What is the first action when a bad fuel sample is confirmed?

When a bad fuel sample is confirmed, retaining the bad sample is the first critical action. This is essential for a few reasons. Firstly, keeping the bad sample allows for further analysis to determine the exact contamination or issue with the fuel, providing valuable data to troubleshoot the problem. This can be important for understanding whether the fuel contamination is a systemic issue that could affect other aircraft or if it's localized to a specific incident.

Additionally, retaining the sample can serve as evidence for accountability and traceability within the process of fuel accountability. It ensures that there is documentation of the problem, which might be necessary for reporting purposes or for conducting further investigations into the source of contamination. This action helps ensure the safety and integrity of the aircraft's operation by preventing compromised fuel from being reused or overlooked.

The other choices do not prioritize the immediate need for evidence regarding the quality of the fuel. Notifying the flight crew or draining additional fuel may follow, but the first step is to secure the bad sample for further examination and investigation.