BMC Monitoring and the BLEED Page

The valves of articles 02–04 largely run themselves. This article is about who watches them: three sensors per side, two computers, one system-display page, and a four-tier alert taxonomy. It is the densest article of the chapter in numbers, and the one that pays off every time you glance at the BLEED page in cruise.

 transferred press (HPV↔PRV) ──► own-side BMC ─┐
 regulated press (after PRV) ─► BMC 1 + BMC 2 ─┤        ┌► SDAC ─► SD (BLEED page) / E/WD alerts
 outlet temp (2 wires) ───────► one wire each ─┼─ BMC ──┼► ENG / APU BLEED pushbutton FAULT lights
 valve position switches (not the HPV) ───────►│        ├► CMC / maintenance
 leak loops (own wing+pylon; APU loop → BMC1) ─┘        └► the other BMC (cross-talk)
 HPV position / EEC data ──► engine interface unit ──► BMC (digital)

1. The BMC's four jobs — and its lopsided wiring

Per AMM 36-11-00, each BMC controls (PRV closure, crossbleed opening, the APU-valve enable signal, temperature-setting selection), monitors, warns, and self-tests. The monitoring sentence contains a routing detail worth underlining:

The BMC transmits to the System Data Acquisition Concentrator (SDAC) the following information relative to the bleed air system status: - positions of the valves (the HP bleed valve position is transmitted by the EIVMU to the BMC). - temperature and pressure of the bleed air - ambient overheat detection.

The HPV never talks to the BMC directly — its position travels EEC → engine interface unit → BMC, consistent with article 01's division of labour (the engine side belongs to FADEC). The warning job has an exact trigger list:

An engine bleed fault output is activated in the following warning conditions: - bleed valve automatic closure disagree - bleed air overheat detection - bleed air overpressure detection - pylon or wing ambient overheat detection.

Those four conditions are the complete set of reasons the amber FAULT can appear on an ENG BLEED pushbutton.

Division of territory — per AMM 36-21-00: two Bleed Monitoring Computers (BMC) which control the pneumatic system of each wing (BMC 1: L wing, BMC 2: R wing). And the wiring is deliberately asymmetric. From the AMM 36-20-00 interface inventory: each BMC receives the opposite wing's leak loops as a discrete (cross-cover), but the APU leak loop is marked not used by BMC 2 on the discrete side and BMC 1 only on the analogue side — and the APU BLEED pushbutton's FAULT drive is likewise BMC 1 only. Digital inputs include the other BMC, the engine interface unit (engine-start data — the source of every "except during engine start" clause), the zone controller (wing-anti-ice data — the 150 °C inhibit of article 03), the CMC and the FADEC.

[!warning]- APU leak monitoring is a BMC 1 solo The APU loop's analogue wiring reaches BMC 1 only. BMC 2 hears about APU leaks second-hand over the cross-talk bus. Remember this when weighing what a BMC 1 failure really costs — and why the pre-opening loop test of article 04 is specifically BMC 1's job.

2. One BMC fails: four alerts saved, three things lost

Per FCOM DSC-36-10-70:

If one BMC is failed the other BMC takes over monitoring of the bleed system and ensures the following ECAM warnings: ‐ ENG BLEED FAULT (overpressure and overtemperature only) ‐ WING LEAK ‐ APU BLEED LEAK ‐ BLEED LO TEMP (if wing anti ice is on)

Nevertheless the associated FAULT light on the AIR panel is lost, and the bleed valve does not close automatically. ENG BLEED LEAK warning is lost for the associated engine.

The logic falls straight out of the wiring: the regulated-pressure transducer and the temperature sensor feed both computers (section 3), and the wing loops cross over — so those alerts survive. But the pylon loop, the FAULT-light discrete and the solenoid-closure line are the dead computer's private wires. The sharpest item on the lost list is the middle one — monitoring still shouts, execution has left the room — and it is precisely why the AIR BMC 1(2) FAULT procedure branches the way it does (from the FCOM procedure: APU bleed selected ON → set the affected engine bleed OFF; APU bleed OFF → leave it ON). With APU air flowing, the affected side's bleed valve should auto-close and now cannot; the crew's finger replaces the dead computer. Full treatment in article 09.

3. Three sensors, three job descriptions

The transferred-pressure transducer (upstream of the PRV, downstream of the HPV; wired to its own side's BMC only). Per AMM 36-21-00:

The main function of the bleed transferred pressure transducer 9HA1(2) is: - To give the ENG HPV NOT OPEN FAULT ECAM warning if the transferred pressure is less than 20 psig during more than 15 sec. and if the HP bleed valve is fully closed. If the transducer 9HA1(2) is defective, there is no effect on the ECAM bleed pressure indication.

Read the logic: HPV confirmed fully closed (via the EEC/EIVMU chain) and barely 20 psig upstream of the PRV — meaning IP isn't carrying the load either, so this engine has no bleed at low power. Hence the alert's laconic STATUS line, NO BLEED 1 (2) AT LOW PWR (article 09). And the closing sentence answers a common question: the pressure you see on the BLEED page is not this sensor's.

The regulated-pressure transducer (downstream of the PRV; wired to both BMCs) holds four jobs. Per AMM 36-21-00:

- To give the AIR ENG BLEED FAULT ECAM warning if the regulated pressure is more than 60 psig during more than 25 sec.

- To give the WING ANTI ICE INOP ECAM status (that comes from the zone controller through the BMC 1 or 2) when there is not sufficient bleed air to supply the wing anti-ice system, (when the regulated pressure is out of the 08/60 psi range).

- To transmit the regulated pressure data for display on the BLEED ECAM page: Green: Normal Values. Amber: if PR<4 psig and/or exist overpressure, PR>60 psig

…plus partnering the transferred sensor for the HPV NOT OPEN case. Note the two 60-psig clocks: the BMC closes the valve at >60 psig sustained 15 s (article 02), and confirms the alert at 25 s — close first, then declare, because if closure worked the pressure would already be falling. And the 8–60 psi window doubles as the definition of "enough air for wing anti-ice".

The pair together yields the chapter's most instructive detection table:

To give the PRV LOW REGULATION FAULT ECAM warning if complete any of the next statements: PR<10 psig AND PT>=44 psig AND T>255 deg. C PR<32 psig AND PT>=44 psig AND T>230 deg. C PR<(PT-12) psig AND 13 psig <= PT<44 psig AND T>230 deg. C

Three rows, one meaning: upstream pressure healthy, downstream abnormally low, and hot. Why must temperature be part of every row? Because a low regulated pressure with high temperature is not always a failure — at 235 °C the control solenoid legitimately cuts the pressure setting (article 02). The detection thresholds (230/255 °C) are drawn tight around that legal cut-back line, separating "solenoid doing its job" from "PRV regulating sick".

The outlet-temperature sensor (downstream of the precooler). Per AMM 36-21-00:

The sensor has two platinum resistance wires. Each wire transmits a signal to the BMCs.

In case of failure of both sensing elements of the exchanger outlet temperature sensor, the BMCs select for their internal logic a substitution temperature value of 200 deg. C.

[!warning]- Blind thermometers assume "normal", not "worst case" Both wires dead → the BMCs substitute exactly 200 °C, the regulation target. Assume-the-worst would trip the overtemperature closure and kill a perfectly healthy bleed system; assume-normal freezes the logic in a benign state and leaves the fault for maintenance. A deliberate inversion of the usual fail-safe instinct, and a fine exam question.

Power supplies split the same way as everything else in this chapter: one side's monitoring rides the essential bus, the other a normal bus.

4. Reading the BLEED page like an owner

The page pops up uninvited for exactly four reasons — per AMM 36-20-00, If a failure occurs (overpressure, overtemperature, incorrect valve position, duct leak) the BLEED page is automatically shown on the SD. Field by field, per FCOM DSC-36-20:

Engine bleed valve symbols. Open in-line green, closed cross-line green, plus two amber disagree states. The note attached to the closed-and-green case prevents a thousand unnecessary radio calls:

Note: In certain circumstances (such as different engine setting, or one minor bleed valve regulation drift), it is possible that one bleed valve on one side closes and will be indicated closed and green on the BLEED SD page. There is no operational impact on the bleed system, provided there is no associated AIR ENG 1(2) BLEED FAULT alert.

With the crossbleed open or thrust split between engines, the higher-pressure side can legitimately push the other side's PRV closed within the 44–52 band. Green-closed is the system's own housekeeping; amber is the only colour worth chasing.

HP valve symbols — five states that encode context: fully closed, not fully closed, a disagree, and two "closed/not-closed with the engine not running or the pushbutton off" combinations. The symbol grammar asks not just "where is the valve" but "should it be there, given the engine state".

The anti-ice arrows. Amber when the valve is open and any of three conditions holds:

The valve is open, and at least one of the following conditions is met: ‐ Bleed air pressure high or low ‐ Wing anti-ice pb is at OFF position ‐ Open for more than 35 s, while the aircraft is on ground.

That 35-second clause pairs with ATA-30's ground rule — the wing anti-ice valves open on the ground for only 30 s (test sequence) per FCOM DSC-30-20-20 — so an arrow still open at 35 s means the test should have ended and didn't.

Crossbleed and APU valve symbols. The crossbleed shows open/closed in green, two amber disagrees, and an in-transit state. The APU valve is displayed only if the APU MASTER SW is ON — a disappearing symbol after APU shutdown is not missing data.

Precooler figures. Inlet pressure: It becomes amber, if lower than 4 PSI, or in case the BMC detects an overpressure (above 60 PSI). Outlet temperature goes amber on the overheat ladder — 290 °C for more than 5 s, or 270 °C for more than 15 s, or 257 °C for more than 55 s — the very same gates as the BMC's closure logic, so an amber temperature means the shutdown countdown is already running; or on low temperature (below 150 °C, in flight, wing anti-ice on, PRV open — However, low temperature may only be due to low outside air temperature.), with the idle-and-cold note covered in article 03.

GND and the memo. A GND flag Indicates that it is possible to connect the HP ground air supply. The green APU BLEED memo appears when the APU is available with its pushbutton on — the timing cue the shutdown flow waits for in article 07.

5. Four tiers of alert, and lights with a memory

The AMM's failure-condition section sorts the chapter's alerts by what accompanies them — a taxonomy that predicts procedure weight better than alphabetical order ever could (per AMM 36-20-00, condensed):

The pattern: the more the crew must act, the louder the annunciation — tier 4 is entirely populated by "a valve has been closed automatically, or needs your hand", and the FAULT light literally marks which pushbutton that hand should find. Which makes the tier-1 quietness of BMC FAULT worth respecting: one line of text, no chime, yet it means the side's automatic protection is gone (section 2).

The FAULT light itself latches. Per AMM 36-20-00:

On the AIR control panel 225VU, when the FAULT legend on an ENG BLEED pushbutton switch comes on (the bleed valves of the related engine close automatically), it stays on until you push the pushbutton switch. When you push the pushbutton switch, the OFF legend comes on. The FAULT legend goes off (if the failure which caused it to come on is cancelled).

Two necessary conditions to extinguish it: the pushbutton pressed to OFF and the causal failure gone. The FCOM panel section adds the operationally vital half-sentence — OFF also means The FAULT light and the autoclosure signal are reset. That reset of the latched autoclosure is the entire mechanism behind the "OFF THEN ON" recovery in article 09. Leak-family lights follow a different master — the hot spot itself — and are covered with their procedures in article 08.

For completeness, the pushbutton's opening conditions (the five-way AND from FCOM DSC-36-20): upstream pressure above 8 PSI; APU BLEED off or APU valve closed; no onside wing/pylon leak, overpressure or overtemperature; ENG FIRE pushbutton not released out; engine start valve closed. The last one is why the bleed valve sits closed throughout a start without anyone commanding it.

6. The daily habit

Cruise SD review is an SOP line item, and the BLEED page is on the list (per FCOM PRO-NOR-SOP-15, review BLEED : BLEED parameters). Five seconds buys the whole picture: pressure green in the forties, temperature green near 200, both valve pairs in-line, crossbleed matching configuration, no LEAK legends. Every abnormal in articles 08–10 announces itself on this page first.

Self-test

[!note]- Q1. Which BMC inputs/outputs are BMC 1 exclusives, and how does the HPV's position reach a BMC at all?

The APU leak loop (analogue) and the APU BLEED pushbutton FAULT drive are BMC 1 only. The HPV has no direct line: its position goes EEC → engine interface unit → BMC as digital data.

[!note]- Q2. One BMC fails. Recite "four saved, three lost" and explain the procedure branch it drives.

Saved by the opposite BMC: ENG BLEED FAULT (overpressure/overtemperature only), WING LEAK, APU BLEED LEAK, BLEED LO TEMP (anti-ice on). Lost: the side's FAULT light, automatic bleed-valve closure, and ENG BLEED LEAK. Because auto-closure is lost, with APU bleed ON the crew selects the affected side's bleed OFF by hand; with APU bleed off it stays ON.

[!note]- Q3. The BLEED page pressure figure — which sensor, and what makes it turn amber?

The regulated-pressure transducer (downstream of the PRV, wired to both BMCs). Amber below 4 psig or above 60 psig.

[!note]- Q4. Why does every row of the PRV LOW REGULATION table include a temperature condition?

Because a deliberately reduced pressure is legal above 235 °C — the control solenoid's cut-back. The 230/255 °C terms separate that lawful behaviour from a genuinely sick regulator.

[!note]- Q5. Both temperature-sensing wires fail. What number do the BMCs use, and why is that the right choice?

They substitute 200 °C — the regulation target. Assuming worst-case would false-trigger the overtemperature closure and cost a healthy bleed system; assuming normal freezes the logic benignly and defers to maintenance.

[!note]- Q6. A bleed valve shows closed and green in cruise. What two questions decide whether you care?

Is there an associated AIR ENG BLEED FAULT alert, and is the symbol amber? Green-closed with no alert is documented normal behaviour (thrust split or minor regulation drift) with no operational impact.

[!note]- Q7. What two conditions must both be true for an ENG BLEED FAULT light to extinguish, and what else does pressing OFF reset?

The pushbutton must be pressed (OFF legend on) and the causal failure must have ceased. OFF also resets the latched autoclosure signal — the enabler of the OFF-THEN-ON recovery.

Key takeaways

References

BMC functions, the four FAULT-output conditions and EIVMU routing per AMM 36-11-00; BMC wing assignment, sensor locations and wiring, the transferred/regulated transducer functions with their thresholds (20 psig/15 s; 60 psig/25 s; 8–60 psi; the PRV LOW REGULATION three-row table), dual-wire temperature sensing and the 200 °C substitution per AMM 36-21-00. Interface inventory (discrete/analogue/digital, BMC-1 exclusives), BLEED-page auto-display conditions, the four-tier failure-condition taxonomy and FAULT-legend latching per AMM 36-20-00. Panel logic, pushbutton opening conditions, all BLEED-page field semantics (valve symbol states, the green-closed note, anti-ice arrow conditions with the 35 s ground clause, precooler amber gates 4/60 psig and 290-5 s/270-15 s/257-55 s, low-temperature display, GND flag, APU BLEED memo) and single-BMC-failure consequences per FCOM DSC-36-20 and DSC-36-10-70; the 30 s ground anti-ice test per FCOM DSC-30-20-20; cruise SD review per FCOM PRO-NOR-SOP-15. The alert-tier table and the "saved/lost" 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.