At what temperature, combined with altitude and power settings, would cause a 6 hours flight time reduction?

The correct choice is -17 °C because lower temperatures generally contribute to denser air, which has a significant impact on aircraft performance, particularly at higher altitudes. When the air is denser, aircraft engines can produce more power, and wings generate more lift. This translates to improved fuel efficiency and potentially greater speed over a longer distance.

In the context of flight time reduction, a temperature of -17 °C suggests that the aircraft can maintain a higher groundspeed and requires less power to achieve the required lift, resulting in an overall reduction in flight duration. The combination of altitude and power settings with such low temperatures creates optimal conditions for performance improvement.

Contrastingly, the other temperatures listed provide less beneficial conditions for performance. At 0 °C, +5 °C, and +15 °C, the air density decreases, leading to an increase in required engine power and a reduction in overall efficiency. These higher temperatures would result in longer flight times due to increased drag and reduced power availability compared to the performance at lower temperatures like -17 °C. Thus, the most significant reduction in flight time is achieved at the lowest temperature, confirming the choice of -17 °C.