Definition
The longitudinal axis of an airplane — an imaginary line running from the nose to the tail — about which the airplane rolls left or right. Movement around this axis is called bank or roll, and it is controlled primarily by the ailerons.
Plain English
The line running from the nose to the tail of the airplane. When the airplane tips one wing down and the other wing up, it is rotating around this line.
Context Anchor
Seen on the electronic attitude indicator when using the display to control pitch and bank.
Derivation
‘Bank’ comes from an old word for a sloping side or edge — the same root as a riverbank. When an airplane banks, one wing slopes down and the other slopes up, just like the sloping sides of a hill. ‘Axis’ comes from the Latin for an axle or pivot line — the line something turns around.
Why Pilots Care
Bank is one of the three primary attitudes a pilot controls. On instruments, recognising rotation around the bank axis — separate from pitch or yaw — is essential for keeping the wings level and for making coordinated turns.
Analogy
Picture a small airplane drawn over a horizon line. If the little airplane’s wings tilt left or right, that wing line is showing the bank axis.
Intuition Check
Do not read bank here as a financial bank or a raised edge of land. In this context, bank means the airplane’s sideways tilt, and bank axis means the display reference for that tilt.
Example Sentence 1
The ailerons control movement around the bank axis, rolling the airplane left or right.
Example Sentence 2
The attitude indicator shows movement about the bank axis as the wings tilt during a turn.