What action should you take when the left MANIFOLD FAIL light comes on at FL 370?

When the left MANIFOLD FAIL light illuminates at FL 370, the appropriate course of action is to accomplish the Manifold Failure checklist. This step is critical because the illumination of the MANIFOLD FAIL light indicates a failure in the fuel manifold system, which can lead to imbalanced fuel distribution among the engines or potential engine reliability issues.

The checklist provides specific procedures and checks that need to be performed to assess the situation, possibly leading to corrective actions to safeguard the aircraft's operational integrity. It is essential to follow the established flight procedures to ensure safety and control of the aircraft, as simply acknowledging the warning without taking further action may not adequately address the potential risk.

This approach aligns with the standard operating procedures for managing system failures in the C-17, focusing on a systematic review and corrective measures to mitigate any adverse effects on aircraft performance and safety.