Any heading system failure will cause the heading Flag to appear on both the RDDMI as well as the HSI. Here is how a heading failure would manifest itself:
The associated RDDMI compass card would have a heading flag and the card would freeze in place with the data from the last good heading. However, the VOR/ADF needles will show the relative bearing.
Heading flags will also be in view while slewing the compasses with the COMPASS panel controls.
The HSI compass cards also provide inputs to the instrument comparator system and any difference between the two cards will trigger the comparator error to be shown. If a difference of between 6º in level flight, and 15º in a 30º bank occurs between the two HSI's, a HEADING warning flag will appear on both comparator panels.
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