Hydraulic Fluid — Specification, Temperature, Thermal Management

The working fluid is not a neutral medium. It has a chemistry (phosphate-ester, the so-called Skydrol family), a measured temperature at each reservoir return port, two distinct overheat thresholds (one for the fluid, one for the electric pump body), and a mandatory post-overheat sampling protocol before the system is cleared for continued service. This article covers each.

The thermal architecture itself — how the fluid stays cool in normal operation — is not formally documented in the hydraulic chapter of the FCOM; what is documented is the monitoring: where the sensors are, what they trigger, and what the procedure does after a trigger. That is the focus here.

1. Fluid chemistry — phosphate-ester (Skydrol family)

The hydraulic fluid is a phosphate-ester compound, commonly referenced by the brand name Skydrol. The A330 maintenance documentation does not state the precise grade in the hydraulic chapter, but the architecture is consistent throughout — placard requirements specify "Skydrol areas" on the airframe, and seal compounds, hose materials, and surface coatings are all selected for compatibility with this fluid family.

Three properties define operational behaviour:

For the pilot, three operational rules follow:

• Hydraulic-fluid stains on the airframe are never cosmetic. They are reported, the source is traced, and dispatch decisions follow the investigation outcome.
• Wrong-fluid events are catastrophic to system integrity. A line technician using mineral-oil-contaminated equipment, or a confused servicing procedure, requires a fleet-grade reseal. Maintenance documentation segregates equipment and storage to prevent this.
• The fluid is hygroscopic and degradation-sensitive. Heat, moisture, and contamination shorten its life. Post-overheat sampling exists for this reason.

2. Where temperature is measured

Each reservoir carries one temperature transmitter at its return port — the inlet through which fluid returning from the consumers re-enters the reservoir.

The transmitters are designated 2JS1 (Green), 2JS2 (Blue), and 2JS3 (Yellow) in maintenance documentation. Each is a platinum-resistance type, with a reference resistance of 100 Ω at 0 °C, calibrated over a range of −60 °C to +120 °C.

Why the return port, specifically. Fluid leaving the reservoir to feed the pumps is cold; fluid returning from the consumers carries the cumulative thermal load of the system. The return port is the hottest point in the loop and the most sensitive monitoring location — temperature anomalies show up there earliest. A sensor placed mid-tank or at the suction port would respond more slowly to a developing thermal problem.

                ┌──────────────────────┐
                │  Reservoir top       │
                │  (gas cushion at     │
                │   4.5 bar absolute)  │
                └──────────┬───────────┘
                           │
                           │  Returning fluid (hottest in cycle)
                           ▼
                ┌──────────────────────┐
                │  Return port ◄────── 2JS sensor (platinum, 100 Ω @ 0 °C)
                │                      │
                ├──────────────────────┤
                │   Bulk fluid         │
                │   (mixing volume)    │
                ├──────────────────────┤
                │  Suction port ──► to pump
                │  (coolest in cycle)  │
                └──────────────────────┘

The HSMU receives the analog signal from each sensor and converts it to a voltage proportional to temperature: 1 V at −60 °C, 9 V at +120 °C, linear in between. The HSMU then applies the comparison logic to generate the OVHT signal and to update the SD HYD page indication.

The linear relationship gives a direct conversion. For a temperature T in °C:

V = 1 + 8 × (T - (-60)) / (120 - (-60))
  = 1 + 8 × (T + 60) / 180

Worked example for the OVHT threshold (95 °C):

V = 1 + 8 × (95 + 60) / 180
  = 1 + 8 × 155 / 180
  = 1 + 6.89
  ≈ 7.89 V

A maintenance technician confirming the temperature circuit's health on the ground can read the HSMU input voltage and compare against the expected value for ambient temperature — for example, with fluid at ~25 °C, the input should read ~4.78 V (1 + 8 × 85/180). A significant deviation from the expected value, with the fluid temperature otherwise known, points to a sensor or wiring fault rather than a real temperature anomaly.

3. The fail-safe default — 150 °C on sensor loss

A characteristic of the sensor circuit worth retaining: if the sensor or its wiring loses continuity, the HSMU treats the input as 150 °C.

The architectural reasoning is fail-safe: a missing temperature reading is treated as the worst case (above the OVHT threshold), generating a spurious OVHT caution. The opposite design — treat missing data as "normal" — would mask a real overheat in the event of sensor failure during a thermal event.

For the crew, this means an OVHT caution may sometimes be a sensor failure rather than a real fluid event. Maintenance can distinguish by checking the SD HYD page reading (an actual 150 °C will display as such; a fault may show as a different anomalous value or no value) and by direct sensor measurement on the ground.

A useful cross-check: in normal operation, the temperature indication on the SD HYD page sits well below the OVHT threshold and changes slowly. An abrupt jump to 150 °C with no preceding rise is more consistent with sensor loss than with a real overheat.

4. Two overheat thresholds — fluid vs. electric pump

The hydraulic system has two distinct OVHT-related triggers, at very different temperatures, measuring very different things.

Reservoir OVHT (95 °C)

When the fluid temperature at the return port reaches 95 °C on a rising trend, the HSMU generates the RSVR OVHT signal. The associated ECAM caution appears, the ENG PUMP and ELEC PUMP FAULT lights illuminate on the affected system, and the procedural response is to switch off both pumps on that system.

The threshold is specified as rising because there is an implicit hysteresis: the signal recovers at a lower temperature. The documentation does not publish the recovery threshold to the crew, but it is below 95 °C. The pilot's observation is that the caution does not flicker on and off as the temperature hovers around 95 °C; it triggers cleanly on a rising condition and clears cleanly after cooling.

Electric pump OVHT (193 °C)

The electric pump itself carries an internal temperature switch in its housing. When the housing temperature reaches 193 °C on a rising trend, the switch opens, the HSMU generates the corresponding FAULT, and the electric pump's FAULT light illuminates.

The threshold is much higher than the fluid OVHT because the electric motor inside the pump runs hot — the housing measures a mixture of motor temperature and fluid temperature, and 193 °C is the design margin against motor insulation failure. Fluid at 193 °C would be a catastrophic system event; pump housing at 193 °C is a localised pump-side overheat with the rest of the system potentially still healthy.

For the pilot, the practical interpretation:

• HYD G (B/Y) RSVR OVHT → fluid problem; both pumps off.
• ELEC PUMP FAULT alone (no RSVR OVHT) → localised pump-side overheat; the procedure typically commands ELEC PUMP off.

5. The FAULT-light latch

A subtle but operationally important behaviour: the FAULT light on the Green and Yellow electric pump pushbuttons does not extinguish when the pump is selected OFF after an overheat event.

Per maintenance documentation, the FAULT light remains illuminated until both of the following are true:

• The reservoir or pump temperature has returned to normal.
• The corresponding circuit breaker has been reset on the ground.

The architectural reason is that the latch ensures the overheat condition is not silently forgotten by the system. On a normal pump FAULT (e.g., low pressure with engine off), the light extinguishes as soon as the pump is switched off — there is no operational hazard to clear. On an overheat FAULT, the underlying condition may persist, and the light keeps the crew aware of it.

For the crew on board, the takeaway is: a FAULT light that persists after a pump is switched off post-OVHT is normal. It is not a second failure, and the pump is not silently refusing the OFF command. The latch will release after cooling and ground reset.

6. The post-overheat sampling protocol

A reservoir overheat triggers a mandatory maintenance follow-up: fluid sampling and laboratory analysis.

Per maintenance documentation, after an SD page overheat warning, the procedure is:

1. Take a fluid sample from the affected system on the ground.
2. Send the sample for laboratory analysis.
3. The aircraft may continue to fly while awaiting results — the OVHT condition itself does not require immediate grounding.
4. If the sample is within acceptable property limits, the system continues in service.
5. If the sample is out of limits, the entire system's fluid is replaced.

The samples are drawn at dedicated fluid sampling valves on each system (one per system, in the engine pylon area). The replacement, if required, uses either a bench-based fluid replacement or operating the relevant electric pump to flush the system through the bench.

The protocol exists because heat degrades phosphate-ester fluid: viscosity, acidity, water content, and additive levels all shift after a sustained thermal event. Fluid that looks unchanged visually can have lost its protective properties. The analysis identifies cases where the degradation is significant enough to require replacement.

       OVHT triggers in flight
                │
                ▼
       Aircraft lands and continues turnaround normally
                │
                ▼
       Maintenance draws fluid sample at the sampling valve
                │
                ▼
       Sample sent for analysis (parallel to next flight if dispatched)
                │
                ▼
       Results:
            │
            ├── Within limits ──► Continue in service, no further action
            │
            └── Out of limits ──► Replace fluid (full system flush)
                                   Maintenance task uses bench equipment
                                   or the system's own electric pump

The "continue to fly while waiting" provision matters operationally. A single OVHT event during a flight does not strand the aircraft at a layover station; analysis can be completed in parallel with the next flight's operation, and the consequence (fluid replacement) is only triggered if analysis confirms degradation.

7. Thermal management — what is documented, what isn't

A330 maintenance documentation does not present a separate thermal-management chapter for the hydraulic fluid. The architecture documents:

• The thresholds (where overheat triggers).
• The sensor locations (return port).
• The response (pumps off, sampling, replacement if needed).

What is not explicitly documented in the hydraulic chapter:

• The detailed cooling path: how fluid temperature stays below 95 °C in normal operation.
• Whether fluid-fuel heat exchange (common on some transport architectures) is part of the design.
• Operational steady-state temperatures expected in cruise vs ground operations.

Maintenance documentation references an Air/Oil Heat Exchanger (AOHE) in the engine pylon area, but this serves engine oil, not hydraulic fluid. The hydraulic system's thermal balance, based on what is documented, is passive — fluid heat is dissipated through the lines, the reservoir surfaces, and ambient airflow, without a dedicated active cooling element.

The pilot's practical handle on thermal state is the temperature indication on the SD HYD page. A normal reading in cruise is in the lower part of the green band; sustained high readings approaching the OVHT threshold are a sign of heavy hydraulic demand (continuous spoiler activity, repeated configuration changes, prolonged abnormal-procedure pump operation) and warrant reduced demand if possible.

8. Putting the numbers in order

Self-test

[!note]- Q1. A reservoir OVHT caution triggers in cruise. The Captain asks whether to divert. What is the correct response, and on what authority?

The aircraft may continue to the planned destination. Maintenance documentation explicitly states that flight may continue while awaiting fluid-sample analysis results. The OVHT itself does not require immediate landing; it triggers a maintenance protocol that begins on the next ground stop. The procedural action in flight is to switch off both pumps on the affected system as the ECAM directs, accept the resulting system loss for the rest of the flight, and continue.

[!note]- Q2. The Yellow electric pump FAULT light is on. The crew switches off the Yellow ELEC PUMP per the ECAM. The FAULT light does not extinguish. Is this a second failure?

No. After an overheat event on the Green or Yellow electric pumps, the FAULT light is latched and remains illuminated until both the temperature returns to normal and the circuit breaker is reset on the ground. This is documented behaviour. The pump has accepted the OFF command; the FAULT light is reporting the persistent overheat memory, not refusing the command. The light will release after the next ground turn-around.

[!note]- Q3. The SD HYD page shows the Green reservoir temperature jumping abruptly from 35 °C to 150 °C in a single update, with no preceding rise. Is this a real overheat?

Almost certainly not — it is consistent with sensor or wiring loss-of-continuity. The HSMU treats sensor failure as a fail-safe 150 °C input, which generates an OVHT caution. The diagnostic cue is the abrupt jump with no preceding gradient. A real thermal event produces a rising temperature trend over seconds or minutes, not a single-step jump from normal to default. The ECAM procedure should still be followed, but the crew should expect the post-flight maintenance finding to be a sensor or wiring fault, not a real fluid event.

[!note]- Q4. Why is the reservoir OVHT threshold 95 °C but the electric pump OVHT threshold 193 °C?

Because the two sensors measure different things. The reservoir sensor measures fluid temperature at the return port — the actual working fluid. 95 °C represents the upper safe limit for the fluid before degradation accelerates. The electric pump sensor measures pump housing temperature, which combines the heat generated by the electric motor inside the pump with the fluid passing through. The motor itself runs hot in normal operation, so the housing sits at a much higher temperature than the fluid. 193 °C is the housing-level threshold above which motor insulation is at risk. The two numbers should not be compared as if they measured the same thing.

[!note]- Q5. The hydraulic fluid is described as "fire-resistant phosphate-ester." A maintenance technician asks if a half-litre of clear automotive hydraulic oil can be added in an emergency at a remote station. What is the issue and what is the correct response?

The two fluids are not compatible at the seal level. Phosphate-ester systems use EPDM or butyl seals; mineral hydraulic oils (typical automotive) cause those seal compounds to swell, deteriorate, and ultimately fail. Even a small contamination volume produces seal damage throughout the system over time. The correct response is unambiguous: no replenishment with non-matching fluid. The aircraft is held until proper Skydrol-grade fluid arrives. The "small quantity, just to get going" reasoning is incorrect — the damage is qualitative, not quantitative, and a half-litre of mineral oil in a phosphate-ester system requires a complete reseal of the affected reservoirs and lines.

References

Per AMM 29-31 (Reservoirs Temperature, Fluid Temperature Indicating System, Electric pump overheat switch, sensor characteristics 2JS1/2/3, HSMU conversion, failure default 150 °C, FAULT light latch logic); AMM 29-00 (post-OVHT fluid sampling protocol, continuation of flight provision); AMM 11-00 (Skydrol-areas placard requirements, confirming fluid family); fluid replacement tasks per AMM 12-36-29 series.

Independent study material, not an Airbus publication. Refer to current operator FCOM, FCTM, AMM, and QRH for operational use.