Under what condition should Flex Thrust not be used?

Flex Thrust should not be used on a contaminated runway because using reduced thrust settings in such conditions can compromise safety. Contaminated runways, which might be wet, icy, or covered in snow, reduce the friction available for the aircraft during takeoff and landing. These conditions require maximum thrust to ensure that the aircraft can achieve sufficient speed for takeoff and maintain control during the initial climb. Utilizing Flex Thrust in these scenarios could result in longer takeoff distances, increased risk of hydroplaning, or inadequate performance, ultimately jeopardizing the safety of the flight.

In contrast, options that suggest clear weather conditions, high altitude airports, or small aircraft do not inherently present the same level of risk associated with thrust settings. Clear weather typically allows for safe operations with various thrust settings. High altitude airports may require considerations for engine performance but do not specifically negate the use of reduced thrust. Small aircraft may have different operational standards but are not directly related to the concerns raised by contaminated runways.