In which situation is the use of FLEX thrust for takeoff prohibited?

Using FLEX thrust for takeoff is prohibited on a contaminated runway due to the increased risk of loss of control during the takeoff roll. A contaminated runway is defined as one that is covered with water, snow, or ice, which can significantly decrease the aircraft’s braking effectiveness and traction. When using FLEX thrust, the engine output is not at full power, which can limit the aircraft's thrust available for takeoff. In conditions where the runway is not in optimal condition, such as wet or icy surfaces, maximizing thrust is crucial to achieve safe acceleration and control during takeoff.

In contrast, while wet runways are also a concern, they do not always meet the specific criteria whereby takeoff thrust should be maximized. At night and in turbulence, there are different operational considerations, but they are not prohibitive conditions for using FLEX thrust. Therefore, the requirement to use maximum thrust on contaminated runways is a critical safety measure that underlines why FLEX thrust is disallowed in that scenario.