Definition
The tendency of an aircraft in a shallow bank (typically less than about 20 degrees) to roll back toward wings-level on its own, requiring the pilot to hold a small amount of aileron pressure into the turn to maintain the bank.
Plain English
In a gentle turn, the airplane wants to level itself out. You have to hold the controls slightly into the turn to keep it banked.
Context Anchor
Seen in instrument flying when setting or correcting the bank for a standard-rate turn.
Derivation
Built from 'under-' (not enough) and 'bank' (the tilt of the wings in a turn). The name describes the airplane's tendency: if left alone, it would end up under-banked — meaning less banked than the pilot wants.
Why Pilots Care
It produces a slower than intended turn rate, which can cause timing errors or course deviations during instrument procedures.
Intuition Check
Underbanking does not mean the airplane is too low or turning too gently for comfort. It means the bank angle is too shallow for the turn rate you need.
Example Sentence 1
In a shallow standard rate turn, the instructor reminded the student to apply a little aileron into the turn to counter underbanking.
Example Sentence 2
Underbanking during the holding pattern entry caused the aircraft to overshoot the inbound leg.