What must be ensured regarding the backup UHF radio before engine start?

Ensuring that the backup UHF radio is off before engine start is crucial for several reasons related to safety and operational efficiency. When the engine is started, various systems on the aircraft can produce electrical noise and interference that might affect radio operations. Keeping the backup radio turned off eliminates the potential for any interference that could disrupt communications during engine start-up.

Moreover, having the radio off conserves battery power, ensuring that it remains operational for use when needed. It also helps in avoiding any unintended transmissions as the crew prepares for the taxi and takeoff phases.

In the context of overall pre-flight checks, turning off unnecessary equipment helps streamline the workflow and allows the crew to focus on the critical tasks at hand, making sure that all systems are poised for operational readiness once engine start is complete.