When is a fuel sample considered invalid?

A fuel sample is considered invalid if it is taken more than 24 hours after the aircraft launch because the fuel’s properties can change over time, and environmental factors may cause contamination or degradation of fuel quality. Fuel stability and cleanliness are crucial for safe aircraft operation, and taking a sample outside of the appropriate time frame can lead to inaccurate assessments of the fuel’s condition. The 24-hour timeframe ensures that the sample reflects the fuel's condition as closely as possible to when it was onboard the aircraft, allowing for proper evaluation and ensuring safety standards are met.

Options that suggest a sample becomes invalid based on presence of a smell or color change, while they may indicate potential issues, do not define a strict time-bound criterion that governs the validity of a fuel sample. Similarly, the 12-hour mark after flight does not provide the necessary allowance for sample integrity compared to the 24-hour guideline. Hence, the most critical factor that determines the validity of a fuel sample is adherence to the maximum time window after aircraft launch.