1. An aviation user interface unit for provision of a user interface for control of a controllable quantity, the user interface unit comprising: a manually actuable selector associated with a controllable quantity; a plurality of value indicators arranged in association with the selector, the value indicators being operable in one of first, and second, visually distinguishable display states and a further visually distinguishable non-display state; a rotary adjuster capable of being manually rotated; and a control unit, the control unit being operable to store a current value for the controllable quantity, to map the stored current value to a corresponding one of a plurality of discrete values in the adjustment range, each discrete value corresponding to a respective value indicator, to assign a display state to a current value indicator of the value indicators, the current value indicator corresponding to the mapped discrete value, and to assign the non-display state to the remaining value indicators; the control unit being further operable to store a control condition of the controllable quantity, the control condition being indicative as to whether the controllable quantity is active or inactive, the control unit being operable to assign the first display state to the current value indicator if the control condition is indicative of inactivity of the controllable quantity; the control unit being responsive to actuation of the selector, if the current value indicator is in the first display state, to assign the second display state to the current value indicator and to assign the active control condition to the controllable quantity; the control unit being responsive to rotation of the rotary adjuster, if the current value indicator is in the second display state, to vary the current value stored by the control unit; the control unit being further operable, when the controllable quantity is in the active control condition, to present an output comprising the current value of the controllable quantity.