FAA Aviation Mechanic Powerplant (AMP)Engine InspectionMedium
A turbine engine's fuel filter is equipped with a differential pressure indicator that pops out when the pressure drop across the filter exceeds 5 psi. During an inspection, the technician measures 42 psi at the filter inlet and 35 psi at the filter outlet, and the indicator has popped. What is the correct interpretation and action?
- AThe differential pressure of 7 psi is within normal limits, so no action is required
- BThe differential pressure of 7 psi exceeds the 5 psi limit, indicating filter contamination; the filter must be inspected and replaced
- CThe 7 psi reading indicates a fuel leak downstream of the filter, not filter contamination
- DThe indicator popped due to a faulty bypass valve, and only the valve needs replacement
Show answer & explanationAnswer & explanation
Correct answer: B. The differential pressure of 7 psi exceeds the 5 psi limit, indicating filter contamination; the filter must be inspected and replaced
The differential pressure is 42 psi − 35 psi = 7 psi, which exceeds the 5 psi threshold that triggers the pop-out indicator; this confirms the filter element is becoming restricted with contaminants and must be removed, inspected, and typically replaced along with checking the bypass valve for proper operation.
Why the other options are wrong
- A. 7 psi does exceed the stated limit, so this is incorrect.
- C. A pressure drop across the filter, combined with the popped indicator, points to filter contamination, not a downstream leak.
- D. The indicator is designed to signal a pressure drop, not a valve fault; the primary cause is a restricted filter.
Fuel Filter Differential Pressure Indicator
A mechanical pop-out (or electrical) indicator signals when the pressure drop across a fuel filter exceeds a set limit, indicating the filter element is becoming clogged.
- Differential pressure = inlet pressure minus outlet pressure
- A popped indicator means the filter is restricting fuel flow
- A bypass valve opens if the filter becomes fully blocked to ensure continued fuel flow
Memory trick: Subtract the pressures — if it's more than 5, the filter's clogged, don't drive!