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Did the Apollo Flight Director Attitude Indicator (FDAI) error needles relate in any direct way to the position of the ball under them, or were they only ever interpreted in relation to the white bars of the scale? ie, if the ball didn't exist, would the attitude error needles still function identically? (like the rate display)

For example, in KSP, the navball has markers for prograde and burn attitude projected directly onto the surface of the ball. It strikes me that the attitude error needles could be used in a similar fashion, to indicate a point on the ball, but this would require a non-linear movement towards the extremes of the scale as the desired point moves towards the edges of the balls visible area. I have not found any mention of the needles being used like this, so can you confirm that they were only ever referenced against the scale, and never the ball? And if that's the case, why put them on front of the ball at all?

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The error needles only showed deflection proportional to the attitude errors, and this deflection was unrelated to the ball below them.

(Although, I guess you could say that the attitude displayed on the ball is related to the error needle deflection in the trivial sense that if the displayed attitude is not the desired attitude the needles will be deflected...)

They were displayed over the ball so that the current attitude (from the ball) and attitude error (from the needles) was available together and simultaneously (at a glance).

The indicator's attitude error needles show the difference between the actual and desired space­ craft attitude. Positive attitude error is indicated by a downward displacement of the pitch error needle, and by a leftward displacement of the yaw and roll error needles. The attitude error needle displacements also are related to the direction of motion required by the rotation control to reduce the error to zero. The ranges of the error needles are 5 degrees or 50 degrees for full-scale roll error, and 5 degrees or 15 degrees for pitch and yaw error. The error scale factors are selected by a switch that also establishes the rate scales. The pitch and yaw attitude error scales contain grad­uation marks at null and full scale, and at 1/3 and 2/3 of full scale. The roll attitude scale contains marks at null, 1/2, and full scale. The attitude error indicators also use servometric meter movements.

Source: CSM News Reference H Missions page 196

Shuttle ADI worked the same way, and when they went to the glass cockpit for Shuttle, the ADI images worked the same way.

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