JT8D Turbine Engine Lubrication System

Lubrication System Overview

• Application: Pratt & Whitney JT-8D turbofan (typical of large turbine engines).
• System type: Dry-sump (oil stored in a separate tank, not in the gearbox).
• Oil type: Synthetic turbine oil
  • NOT interchangeable with reciprocating-engine mineral oil.
  • Advantages: produces less lacquer / coke deposits.

Regulatory / Design Requirements

• Tank is external (because dry-sump)
  • Must contain an ullage (air space) of 10\% of total capacity (≈ ½ gal minimum) to absorb thermal expansion.
• Marking: FAA FAR 33.71(c)(5) — the word “OIL” must be stamped/painted on the filler cap.
• Tank proof-pressure test: operational pressure + 5\,\text{psi} (also referenced in FAR 33.71(c)(5) in class notes).

Oil Flow Path (Supply Side)

1. Oil tank →
2. Engine-driven oil pump (gear-type)
   • Positive-displacement gears create both flow & pressure.
3. Main oil filter (Kundert / “Kundo” disc type)
   • Flow path: outside→bottom→inside→up.
   • Disc spacing determines particle size removed.
   • Differential-pressure (ΔP) pop-out indicators warn of clogging.
4. Oil pressure regulating valve (OPRV)
   • Uses breather air pressure tapped from the N₁ (low-pressure compressor) section to sense system pressure.
   • If pressure exceeds set point, valve routes oil back to inlet side of the pump (orange return line).
5. Fuel–oil cooler (FOC)
   • If OPRV is already open but pressure still high, excess hot oil is diverted through the cooler.
   • Fuel flows inside the tubes; oil flows outside around baffles.
     • ≈ 200 thin, straw-like baffles ↑ surface area; heat exchange occurs on tube walls.
   • Results: oil is cooled, fuel is warmed (prevents icing, improves atomization).
   • Cooler bypass valve opens for highly viscous / congealed oil, routing oil back to accessory section so gears can shear/thin it.
6. Last-chance filters
   • Five (5) small “S”-shaped screens located immediately upstream of the bearing spray nozzles.
   • Only serviced at overhaul.
7. Spray nozzles direct pressurized oil onto bearing balls & races.

Bearings, Air System, & Pressure Balance

• Bearing compartments receive both oil spray and breather air.
  • Air functions: cooling, cushioning, and pressure balancing.
• Loads on bearings: radial + axial → causes micro-oscillation (very small amplitude, very high frequency due to RPM).
• To avoid over-pressurising the compartment, each cavity is vented through the breather / vent system.
  • Portion of vent air is routed to accessory gearbox; OPRV uses this air reference.
• Oil / air separation inside cavity: metal seals & gravity segregate liquids from gases.

Scavenge System

• Gravity + seal geometry push used oil downward into scavenger pumps.
  • Scavenger pumps are larger-capacity, electrically driven (per class notes) to guarantee removal of air-entrained, foamy oil plus contaminants.
• Typical contaminants carried: metal shavings, rubber particles, moisture, dirt, acids → elevate back-pressure.
• Downstream of each scavenge pump: Magnetic chip detector (“C” symbol)
  • Detects ferrous debris, completes an electrical circuit when bridged, triggers cockpit advisory via EICAS/CRT.

Indication & Cockpit Interface

• Sensors: oil pressure (Bourdon tube or Wheatstone bridge), oil temperature (Weston bridge), ΔP indicators, chip det.
• Avionics path:
  • Sensors → Cathode Ray Tube (CRT) converts analog inputs to alphanumeric symbols.
  • Symbol Generator (SG) produces the visual symbols shown on display.
  • Data/Digital Generator (DG) allows pilot selection & formatting.

Return to Tank, De-aeration & Venting

1. Scavenged oil collects at bottom of accessory gearbox.
2. Passes through scavenge filter (return-side last-chance screen).
3. Enters tank through an aerator / oil chamber that strips entrained air from oil.
4. Separated air flows to a de-oiler (air-oil separator)
   • Removes any residual oil mist; recovered oil drains back to tank.
   • Dry air vents overboard or is routed to OPRV reference port.
5. Cycle repeats.

Special Conditions: Burned / Over-heated Oil

• Areas near bearing #4, #5, and turbine cavities may over-heat oil (“burned oil”).
• Scavenge system routes this high-temp oil to mix with warmer, less-degraded oil upstream, reducing viscosity mismatch before cooler/tank.

Key Numbers & Facts for Exam

• Dry-sump → external tank with 10\% ullage (≈ ½ gal min).
• Proof-pressure test: operating pressure +5\,\text{psi}.
• Five last-chance filters.
• ≈ 200 baffles inside F/O cooler.
• Gear-type pressure pump; larger-capacity scavenger pumps.
• FAR reference for filler‐cap marking & pressure test: \text{FAR}\;33.71(c)(5).

Concepts to Emphasize

• Purpose of synthetic oil: high-temp stability, less coke.
• Sequence of components from tank → bearings → tank.
• Dual role of breather air (bearing ventilation & pressure reference).
• How OPRV, F/O cooler bypass, and pump recirculation protect against high pressure or congealed oil.
• Chip detector logic & cockpit display chain (sensor → CRT → SG/DG).
• De-aeration path and reason for ullage space.