What does assumed/flexible takeoff refer to?

Assumed or flexible takeoff refers to the practice of adjusting the fuel-air mixture to optimize performance based on varying conditions, such as changes in air density experienced at different altitudes and temperatures. This adjustment is crucial because the efficiency of combustion within the engine is affected by the density of the air; thus, a proper fuel-air ratio must be maintained for effective engine performance and optimal thrust during takeoff. By adapting to these conditions, pilots can ensure the aircraft achieves the necessary performance metrics required for a safe and efficient takeoff.

In contrast, using a constant fuel flow does not take into account the changing environmental conditions, which could lead to inefficient engine performance. Similarly, employing a specific takeoff speed without regard for air conditions would not provide the best performance, as it could compromise safety and efficiency. Maintaining a fixed thrust regardless of altitude would also ignore the fact that different altitudes have varying air densities, which influence engine performance and aircraft lift abilities.