When does the L/R Fuel PESS LO annunciator illuminate?

The L/R Fuel PESS LO annunciator illuminates specifically when the pressure in the fuel feed line drops below 5.0 +/- 0.5 psi. This low-pressure indication is crucial for ensuring that the fuel system is functioning properly and can maintain adequate fuel delivery to the engine. A drop in pressure could indicate that the fuel feed system is compromised, such as a blockage, malfunction, or insufficient fuel being supplied from the tanks.

Understanding fuel pressure dynamics is essential for flight safety, as low pressure might lead to engine fuel starvation or failures, which could compromise the overall operation of the aircraft. Monitoring systems that involve pressure readings are integral to maintaining safety and performance protocols during flight operations.

The other options do not align with the specific function of the L/R Fuel PESS LO annunciator, as they refer to different aspects of fuel management and system performance. For example, having a full fuel feed chamber or a significant fuel leak would not trigger this specific warning, nor would simply activating the boost pump, which is a separate operation in the fuel system.