Lubrication, Fuel and Ignition Systems
Domain focus: This section tests the oil, fuel and ignition systems that keep the engine running: how oil is circulated and monitored, how fuel is metered and delivered, and how ignition is used for starting and for relight.
Lubrication
- A dry-sump system uses a pressure pump to feed oil to bearings and gears, and scavenge pumps to return it to the tank. Filters, a cooler, chip detectors and magnetic plugs protect the system; oil pressure and temperature are limiting parameters.
- Oil seals keep oil out of the air path. Synthetic oils are used for high temperatures. Low oil pressure, high oil temperature, metal debris or rising consumption require investigation.
Fuel system
- Fuel passes through a low-pressure pump, a high-pressure pump, a fuel/oil heat exchanger, a fuel control unit or FADEC-controlled metering valve, a flow divider and nozzles. The schedule is a function of throttle position, compressor speed, altitude and temperature.
- Fuel nozzles atomise fuel; blockage causes uneven combustion, EGT spread and possible hot streaks. Fuel contamination and water are major hazards.
Ignition
- An exciter supplies high voltage to igniter plugs. Ignition is used for starting, and continuous ignition may be selected for adverse weather, low power or relight. Too lean or too rich a mixture can cause blowout or a hot start.
Worked example
During a start, the fuel control is rigged so acceleration fuel flow is 5% above schedule. Light-off EGT peaks 60 °C above normal. The reasoning: extra fuel raises combustion temperature before airflow has accelerated; the schedule must be checked against rigging data, and the start aborted if limits are exceeded. A rich schedule can also cause carbon build-up and turbine damage.
Traps
Do not confuse oil pressure with fuel pressure; do not leave continuous ignition on unnecessarily; do not assume a blocked nozzle affects all cylinders equally; do not ignore fuel contamination; do not adjust the FCU without approved data.