FAA Aviation Mechanic Powerplant (AMP)Fuel, Induction and LubricationMedium
A mechanic is studying the fuel nozzles installed on a turbine engine combustor and notes that they are duplex (dual-orifice) rather than simplex nozzles. What is the primary reason for using duplex nozzles?
- ATo provide proper fuel atomization across the wide range of flows from idle to full power
- BTo allow fuel to be shut off at only one orifice during engine shutdown
- CTo allow the nozzle to also inject compressor bleed air for cooling
- DTo reduce the total number of fuel manifolds required in the engine
Show answer & explanationAnswer & explanation
Correct answer: A. To provide proper fuel atomization across the wide range of flows from idle to full power
A duplex nozzle has a primary (pilot) orifice for fine atomization at low fuel flows (idle/start) and a secondary orifice that opens at higher flows for full-power operation, ensuring good atomization and combustion efficiency across the entire operating range. A simplex nozzle atomizes well only over a narrow flow range.
Why the other options are wrong
- B. Shutoff is controlled by the fuel control/shutoff valve, not nozzle design.
- C. Nozzles inject fuel, not compressor bleed air, though some designs use air-assisted atomization separately.
- D. Duplex nozzles typically require more, not fewer, fuel manifold lines (primary and secondary).
Duplex Fuel Nozzle
A turbine engine fuel nozzle with two orifices (primary and secondary) designed to maintain proper atomization across the full range of engine power settings.
- Primary orifice handles fine atomization at low flow (idle/start)
- Secondary orifice opens at higher fuel pressures for high power
- Flow divider routes fuel appropriately between primary and secondary manifolds
Memory trick: Two doors for fuel: a small one for a whisper, a big one for a roar.