JT8D Fuel System
Revised 30-Jul-2001
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The fuel system supplies clean, metered fuel to the combustion chambers during all conditions of engine operation.
Boost pumps send fuel from the aircraft tanks to the fuel pump inlet. The inlet fuel pressure is typically in the range of 15 - 25 psig. It must be 5 psi higher than the pressure of the fuel but no more than 50 psi. The fuel flows thorough the (impeller) boost stage which increases its pressure by approximately 10 to 60 psi from start to takeoff power.
If there is a boost stage failure the delta P across the impeller bypass valve increases. The bypass valve opens permitting fuel to pass directly to the pumps gear stage (if this occurs fuel does not pass through the heater and main fuel filter). After passing through the pump it goes to the fuel heater. The fuel is heated as it passes the heater providing the fuel heat is on. If the pressure drop across the fuel heater is large the fuel can bypass the heater. From the heater the fuel flows to a fuel filter in the fuel pump. If this filter becomes clogged by dirt or ice particles the delta P across the filter increases. If delta P increases the differential pressure switch sends a signal which turns on the icing light in the flight deck. If delta P increases further the filter bypass opens. This permits the fuel from the heater to go directly to the gear stage. (Fuel that goes from the boost stage through the fuel heater and filter is called interstage fuel).
Fuel goes from the filter to the pump gear stage which increases it's pressure. Approx. 150 psi when the engine is motored, approx. 900 psi at takeoff power (1,000 psi for the -17/R/AR engines). If the output pressure of the gear stage increases to more than 950 psi (1,050 psi -17/R/AR) the high pressure relief valve opens. Returning some fuel back to the gear stage inlet.
From the fuel pump gear stage fuel passes to the fuel control unit. The FCU has inputs to it from some of the engine parameters and the power level requested by the flight crew. It schedules the correct fuel flow rate and pressure that is necessary for combustion. The metered fuel then passes to the fuel flow transmitter, unneeded fuel is sent back to the gear stage. (Fuel that goes from the FCU thorough the FF Transmitter, fuel / oil cooler, P&D valve to the nozzles is called metered fuel). The metered fuel goes through the fuel flow transmitter which passes flow rate information to the flight deck. After the transmitter fuel passes through the fuel / oil cooler which reduces the oil temperature. Fuel now passes to the P & D valve (Pressurising & Dump). When it enters the P & D valve it goes through a filter. This filter has a bypass facility. The P&D valve divides fuel into primary and secondary fuel flows and send it to the fuel manifolds. Above idle (higher power settings) it also sends fuel to the secondary manifolds. Fuel from the primary and secondary manifolds passes through the nozzle supports and into nine fuel nozzles Here it is atomised and sent to the combustion chambers.



  • Fuel / Oil Cooler
    • Located between flanges B and D at the 8:00 position, attached to flange B Eng LH side


  • Fuel Flow Transmitter
    • Located under the fuel / oil cooler, attached to the fuel / oil cooler. Eng LH side


  • Fuel Heater
    • Located between flanges C and E at the 6:00 position, attached to the left side of the fuel pump. Note that the tubing with 13th stage air for the fuel heater passes on the right side of the engine. Eng underside LH side"ish".


  • Tt2 Probe
    • Located at the 7:00 position in the inlet case, connected to the FCU which is on the RH side. Eng Inlet.

  • Pressurising and Dump Valve (P&D)
    • Located between flanges H and J at the 7:30 position, attached to the fan discharge diffuser outer duct. Eng LH side.


  • Fuel Pump
    • Located between flanges C and F at the 5:30 position, attached to the front of the accessory gearbox. Eng RH side


  • Fuel Control Unit (FCU)
    • Located between flanges B and D at the 5:30 position, attached to the front of the fuel pump. Eng RH side.


  • Moisture Trap (Condensation Trap)
    • Located on the front of the FCU


  • Fuel De-icing Air Shutoff Valve and Actuator
    • Located between flanges D and F at the 1:30 position, attached flange E. Eng RH side.


  • External Tubing
    • Two tubes with 13th stage air come out of the fan discharge diffuser outer duct at the 1:00 and 11:00 positions. They connect to one tube at the top of the engine, and goes to the fuel heater. A tube with Ps4 air comes out of the fan discharge combustion outer duct at the 12:30 position. It goes to the FCU (Routing varies on JT8 models)


  • Fuel Manifolds
    • Two short fuel manifold tubes come out of the P&D valve and are divided into two pairs of tubes. One pair of tubes goes into the diffuser outer fan duct at the 5:00 position. The other pair of tubes goes into the diffuser outer fan duct at the 7:00 position. The two pairs of fuel manifold tubes go from the diffuser outer fan duct to the diffuser case. They are connected to the fuel manifolds which are installed on the diffuser case.


  • Fuel Nozzles and Supports
    • Nine fuel nozzles and support assemblies, located internally on both sides of the engine.


The following components are all LRU's. Fuel Pump, Fuel Control Unit, Fuel De-icing Air Shutoff Valve and Actuator, External Fuel Manifolds.

Typical 100 series system shown below

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