Under what condition does the L/R FUEL FILTER BYPASS annunciator illuminate?

The L/R FUEL FILTER BYPASS annunciator illuminates when the pressure differential across the fuel filter exceeds 2.3 +/- 0.2 psi. This condition indicates that the fuel filter is becoming clogged or restricted, which creates a significant pressure difference between the inlet and outlet sides of the filter.

When the differential pressure reaches this threshold, it signals to the pilot that the filter may be obstructed and that fuel flow could be compromised, which is critical for maintaining engine performance and safety. Recognizing this alert allows the crew to take appropriate action, such as checking the condition of the fuel filter or preparing for potential fuel flow issues.

The other options do not specifically indicate a direct relation to the filter bypass condition. For example, a drop in pressure in the fuel feed line doesn't necessarily mean the filter is bypassing; it could be caused by other factors. Similarly, while a malfunctioning boost pump can affect fuel flow, it would not trigger the bypass warning directly. Finally, fuel being returned to the outboard wing bay does not pertain to the health of the fuel filter itself and does not impact the bypass condition directly.