Before flight, the airplane is run on the runway

Option C: Before flight, the airplane is run on the runway to reduce the friction force between the ground and the aircraft. This process is crucial for enabling the aircraft to accelerate smoothly and achieve takeoff speed. When the airplane moves, the friction between its wheels and the runway generates a force called rolling friction. By running the airplane on the runway, the initial friction helps to overcome inertia and resistance, gradually reducing as the aircraft gains speed. Reducing this friction ensures that the aircraft can accelerate effectively without excessive resistance, allowing it to reach the required takeoff velocity. Once the aircraft reaches sufficient speed, the lift generated by its wings surpasses the gravitational force, enabling it to become airborne. Options A (to reduce the working air pressure) and B (to increase the working air pressure) are incorrect, as the running of the airplane on the runway primarily affects friction force and has minimal impact on air pressure. Option D (to increase the friction force between the ground by the aircraft) is also incorrect, as increasing friction would hinder the airplane’s movement rather than facilitating takeoff. Therefore, option C accurately describes the purpose of running the airplane on the runway before flight.