside of the instrument, not the part that moves, so you don't have to wait for it to spin up or find flat ground. I do it as part of my pre-start instrument checks, after adjusting my seat.

All mechanical AIs should have an adjustment mechanism, but many modern ones are digital, with a flat screen. You can't have parallax error with a screen because there is no distance between the needles and the reference measurements.

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  • $\begingroup$ So pre-start, after adjusting your seat, you just put the symbol on the horizon line and you're done? $\endgroup$ – ExternalUse Jul 24 at 8:37
  • $\begingroup$ Exactly @ExternalUse. I'll edit to point out it is post seat adjustment. $\endgroup$ – GdD Jul 24 at 9:38
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    $\begingroup$ @ExternalUse - Actually, I’ve always adjusted the Symbolic Airplane after I start the actual airplane engine, usually during runup. The AI needs at least five minutes to spin up its gyroscope and stabilize before I cage it in the level-with-the-horizon position. $\endgroup$ – Dean F. Jul 24 at 13:31
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    $\begingroup$ @DeanF. , you are supposed to adjust it to the horizon bar on the outside of the instrument, not the attitude card. $\endgroup$ – GdD Jul 24 at 14:09
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    $\begingroup$ You may need to adjust it in cruise flight. If the attitude on the ground differs from that needed to hold altitude at your chosen airspeed. $\endgroup$ – Michael Hall Jul 24 at 14:11

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