JT8D Oil System
Revised 02-Mar-2001
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The engine oil system can be divided into three subsystems.
Pressure subsystem
Scavenge subsystem
Breather subsystem
Firstly I'll cover the components installed.

External components:

  • Oil Tank, attached to the left font of the accessory gearbox.
  • Oil Pump, at the bottom of the accessory gearbox
  • Oil Filter, lower left side of accessory gearbox, three micron sizes available
  • Oil Pressure Regulating Valve, rear of accessory gearbox
  • Fuel Oil Cooler, located between flanges B and D at the 7:00 position
  • Magnetic Chip Dectors

Internal Coponents:

  • Deaerator, in the oil tank
  • Deoiler, in the accessory gearbox
  • Oil Filter Bypass Valve, in the oil filter
  • Four Scavange Pumps, in the gearbox and at bearing compartments 1, 4, 5 and 6
  • Strainers and nozzels, in the oil flowpath of the engine

Oil Specifications:

  • PWA 521 gives the type of syntheic oil that can be used
  • SB 238 lists the approved oils by brand name.
  • *Note* Some of this information is available from this web site

Oil Consumption Guidelines:

  • If the No 4 bearing seal is labyrinth, .2 to 2 quarts an hour
  • If the No4 Bearing seal is carbon .1 to 1 quats per hour

The following components are LRU's (Line Replacable Units)

  • Oil Tank
  • Oil Pump
  • Oil Filter
  • Oil Pressure Regulating Valve
  • Fuel / Oil Cooler and Bypass Valve
  • Scavange Pump in the gearbox
  • Scavange Pump for No1 Bearing
  • Scavange Pump for No6 Bearing
  • Magnetic Chip Dectors

Oil System Limits

  • Normal Oil Pressure 40 to 55 psi
  • Temperature
    • 120'C , -1, -7, -9 (Continuous)
    • 157'C ,-1, -7, -9 (Transient)
    • 130'C , -11, -15, -17 (Continuous)
    • 165'C , -11, -15, -17 (Transient)
  • Breather
    • 8.0 in/Hg Max, -1, -7, -9
    • 8.5 in/Hg Max, -11, -15, -17
    • 7.13 in/Hg Max, on all models incorperating No4 carbon seal

Pressure Oil Subsystem

The oil pressure subsystem supplies pressurised clean cooled oil to the engine bearings and accessory drives. The flow sequence is as follows.
Oil is gravity-fed from the oil tank Tao the oil pump. It enters the pump through the pressure stage inlet. The oil pump increases the oil pressure and sends it to the oil filter. If the oil filter is clogged, or if the oil is very cold, the oil filter bypass valve opens. This allows the oil to flow to the pressure regulating valve and to the fuel / oil cooler. This regulating valve can be closed in which case it does not bypass any oil, so all of the filtered oil from the pump goes to the fuel / oil cooler. If it's partially or fully open it bypasses some of the oil and sends it back to the oil pump inlet. The pressure of the non bypassed oil is reduced and passes to the Fuel / Oil cooler. In the fuel oil cooler the oil flows around the heat exchanger tubing to transfer some of it's heat to the fuel. Thus the oil is cooled before it flows to the bearing compartments.
A fuel / oil cooler bypass valve permits the oil to bypass the cooler if it becomes clogged or the oil is cold. Pressurised oil then flows to the bearing compartments, passing through last chance strainers and then to the nozzles. Each bearing receives cleaned, cooled oil for lubrication.

Scavenge Subsystem

The scavenge system collects oil from the bearing compartments and accessory gearbox and returns it to the oil tank. The flow sequence is as follows:
A gear type pump scavenge pump sends the oil from the No1 bearing compartment to the accessory gearbox. Oil from the No2 and No3 bearing compartments drains by gravity through the tower shaft housing to the accessory gearbox. A gear type scavenge pump sends oil from the No6 bearing compartment through an internal tube to the sump for the No4, 4 1/2, 5 bearing compartments. Scavenge oil from the compartments is sent by a dual element gear type scavenge pump to the accessory gearbox. It then flows to the oil tank through an internal passage. There is a scavenge pump in the accessory gearbox sump that sends oil to the oil tank. When the scavenge oil enters the oil tank it goes through a tube to a deaerator that removes the air from the oil. The JT8D oil system is referred to as a "hot tank" system as hot scavenge oil is returned to the tank. Magnetic chip detectors can be installed in the scavenge lines and gearbox.

Breather Subsystem

The breather subsystem prevents excessive air pressure in the bearing compartment so that the flow of oil to the bearings and the operation of the scavenge system is not impaired. It also affects the operation of the oil pressure regulating valve which controls oil pressure.
A controlled amount of air from the primary gaspath leaks past the bearing compartment airseals into the bearing compartments. This air prevents the leakage of air into the gaspath, pressurises the bearing compartments and mixes with the very small oil particles in these bearing compartments.
The air / oil mist is referred to as breathe air. The word "Breather" is used because the air goes into and out of the bearing compartments, which is similar to breathing. The breather air flows from the bearing compartments to the accessory gearbox as follows.
Breather air from the No1 bearing compartment goes through an external breather tube to the accessory gearbox. Breather air from the No6 bearing compartment goes through an internal tube to the No4, 4 1/2, 5 bearing area. Breather air from these then goes through an external breather tube to the accessory gearbox. Breather air from the No2 and No3 goes through the tower shaft to the accessory gearbox. Similar to the scavenge system as described above.
The accessory gearbox also gets air through an internal passage from the oil tank. (Air from the Deaerator via the scavenge oil). In the accessory gearbox this air mixes with the air from the bearing compartments. The air / oil mist in the gearbox passes through a deoiler that separates the air from the oil. Then the oil with the air removed drains to the gearbox sump and returned via a pump to the oil tank. The air with the oil removed is sent through the gearbox breather port. It then flows ducts and vented overboard.


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