Definition
The instrument that gives the most direct and immediate indication of the airplane's bank in a given flight condition. In straight-and-level flight and in steady level turns, the heading indicator is the primary bank instrument because it shows whether the airplane is holding a constant heading (wings level) or turning at a constant rate.
Plain English
The one instrument you watch most closely to know if the wings are doing what you want them to do — staying level or turning at the rate you've chosen.
Context Anchor
Seen in instrument flying when learning the primary-and-supporting method for scanning analog cockpit instruments.
Derivation
Primary' comes from the Latin primus, meaning 'first.' In this context it does not mean 'most important instrument overall' — it means the first instrument to look at for direct information about bank in this specific flight condition.
Why Pilots Care
Knowing which instrument is primary for bank tells the pilot exactly where to look for the most accurate, direct answer about whether the wings are level or turning correctly. Without that discipline, the scan becomes scattered and small heading or bank errors grow.
Intuition Check
“Primary” does not mean the same instrument is always most important. It means this is the main reference for bank for the maneuver or condition being flown. “Bank” here does not mean money; it means the airplane’s wings are tilted left or right.
Example Sentence 1
In straight-and-level flight, the heading indicator is the primary bank instrument because a constant heading confirms the wings are level.
Example Sentence 2
During the standard-rate turn, the student adjusted the yoke while watching the primary bank instrument to hold the correct angle.