keep it on the barber pole
From the artificial horizon used in aircraft, which is similar to a barber's pole
It is a variation of the colloquial “straighten up and fly right” expression with a similar meaning. Its origin is that in many airplanes the artificial horizon has a solid pattern above the horizon indicator and strips (similar to a barber pole’s) below this line. When the plane indicator is sitting “on the barber pole” the airplane is flying level.