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JT8D Turbine Engine Lubrication System
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)
Oil tank
→
Engine-driven oil pump
(gear-type)
Positive-displacement gears create both flow & pressure.
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.
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).
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.
Last-chance filters
Five (5) small “S”-shaped screens located immediately upstream of the bearing spray nozzles.
Only serviced at overhaul.
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
Scavenged oil collects at bottom of accessory gearbox.
Passes through
scavenge filter
(return-side last-chance screen).
Enters tank through an
aerator / oil chamber
that strips entrained air from oil.
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.
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.
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