Engine Bleed II: Temperature Control and the Precooler

Article 02 delivered air at 44–52 psig — but straight off the 14th stage it can be scorching. Composite ducting, fuel-tank neighbourhoods and every downstream consumer need it tamed to 200 °C, sometimes 150 °C. That is the work of a three-piece kit: the precooler exchanger (PCE), the fan air valve (FAV) that meters its cooling flow, and the control thermostat that commands the FAV without a single electronic component. This article also assembles the full four-tier temperature defence that articles 02, 05 and 06 each own a piece of.

   Engine fan (bypass air, cool)          Hot path: bleed air from PRV
        │                                        │
        ▼                                        ▼
   FAV (8 in butterfly) ──cool flow──► PRECOOLER (air-to-air) ──► ≤200 °C (or 150 °C) ──► wing
        ▲ motive pressure                tubular hot side /            │
        │                                crossflow cold side           ├─► outlet temperature sensor (to both BMCs)
   CONTROL THERMOSTAT ◄──── senses & taps pressure at PCE outlet ──────┘
        ▲ solenoid: de-energised = 200 °C · energised = 150 °C
        │
   BMC ◄── zone controller (pack low-temperature demand) ; inhibited when wing anti-ice ON

1. The precooler — chilling compressor air with fan air

Per FCOM DSC-36-10-20:

The temperature regulation of bleed air is achieved by a precooler, that is mounted downstream of the bleed valve. The precooler is an air-to-air heat exchanger, which uses cooling air that is bled from the engine fan, to regulate the temperature to 200 °C.

Why fan air is the perfect coolant: it is right next door (the whole bleed kit lives in the pylon region), it is cold, it is abundant — and it was leaving anyway. Borrowing a sliver of bypass flow costs far less than any active cooling would, and the spent air exits overboard, incidentally ventilating the pylon bay.

Construction, per AMM 36-11-00:

The precooler exchanger is tubular on the hot air side and the cooling air flows over the tubes in a crossflow configuration.

Hot bleed inside the tubes, fan air sweeping across them — a crossflow layout that packs a lot of exchange area into pylon-sized real estate. The AMM also names the clientele: To meet the required temperature for the air conditioning, wing anti ice and other sub-systems, the bleed air has to be cooled — everyone downstream depends on this one box.

2. The fan air valve — the coolant tap

Per FCOM DSC-36-10-20:

Fan airflow is controlled by the fan Air Valve.

The fan air valve is spring-loaded closed, in the absence of pressure.

Per AMM 36-11-00:

The airflow rate from the engine fan is controlled by the fan air valve (8 in. dia) to regulate the engine bleed air temperature.

A spring holds the fan air valve in the closed position as long as the threshold pressure for initiation of valve opening is not reached. A thermal fuse causes the valve to close in the event of fire and a test intake is provided to check valve correct operation on the aircraft. Microswitches indicate the valve fully open and fully closed positions.

Put the FAV next to the PRV from article 02 and the family resemblance is obvious: spring-closed, thermal-fused for fire, lockable, position-reported. In a fire both fuses melt — the PRV cuts the hot path, the FAV cuts the cold path, and the burning zone is left with no fresh airflow of either kind.

[!warning]- Why the FAV reports TWO end positions when the PRV reports one The PRV's microswitch reports fully-closed/not-fully-closed. The FAV reports fully open and fully closed — because it is a modulating valve that lives mid-stroke, and what monitoring cares about is whether it has jammed at either end. Jammed open means over-cooling (the low-temperature alert family); jammed closed means no cooling (the overheat family). Both endpoint switches feed the BMC's built-in test logic.

3. The control thermostat — a regulator with no electronics

Per AMM 36-11-00:

The control thermostat consists of : - an assembly comprising two temperature probes - a pressure reducing assembly.

The control thermostat supplies and controls the fan air valve motive pressure depending on the precooler exchanger outlet temperature.

The pressure used by the control thermostat is picked off from the outlet of the precooler exchanger. The pressure sent to the fan air valve opening chamber is modulated by the expansion of the temperature probes.

Trace the loop and notice there is not one electronic part in it: outlet air bathes the probes → thermal expansion strokes the pressure-reducing assembly → the FAV's opening-chamber pressure changes → the butterfly swings → cooling flow changes → outlet temperature returns to target. A completely pneumatic closed loop, self-powered by the very air it is regulating. Bleed temperature control keeps working with the aircraft electrically dark. Electricity enters at exactly one point: the setting-change solenoid.

4. Two settings, one trick: isolate a probe

Per AMM 36-11-00:

The thermostat provides two temperature settings selected by means of a solenoid: - solenoid not energized: 200 plus or minus 15°C - solenoid energized: 150 plus or minus 15°C.

When the solenoid is not energized,the probe adjusted to 150°C is isolated and the control thermostat regulates the temperature to 200°C. When the solenoid is energized,the probe adjusted to 200°C is isolated and the control thermostat regulates the temperature to 150°C.

Each probe is factory-set to one value; the solenoid's only job is to silence one of them. De-energised defaults to 200 °C — so any wiring failure leaves the system in the high setting, which every consumer tolerates (it is merely less cool). Another quiet fail-safe.

Who is allowed to ask for 150 °C? Per AMM 36-11-00:

A second level of control at 150°C (adjustable) is available. It is activated by the BMC according to the demand of the air conditioning system.

And the FCOM's crew-facing version — per FCOM DSC-36-10-20: When wing anti-ice is selected off, the temperature may be regulated to 150 °C, upon zone controller demand. The chain is zone controller (pack wants cooler inlet air, typically flat-out cooling on a hot ramp) → BMC → thermostat solenoid. But the request is confiscated the moment anti-ice enters the picture. Per AMM 36-11-00:

This solenoid is energized by the BMC in case of pack low temperature demand. This function is inhibited when the wing anti ice system is selected on.

[!warning]- Wing anti-ice ON locks out TWO economies at once The 150 °C setting (this article) and the EEC's fuel-saving early transfer to the IP stage (article 02) are both inhibited when wing anti-ice is selected on. One philosophy, two implementations: anti-icing wants air that is hot and plentiful — passenger-comfort and fuel economies wait. The reverse view explains the BLEED LO TEMP alert: below 150 °C with anti-ice on, the air is officially too cool to de-ice properly (article 09).

5. ±15 or ±20? Reading the gauge honestly

The AMM adds a note aimed squarely at people staring at the BLEED page:

NOTE: In addition, ECAM Display introduces a tolerance for temperature indication of plus or minus 5°C. Therefore, the regulated temperature downstream of the precooler exchanger shown on the ECAM, must be within these tolerances. - solenoid not energized: 200 plus or minus 20°C. - solenoid energized: 150 plus or minus 20°C.

Regulation tolerance ±15, display tolerance ±5, so the on-screen acceptance band is ±20. A cruise indication of 215 °C is not a sick thermostat — it may be 15 of genuine regulation scatter plus 5 of display. Anything inside 180–220 is simply "regulating at 200". Judge the screen with the screen's own tolerance.

And the winter companion note — per FCOM DSC-36-20:

Note: When the engines are at idle, and depending on the ambient temperature, the precooler outlet temperature may be below 150 °C (displayed in amber).

Idle offtake on a cold day just isn't hot; the FAV can be fully closed and the outlet still won't reach the target. Amber, yes — abnormal, no. The BLEED LO TEMP procedure's own logic acknowledges the same physics (article 09).

6. The four-tier temperature defence

Assembled across three articles, the whole ladder in one table:

Tier 2 deserves a second look: the hotter the air, the shorter the patience — nearly a minute of grace just above 257 °C, five seconds at 290. And the same three time-temperature gates double as the BLEED page's amber logic, so when the number turns amber, the closure countdown is already running.

Self-test

[!note]- Q1. What is the precooler's coolant, and why is that choice inherently cheap?

Air bled from the engine fan (bypass flow) — adjacent, cold, abundant, and already on its way overboard. Metering a little of it through the exchanger costs almost nothing compared with any active cooling scheme.

[!note]- Q2. How does the thermostat switch between 200 °C and 150 °C, and which setting survives an electrical failure?

Two factory-set probes; the solenoid isolates one of them. De-energised isolates the 150 °C probe → regulation at 200 °C; energised isolates the 200 °C probe → regulation at 150 °C. Loss of power therefore defaults to the safe high setting, 200 °C.

[!note]- Q3. Who requests 150 °C, who approves it, who executes it — and when is the request refused?

The zone controller (pack low-temperature demand) requests, the BMC approves and energises, the thermostat solenoid executes. The function is inhibited whenever wing anti-ice is selected on.

[!note]- Q4. Is 217 °C on the BLEED page abnormal? Show the arithmetic.

No. Regulation tolerance is ±15 °C and the ECAM display adds ±5 °C, so the on-screen band is 200 ± 20 °C. 217 sits inside 180–220.

[!note]- Q5. Recite the four-tier temperature defence with thresholds and actors.

Tier 0: thermostat/FAV regulate to 200/150 °C. Tier 1: >235 °C, PRV solenoid cuts the pressure setting. Tier 2: BMC closes the PRV on the 257 (55 s) / 270 (15 s) / 290 (5 s) ladder. Tier 3: duct-external leak loops at 124 °C (wing/fuselage) or 204 °C (pylon) declare a leak and isolate the side.

[!note]- Q6. Cold morning, idle taxi, outlet showing 138 °C in amber. Diagnosis?

Physics, not failure: at idle with low ambient temperature the outlet may sit below 150 °C — the FCOM notes it explicitly. Confirm no associated alert and carry on.

Key takeaways

References

Precooler function, fan-air cooling to 200 °C, the 150 °C zone-controller case and the FAV spring-closed characteristic per FCOM DSC-36-10-20; idle low-temperature display note per FCOM DSC-36-20. Temperature-limitation sub-system description (200 °C normal, 150 °C second level activated by the BMC on air-conditioning demand), precooler crossflow construction, FAV construction (8 in, thermal fuse, endpoint microswitches, test intake), control-thermostat construction and operation (two probes, pressure-reducing assembly, probe-isolation switching, ±15 °C settings with the ±5 °C ECAM display note, wing-anti-ice inhibit) per AMM 36-11-00 (Description and Operation). The BMC closure ladder cited in the defence table per AMM 36-11-00 with its display counterpart per FCOM DSC-36-20; leak-loop trigger values per AMM 36-22-00. The four-tier ladder table and the "isolate a probe" framing are integrative syntheses of the referenced text.

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.