What is a key factor affecting aircraft speed during flight?

Engine power output is a crucial factor affecting aircraft speed during flight because it directly influences the thrust produced by the aircraft's engines. Higher engine power typically results in increased thrust, allowing the aircraft to accelerate and maintain higher speeds during cruise flight.

When pilots want to achieve a specific speed, they adjust the throttle to manage the engine output. For example, during takeoff and climb, maximum power is often utilized to gain altitude efficiently. Conversely, cruising speed can be adjusted by altering the engine power output, aiming for a balance between speed, fuel efficiency, and aircraft performance.

The other factors mentioned can have impacts as well but do not have as direct an effect on speed as engine power output. For instance, while altitude affects air density and subsequently the aircraft’s performance, the actual speed is primarily determined by how much thrust is being produced. Similarly, crew member weight and fuel quality can influence overall aircraft performance but are not primary drivers of flight speed. Therefore, engine power output stands out as the key factor in this context.