Why should you wait after fueling an aircraft before checking the fuel sumps?

Waiting after fueling an aircraft before checking the fuel sumps is important because it allows time for water and contaminants that may have been disturbed during refueling to settle. When an aircraft is fueled, especially with multiple types of fuel or from different sources, there is a chance that water—or other contaminants—may be present. These heavier substances usually settle at the bottom of the fuel tanks.

By allowing some time for these particles and water to settle to the drain point, the inspection can yield a more accurate and informative result about the condition of the fuel. This process aids in ensuring that the fuel that will be used for flight is clean and free of impurities that can affect engine performance and safety.

The other considerations, while related to the fuel system, do not directly focus on the critical nature of contaminant settlement after refueling. Accurate quantity readings could be influenced by other factors besides waiting, evaporation of contaminants could lead to misleading results, and preventive measures to avoid spills while essential, do not correlate specifically with the timing of checking sumps post-fueling.