How does the cleanliness of an aircraft affect its performance?

The cleanliness of an aircraft directly impacts its performance primarily due to the aerodynamics involved. When an aircraft is dirty, it accumulates contaminants such as dirt, oil, and grease on its surfaces, which can disrupt the smooth airflow over the wings and fuselage. This disruption increases drag and can lead to a decrease in overall lift and efficiency. Consequently, a dirty aircraft may underperform compared to performance charts that are based on clean aircraft data.

In contrast, the other options do not hold true in this context. A clean aircraft is designed to minimize drag and optimize fuel efficiency, thus should perform better, not worse. While engine condition is certainly important, it is only one aspect of overall aircraft performance; aerodynamic factors also play a crucial role. Lastly, while weather conditions are vital for flight safety and planning, they do not negate the fact that an aircraft's cleanliness can significantly influence its efficiency and performance.