When should the MCDU be set to ENG in the TO DATA SET MENU?

The correct choice indicates that the MCDU should be set to ENG when the outside air temperature (OAT) is between 5°C and 10°C and there is visible moisture present. This is crucial because condensation at these temperatures can increase the risk of ice formation on the aircraft's components, which might affect performance during takeoff.

When the temperature is between 5°C and 10°C, the risk of ice is particularly significant if there is moisture in the environment, such as rain or fog. Setting the MCDU to ENG in this scenario helps ensure that the aircraft’s anti-ice systems are activated, thus preventing ice accumulation on critical surfaces like the wings and engines, ensuring safety during takeoff.

The other options do not adequately address the specific conditions that necessitate setting the MCDU to ENG. For instance, while an OAT below 5°C does indicate the potential for icing, it is not sufficient to account for other moisture conditions. Always setting the MCDU to ENG when on the ground ignores the need to evaluate moisture presence, and setting it to ENG only when clear of clouds could lead to significant risks if moisture is still present at low temperatures. Therefore, the conditions described in the correct choice effectively cover the