Definition
In the primary/supporting method of attitude instrument flying, the instrument that provides the most direct and accurate indication of bank in a given flight condition. In straight-and-level flight, the heading indicator is the primary instrument for bank, because any change in bank will produce a change in heading.
Plain English
The one instrument you watch most closely to keep the wings level. In straight-and-level flight, that's the heading indicator -- if the heading starts drifting, you know a wing has dropped.
Context Anchor
Seen in instrument scan discussions, especially when learning which flight instrument to trust most for pitch, bank, and power during straight-and-level flight.
Derivation
"Primary" comes from the Latin primus, meaning "first" -- the first instrument you reference for that piece of information. It does not mean the only instrument, just the leading one for that specific control input.
Why Pilots Care
Correct bank control keeps the aircraft on the assigned heading and prevents unintentional turns that could lead to navigation errors or loss of situational awareness.
Intuition Check
Do not read “primary” as “the only instrument that matters.” It means the main reference for that job. Do not read “bank” as anything financial; here it means the airplane’s sideways tilt.
Example Sentence 1
While holding straight-and-level flight on instruments, the student kept returning to the heading indicator as the primary for bank.
Example Sentence 2
When the heading indicator begins to drift, the pilot applies aileron to level the wings while still referencing the heading indicator as primary for bank.