Where are Modes 1 and 3/A selected on the AN/APX-72 transponder set?

Modes 1 and 3/A on the AN/APX-72 transponder set are selected using thumbwheel switches located on the front of the control box. This design allows for quick and precise adjustments by the operator, which is crucial during the dynamic operations typically encountered in aviation environments. The thumbwheel switches provide tactile feedback and are easy to manipulate, allowing the crew to effectively select the necessary transponder modes without diverting significant attention from their primary flying duties.

Other interfaces, like a touchscreen or onboard computer systems, might be present in other advanced avionics but are not suited for the immediate mode selection demands found in transponder operations. The simplicity and reliability of thumbwheel switches make them the preferred method for this specific function.