Troubleshooting & Diagnostics Flashcards

An aircraft with a constant-speed propeller experiences RPM fluctuation at cruise. After verifying the engine is running smoothly, the technician should suspect:

Answer: Propeller governor hunting or a faulty governor oil seal

RPM fluctuation with a constant-speed prop and smooth engine operation points to the governor failing to maintain stable pitch control (hunting) or leaking internally.

A fuel quantity indicator reads full even though the tanks are known to be partially full. With the aircraft de-powered, resistance measured at the fuel sender is 30 ohms where 90 ohms is expected for the current fuel level. The fault is in the:

Answer: Fuel sender (float/rheostat) unit in the tank

An incorrect resistance reading at the sender means the sender itself is faulty; the indicator and wiring are bypassed by measuring directly at the sender.

A reciprocating engine produces a consistent metallic knock that increases with engine load but decreases at higher RPM. This symptom most closely resembles:

Answer: Connecting rod bearing (big-end) knock

Big-end bearing knock typically increases with load (higher cylinder pressure forces) and can decrease at high RPM as oil centrifugal force improves bearing film.

A pressurization system controller is unable to maintain cabin altitude below 8,000 feet, yet outflow valve travel is verified as correct. The MOST likely remaining cause is:

Answer: A cabin pressure leak exceeding bleed air supply capacity

If the outflow valve is working correctly but pressurization cannot be maintained, the pressurization demand exceeds supply — indicating a structural or seal leak.

An ELT performs a self-test successfully but does not transmit when subjected to a simulated crash G-force. The MOST likely fault is:

Answer: Faulty G-switch (inertial switch) that is not activating under impact

The self-test bypasses the G-switch; a unit that passes self-test but fails G-activation points directly to a faulty inertial switch.

When performing a fuel system operational check, fuel is found leaking from the sump drain after the drain valve is closed. The MOST likely cause is:

Answer: Contamination or damage to the drain valve seat or O-ring

A sump drain that continues to leak after closing indicates the valve seat or sealing O-ring is damaged or contaminated, preventing a proper seal.

A transponder passes bench tests perfectly but ATC reports intermittent target loss in flight. The MOST productive next troubleshooting step is to check:

Answer: Antenna coaxial cable integrity and connector security through the full range of aircraft motion

Intermittent failure in flight that is absent on the bench often results from a coaxial cable or connector that flexes and loses continuity during airframe movement.