Engine Oil System

"The oil system lubricates the engine components. It contains the: ‐ Oil tank ‐ Lube and scavenge pump modules ‐ Fuel/oil and/or air/oil heat exchanger ‐ Filters, pressure relief and bypass valves."

The FCOM gives the oil system one page; the real weight of this chapter lives in the AMM. Oil is the engine's blood: it lubricates (friction), cools (carries bearing heat away), damps (the oil film under the bearing outer races) — and it is also the engine's continuously sampled blood test: seven magnetic chip detectors and four indication chains translate the machine's internal health into cockpit data. This article follows one drop of oil around the closed loop — tank → pressure side (pump, filter, two heat exchangers, bearings and gears) → scavenge side (six lines, chip detectors, scavenge filter, back to tank) — and then examines how the four "laboratory" chains read the numbers.

1. The closed loop

 ┌──── OIL TANK (magnesium casting, right front of LP case) ── QTY ─► FADEC ─► ECAM
 │          │ (bottom strainer)
 │     OIL PRESSURE PUMP ──[PRESSURE RELIEF VALVE 535 PSI]
 │          │
 │     OIL FILTER (145 μ, cleanable) ── CLOG → FADEC → ECAM ──[bypass]
 │          │
 │     AIR/OIL HEAT EXCHANGER ──[bypass]          (air side → article 03)
 │          │
 │     FUEL/OIL HEAT EXCHANGER ──[BYPASS VALVE ◄┄ FADEC]  (fuel side → article 09)
 │          │  ⓟ OIL PRESS → FADEC → ECAM ‖ OIL LOW PRESS → ECAM (independent switch)
 │          ▼
 │    six users (each with its own strainer):
 │    front bearing chamber │ internal gearbox │ intermediate gearbox (IDBG) │
 │    HP/IP turbine bearing chamber │ LP turbine bearing chamber │ external gearbox
 │          │
 │     six SCAVENGE PUMPS (one per line; an MCD seat ahead of each)
 │          │
 │     MASTER CHIP DETECTOR
 │          │
 │     SCAVENGE FILTER (30 μ → post-SB 15 μ, disposable) ── CLOG → FADEC → ECAM ──[bypass]
 │          │  ⓣ OIL TEMP → FADEC → ECAM  (2 thermocouples; same data drives the AOHE)
 └──────────┘  through the deaerator back to the tank (air → breather mast, article 02)

Three reading points from the FCOM loop diagram. The pressure-side relief valve carries a number: 535 psi — the pump outlet's last insurance (a separate cold-start relief valve handles thick, cold oil, §3). OIL PRESS and OIL LOW PRESS are drawn as two separate lines — the indication chain and the warning chain are deliberately divorced, the diagram's own proof of §6. And the scavenge side is laid out as six users, six pumps, one chip-detector seat per line — a reconnaissance network organised by line (§5).

2. The tank: a visible 19 quarts and an invisible 5.4-quart basement

The tank is a magnesium casting on the right front flange of the LP compressor case, with a sight glass (a walkaround-readable mechanical truth), a gravity filler with anti-spill valve, and a pressure-fill connector. Inside the cap sits the deaerator: scavenge oil returns mixed with bearing-chamber sealing air, is spun free of its bubbles, and only then rejoins the tank — the air leaving through the vent to the breather mast (article 02). The quantity bookkeeping contains a counter-intuitive detail worth quoting:

"Output of acquisition system varies linearly with true oil quantity in the tank from 5.4 to 25 US quarts (5.11 to 23.7 L) corresponding 0 to 20 US quarts (0 to 18.9 L) indicated on the system display. The corresponding indicated oil quantity in the cockpit, taken into account an unrecorded oil quantity of 5.4 plus or minus 1.02 US quarts … is 19 US quarts plus or minus 1.02 quarts."

An indicated zero does not mean an empty tank — the transmitter's lower end cannot see the bottom 5.4 ± 1 quarts. This is no licence to fly with low oil (the 15-quart rule of article 00 is computed on indicated values as always), but it explains the physical buffer between "indicating low" and "actually dry".

3. The pump block: one feeds, seven collect

"The pumps assembly contains vane-type pressure and scavenge pumps. These are eight vane units attached along two parallel shafts. … The drive shaft has five scavenge units. The other shaft has the pressure unit and two other scavenge units."

The lopsided 1-feeds-7-collect ratio has two reasons (synthesis). Scavenge oil is a froth of oil and sealing air — volumetrically inflated, so the collection side must be sized for the swollen flow. And the six chambers sit at different heights with no cooperation between them; only a dedicated pump per line guarantees that no chamber ever pools. Two supporting components: the cold-start relief valve (cold, viscous oil spikes the pump outlet; the valve recirculates part of the flow to the inlet and re-seats itself as the oil warms — factory-set, not adjustable) and the 535-psi system relief valve from the loop diagram.

4. Two filters: coarse in, fine out

Why is the return filter the finer one (synthesis)? The pressure path only needs to stop particles big enough to block oil jets — 145 μ does that with little pressure loss. The scavenge path is the system's only purification gate: bearing-wear debris not caught before the tank would circulate through the whole body. Both filters carry bypass valves — when clogged, the oil flows dirty rather than not at all — and each filter has its own differential-pressure switch (pressure-filter and scavenge-filter switches, either of which raises the same CLOG caution). One display detail rewards attention: on the SD, the amber CLOG legend appears directly beneath the oil-pressure indication — the alert literally points at the parameter its procedure will tell you to monitor (article 30).

5. Scavenge reconnaissance: six lines and seven magnetic sentries

The six scavenge lines drain the six lubricated chambers: front bearing chamber, internal gearbox, gearbox input drive, HP/IP turbine bearing chamber, LP turbine bearing chamber, external gearbox. The chip-detector deployment is "one master gate plus six precinct posts":

"There are seven MCD locations in the oil system. A master MCD is found immediately upstream of the scavenge filter and is permanently installed. … Six other MCD locations are found (one for each scavenge line) immediately upstream of the scavenge pumps."

The diagnostic logic follows directly: debris on the master MCD → install and inspect the six line MCDs → whichever line shows debris identifies the chamber whose bearing or gear is shedding metal. Blood test first, then biopsy by organ. The thread planted in article 01 closes here: a service bulletin activates the electronic master chip detector (EMCD) function, letting debris report electronically in real time rather than waiting for a plug inspection — the configuration basis of the OIL CHIP DETECTED alert (article 30).

Between the heat exchangers, the pressure path runs pump → pressure filter → AOHE → FOHE → users. The AOHE cools oil with fan air, its rotary airflow valve driven by a fuel servo piston under a dual-circuit torque motor (one circuit per EEC channel) with dual LVDT feedback; the EEC's control inputs are the oil temperature sensors plus the fuel thermocouple — it manages oil temperature and oil's downstream effect on fuel temperature. The FOHE has the double identity met in article 09: cooling oil while warming fuel against icing — plus an anti-siphon hole that stops the oil system siphoning itself empty after shutdown.

6. Four indication chains — a differential, a correction, and a divorced warning

Quantity: a float rides an aluminium tube containing a PCB with 51 reed switches and 50 series resistors — the float's magnet closes the switch at its level, the resistor chain divides the voltage, the EEC digitises (6 V DC supply) and sends it to the SD.

Temperature: two thermocouples, one per channel, at the scavenge return into the tank — measuring the oil that has just finished work, the hottest point in the loop; the same data closes the EEC's AOHE control loop.

Pressure — the chapter's most important counter-intuitive, quoted in full:

"The oil pressure transmitter senses the difference between oil pressure and scavenge oil pressure. … When N3 is more than 70 percent the indication in the cockpit does not show the correct pressure. The pressure indication is adjusted in relation to N3. This is to make sure that the in-flight low oil pressure advisory comes on at the same time as the low oil pressure warning. The low oil pressure warning is given when the engine oil pressure indication is 25 psi or less."

Unpack it in two layers. First, it is a differential: the bearing chambers carry sealing-air back-pressure (article 03), so supply pressure minus scavenge pressure is the only number that represents the effective head actually forcing oil into the bearings. Second, the N3 correction: above 70 % N3 the pump pressure rises with shaft speed, and a raw reading would let "dangerously low" hide inside "normally high" — normalising the indication against N3 keeps the SD advisory and the 25-psi warning triggering at the same true danger level. The cockpit oil pressure is a normalised health index, not a gauge reading. Two transmitters, one per channel; if they fail, the low-pressure switch still stands behind them —

The warning chain, divorced from the indication chain:

"The low oil pressure switch gives an indication of a decrease in main oil pressure compared to scavenge oil pressure. … The power for the switch is supplied from the aircraft. The signal from the low oil pressure switch is sent to the EIVMU, FWC1, FWC2 and other aircraft indicating system."

Three deliberate differences: not through the EEC, not on EEC power, straight to both FWCs. The single most lethal oil message travels a hardwired path completely independent of the indication chain — with the EEC dead and every indication lost, this bellows switch still raises the alarm.

The wiring schematic for that warning (read directly) names every recipient that the AMM's "other aircraft indicating system" summarises. The low-oil-pressure switch's signal fans out to six classes of user: both FWCs (the warning), both FMGECs as a Q ENG STOP discrete — one of the autoflight system's sources for "this engine has stopped" — the EIVMU, the weather-radar transceivers (as an engine-running qualifier, where fitted), a relay feeding the avionics-ventilation computer, and the door/slide management side. The bellows switch's true identity is thus complete: it is not merely an oil warning sensor — it is the aircraft's agreed hardwired source of "engine running / engine stopped" (article 06 met the same truth from the broadcast side). The schematic's annotation also restates the full 25-psi effect chain verbatim — red oil-pressure indications on the SD, the OIL LO PR warning on the E/WD, the flashing MASTER WARN, the continuous repetitive chime, and the relay energising — making 25 psi triple-confirmed across the limitations, the procedures and the schematic.

7. One under-rated sentence: the oil film that damps the engine

"Pressure oil is supplied between the bearing housing and the outer race of the rotor-shaft roller-bearings. This layer of oil keeps engine vibration to a minimum."

The bearing outer races are not pressed rigidly into their housings — they float on a pressurised oil film (a squeeze-film damper, to give it its engineering name). The oil is itself the vibration absorber. This single sentence explains a cross-chapter pattern: oil-system anomalies and vibration changes often arrive as a pair, which is why the vibration article (29) lists the oil system among its correlation checks.

8. Where the oil system meets the failure chapters

Self-test

[!note]- Q1. How is the pump block divided, and why so many scavenge units? One pressure unit plus seven scavenge units on two parallel shafts (5 + 2/1). Scavenge flow is an oil-air froth — volumetrically inflated — and six independent chambers each need a dedicated pump so that none ever pools.

[!note]- Q2. The cockpit shows 60 psi. Is that the pump outlet pressure? No. It is the differential (supply minus scavenge) — the effective head into the bearings — and above 70 % N3 it is further normalised against N3 so that the advisory and the 25-psi warning trigger at the same true danger level. A health index, not a gauge.

[!note]- Q3. Both EEC channels are dead. Does a low oil pressure still announce itself? Yes. The low-pressure switch is aircraft-powered and hardwired directly to the EIVMU and both FWCs — a warning chain fully independent of the indication chain.

[!note]- Q4. The master MCD shows debris. How does maintenance localise the source? By installing and inspecting the six line MCDs (ahead of each scavenge pump): the line that shows debris names the chamber — front bearing, internal gearbox, input drive, HP/IP turbine, LP turbine or external gearbox.

[!note]- Q5. Why is the scavenge filter finer than the pressure filter, and why is it never reused? The scavenge filter is the system's only purification gate — it must catch fine wear debris before the tank, hence 30 μ (15 μ post-SB). Its dirty element is the evidence and is discarded after inspection. The pressure filter merely protects the oil jets from blockage — 145 μ, cleanable metal.

Key takeaways

References

• FCOM DSC-70 (oil system) — component list; loop schematic with the 535-psi relief, divorced warning lines and six-line scavenge layout (read directly).
• AMM 79-11 (oil storage, D/O) — tank construction, deaerator, the 5.4-to-25-quart acquisition window and 19-quart indication.
• AMM 79-21 (oil distribution, D/O) — eight-unit pump block, cold-start relief, filter ratings and dispositions, AOHE/FOHE descriptions and anti-siphon hole, six scavenge lines, seven MCD locations and the localisation logic, squeeze-film sentence.
• AMM 79-31/-32/-33/-34 (indicating, D/O) — quantity reed-switch chain, dual temperature thermocouples at the scavenge return, the differential and N3-corrected pressure indication with the 25-psi warning, the independent low-pressure switch and its recipients.
• AMM 79-35 (filter clogging warning, D/O) — twin differential-pressure switches and the SD CLOG presentation.
• ASM (oil low-pressure warning schematic, read directly) — the six-user fan-out (FWCs, FMGEC Q ENG STOP, EIVMU, weather radar qualifier, ventilation relay, doors) and the 25-psi effect-chain annotation.
• Integrative synthesis (marked in text): the froth/feeds-collects reasoning; the purification-gate argument; the health-index framing; the blood-test analogy.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.