Oil System — Three Sumps, the De-Oil Valve, and Oil-Triggered Auto Shutdowns
The oil system both lubricates and cools the APU and the APU generator. Three of its features are pilot-relevant: the shared gearbox shaft that also drives the fuel control, the de-oil valve that unloads the starter for a cold or high-altitude start, and the oil pressure/temperature limits that command an automatic shutdown.
The oil system supplies the oil which lubricates and cools the APU and the APU generator... The APU has three oil sumps: the gearbox sump, the mid sump, the aft sump. The gearbox sump is a wet sump which is also the integral oil reservoir. The mid and the aft sumps are scavenged sumps. The three sumps have no direct connection between each other. — AMM 49-90-00 / 49-91-00
1. Three sumps and the reservoir
Per AMM 49-91-00, there are three sumps with no direct connection:
- the gearbox sump — a wet sump that is also the integral oil reservoir (lower part of the accessory drive gearbox);
- the mid sump and the aft sump — both scavenged.
The reservoir reads on a sight glass between FULL (7.3 l) and ADD (4.1 l). At 4.1 l or less the Low Oil Quantity (LOQ) switch signals the ECB; with the APU stopped on the ground, the APU page shows a LOW OIL LEVEL advisory in pulsing green. A chip detector sits in the reservoir drain plug.
2. The oil pump shares its shaft with the fuel control
The gearbox drives the central pump shaft of the oil pump module... The central pump shaft transmits also the mechanical power which is necessary to operate the Fuel Control Unit (FCU).
Per AMM 49-91-00, the gearbox-driven central pump shaft drives both the oil pump module and the FCU.
[!note]- Oil and fuel hang off one shaft (integrative synthesis) The same central shaft turns the oil pump and the fuel control unit (AMM 49-91-00). The accessory drive gearbox is the common mechanical root for lubrication and fuel metering — a reminder that the APU's "support" systems are not independent boxes but limbs off one geartrain.
The pump draws from the reservoir, sends oil through the cooler and filter, regulates pressure, and lubricates the engine, load compressor, gearbox and APU generator. It scavenges the mid sump, aft sump and generator. The generator scavenge oil is filtered separately, then routed to the starter-clutch sprags — so the generator never contaminates the reservoir, and the clutch is lubricated even during shutdown.
3. The de-oil valve — unloading a cold or high start
When the ECB receives the start signal... and finds that the oil temperature is less than 20 deg.F (-6.67 deg.C) or the altitude is above 31000 ft., it energizes the de-oil solenoid and the normally closed de-oil valve opens. Oil from the oil pump outlet returns to the pump inlet... the pump outlet is depressurized. This reduces the load of the oil pump module and thus also for the APU starter motor which is caused by the high viscosity of the cold oil... The ECB 59KD also carries out de-oiling at every APU shutdown between 95 % and 7 % APU speed.
Per AMM 49-91-00, the de-oil valve depressurises the pump outlet to unload the starter from thick cold oil. The ECB de-oil solenoid pilots it electrically, and (per 04) high-pressure fuel is the hydraulic medium that actually operates the valve. It opens in two situations:
- at start, when oil temp < 20 °F (-6.67 °C)** *or* altitude **> 31000 ft — closed again after 60 % (65 % at high altitude);
- at every shutdown, between 95 % and 7 %, to clear the scavenge elements and ease the next start.
[!warning]- The de-oil valve is the cold/high-altitude start enabler (integrative synthesis) A high-altitude or cold-soak start is the demanding case for ignition and the starter. The de-oil valve attacks the oil-drag side of that problem: it dumps pump-outlet pressure so the starter isn't fighting viscous cold oil (AMM 49-91-00). Note the trigger is the same > 31000 ft that raises the starter cut-out speed to 65 % — the systems are co-designed for the high start.
4. The oil cooler keeps oil hot enough too
Per AMM 49-91-00, the oil cooler (oil/air, fed by the cooling fan, 49-52) keeps oil below 140 °C. But a thermal bypass valve stops any oil below 60 °C from going through the heat exchanger — warming the oil quickly and keeping it hot enough to prevent moisture intrusion. It also bypasses a blocked cooler.
The lubrication oil filter has a bypass valve (a clogged filter still passes oil to the APU); the generator scavenge filter has a relief valve. The oil heater 59KT16 adds heat on the ground: its switch closes below 70 °F (21.1 °C) for 115 V AC heating and opens above 110 °F (43.3 °C) — keeping the oil above compartment temperature in cold soak.
[!warning]- A "cooler" that refuses to cool cold oil The thermal bypass valve blocks the heat exchanger until the oil is above 60 °C (AMM 49-91-00) — counterintuitively, the cooler is bypassed when oil is cold, to drive temperature up and keep moisture out. Cooling and warm-up are the same valve's job.
5. Oil-triggered automatic shutdowns
When the ECB 59KD receives the low oil pressure or a high oil-temperature signal, it starts the APU automatic shutdown-sequence (if it is not inhibited).
Per AMM 49-94-00, the ECB commands an automatic shutdown (if not inhibited) on:
- low oil pressure — LOP switch closes at 28 ± 2 psi; above 95 % speed, shutdown after a low-pressure signal held 15 s (oil < 25 °F) or **1 s** (oil > 25 °F);
- high oil temperature — HOT sensor (downstream of cooler) above 297 °F (147.2 °C) for 10 s;
- high generator scavenge-oil temperature — above 365 °F (185 °C) for 1 s (protects the generator).
[!warning]- "If not inhibited" is the backup-source logic (integrative synthesis) Every oil auto-shutdown carries the qualifier if it is not inhibited (AMM 49-94-00). That inhibit is the heart of the chapter: in flight, or with an engine running, the APU may be the critical backup source (00, 13), so the ECB will tolerate an oil fault rather than shed the source. The same low-oil-pressure condition that auto-shuts the APU on the ground may be kept running in the air.
6. Counterintuitive points
[!warning]- Oil and fuel share one gearbox shaft The central pump shaft drives the oil pump and the FCU (AMM 49-91-00) — they are not independent.
[!warning]- Oil faults command auto shutdown — but only if not inhibited Low oil pressure / high oil temp / hot generator oil each start an auto shutdown if not inhibited (AMM 49-94-00); the inhibit preserves the APU as a backup source in flight.
Self-test
[!note]- Q1. The three sumps? Gearbox (wet sump = integral reservoir), mid and aft (both scavenged) — no direct connection between them.
[!note]- Q2. What does the de-oil valve do, and when does it open? Depressurises the pump outlet to unload the starter from cold viscous oil. Opens at start when oil < 20 °F or altitude > 31000 ft, and at every shutdown 95–7 %.
[!note]- Q3. The oil-triggered auto-shutdown limits? Low oil pressure (LOP closes 28 psi; >95 %, 15 s cold / 1 s warm); oil temp > 297 °F / 10 s; generator oil > 365 °F / 1 s — auto shutdown if not inhibited.
[!note]- Q4. Why does the thermal bypass valve block cold oil from the cooler? To warm the oil quickly and keep moisture out — oil below 60 °C bypasses the heat exchanger.
Key takeaways
| Point | Detail |
|---|---|
| Function | lubricates and cools APU + APU generator |
| Sumps | gearbox (wet/reservoir), mid, aft (scavenged) — no interconnection; FULL 7.3 l / ADD 4.1 l; LOQ → pulsing-green LOW OIL LEVEL |
| Shaft | central pump shaft drives oil pump + FCU |
| Generator oil | scavenged, separately filtered → starter-clutch sprags (lubed during shutdown, no reservoir contamination) |
| De-oil valve | unloads starter; opens at start (< 20 °F or > 31000 ft) and every shutdown 95–7 %; solenoid-piloted, fuel-actuated |
| Cooler | < 140 °C; thermal bypass blocks oil < 60 °C (warm-up/moisture) |
| Auto S/D (if not inhibited) | LOP 28 psi (>95 %, 15 s/1 s); oil temp 297 °F/10 s; gen oil 365 °F/1 s |
References
- AMM 49-90-00 / 49-91-00 (Oil; Storage and Distribution — Description and Operation) — oil lubricates and cools APU + APU generator; three sumps (gearbox wet/reservoir, mid, aft scavenged, no interconnection); FULL 7.3 l / ADD 4.1 l, LOQ switch → pulsing-green LOW OIL LEVEL on the ground; central pump shaft drives oil pump module + FCU; generator scavenge oil separately filtered → starter-clutch sprags; de-oil valve (ECB de-oil solenoid, fuel-actuated) opens at start (oil < 20 °F or altitude > 31000 ft, off after 60/65 %) and every shutdown 95–7 %; oil cooler < 140 °C with thermal bypass blocking oil < 60 °C; lubrication-filter bypass valve; oil heater 59KT16 (close < 70 °F / open > 110 °F, 115 V AC).
- AMM 49-94-00 (Low Oil Pressure and High Oil Temperature Warning) — auto shutdown if not inhibited: LOP closes 28 ± 2 psi, >95 % speed 15 s (oil < 25 °F) / 1 s (> 25 °F); HOT sensor > 297 °F (147.2 °C) for 10 s; generator scavenge oil > 365 °F (185 °C) for 1 s.
- FCOM DSC-49 — APU oil indications (LOW OIL LEVEL advisory).
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.