Engine Bleed I: Stage Selection and Pressure Regulation

Each engine bleed system has three jobs, and the FCOM states them in one breath. Per FCOM DSC-36-10-20:

Each system is designed to: ‐ select the air source compressor stage ‐ regulate bleed air pressure ‐ regulate bleed air temperature

This article covers the first two — which compressor stage the air comes from, and how its pressure is tamed on the way out. Temperature is article 03. One theme runs through everything here: the pressure chain is almost entirely pneumatically self-governing. Springs, diaphragms and sense lines do the regulating; the computers (BMC on the wing side, EEC on the engine side — article 01) intervene only to shut things off. That is why these valves fail "stuck open" or "stuck closed" rather than "regulating wildly", and why the MEL can be surprisingly generous with them (article 11).

 Three-shaft compressor
 ┌─ IP port (8th stage) ──► IPC (IP check valve, 5.5 in, split flapper) ──┐
 │                                                                        ├─► PRV (bleed valve, 6 in)──► precooler ──► wing
 └─ HP port (14th stage) ─► HPV (HP bleed valve, 5 in butterfly) ─────────┘        ▲                          │
                              ▲ regulates 40 psig                    regulates 44-52 psig            OPV (6 in, normally open)
                              │ EEC ch A/B dual-coil solenoid              │ control solenoid            closes >85 psig
                              └── relay ◄── BMC ───────────────────────────┘ (shutoff; >235 °C pressure cut-back)

1. IP by preference, HP by necessity

Per FCOM DSC-36-10-20:

Air is normally bled from the intermediate pressure stage (IP) of engine HP compressor, to minimize fuel penalty.

When pressure from IP is not sufficient (low engine speed), air is bled from the high pressure (HP) stage thru the HP valve which limits downstream pressure to 40 ± 4 PSI .

Two intermediate pressure check valves, mounted downstream of each IP port, close to prevent air from HP stage being circulated to the IP stage.

Why IP is cheaper: air tapped after 14 stages of compression cost the engine 14 stages of work; air tapped after 8 stages cost 8. Same kilogram of bleed, very different fuel bill. So the system's instinct is IP whenever possible, HP only when IP sags — and IP sags at low engine speed, classically in an idle descent. The AMM turns this into a three-case table worth memorising. Per AMM 36-11-00:

There are three cases of pneumatic operation: - HP stage pressure lower than 40 psig (average value): Air is bled from the HP port through the HP bleed valve which is fully open. The IP bleed check valve is closed to prevent any air recirculation through the engine. - HP stage pressure higher than 40 psig and lower than 185 psig, and IP stage pressure lower than 40 psig: Air is bled from the HP port through the HP bleed valve which regulates the downstream pressure at 40 psig. The IP bleed check valve is closed to prevent any air recirculation. - IP stage pressure higher than 40 psig: If the solenoid of the HP bleed valve is not energized, air bleed transfer from the HP port to the IP port is pneumatically achieved. The IP bleed check valve is open.

The handover is seamless and purely pneumatic: When the IP port pressure exceeds the HP bleed valve target value, the HP bleed valve closes. Air bleed transfer from the HP port to the IP port is pneumatically achieved (AMM 36-11-00). And reverse-flow protection is double-banked: A reverse flow protection is ensured by the IP bleed check valve and the HP bleed valve which close when the downstream pressure is higher or equal than the upstream pressure. What that guards against becomes obvious the first time another source pressurises the manifold — APU, opposite engine or ground cart pushing 40-plus psi backwards toward a compressor. It is also the mechanical reason the crossbleed-start procedure switches the receiving engine's bleed off (article 07).

2. The HP valve — a triple-natured butterfly

Per AMM 36-11-00, the HPV's four duties:

This valve mainly provides: - control of the engine compressor HP stage air bleed - downstream limitation of the relative static pressure of the air bled from the HP stage - reverse flow protection of the engine compressor HP stage - ON/OFF control of the HP air bleed.

And its construction:

The HP bleed valve is a 5 in. dia butterfly type valve. The HP bleed valve is normally spring-loaded closed in absence of upstream pressure. The valve is not fully closed when upstream pressure is higher than 12 psig. Valve operation is pneumatic (pressure control), mechanical (safety) or electropneumatic (closure).

Three natures: pneumatic for the regulating (40 ± 2 psig at minimum airflow, 40 ± 4 at maximum), mechanical for the built-in safeties, electro-pneumatic only for commanded closure. Two of the safeties need no computer at all. Per AMM 36-11-00:

The valve is pneumatically controlled to the closed position when upstream pressure is higher than 185 plus or minus 5 psig. The valve is pneumatically controlled closed in case of potential reverse flow.

185 psig is the ceiling of Case 2: at high power the HP stage delivers enormous pressure, and the valve simply shuts its own door — IP is more than sufficient by then anyway.

Commanded closure comes through a dual-coil solenoid powered redundantly by EEC channel A or B. When does the EEC act? Per AMM 36-11-00:

1 Automatic closure control by the redundant EEC channel A or channel B when: - the bleed valve closure is controlled by the BMC (through 14HA relay) except in case of action on the ENG Fire Pushbutton. - the HP compressor outlet pressure P30 is higher than 85 psig, the wing anti ice system is not selected on and the altitude is higher than 26.000 ft - the HP compressor outlet pressure P30 and the HP compressor exit temperature T30 is higher than 75 psig and 430°C.

Read the three conditions as three personalities. The first is guilt by association — if the BMC has shut the downstream PRV, the EEC closes the HPV too, via the relay. The second is a high-altitude economiser — cruise with anti-ice off and healthy P30 means HP extraction is locked out. The third is a temperature fuse — air that is both that hot and that pressurised has no business entering the manifold. The EEC can also force an early transfer to IP by energising the same solenoid, but with one veto: per AMM 36-11-00, The transfer to the IP stage controlled by the EEC closing the HP bleed valve, is inhibited when the wing anti-ice is selected ON. Anti-ice wants air that is generous and hot; fuel-saving transfers wait.

[!warning]- The fire-pushbutton exception Look again at condition 1: except in case of action on the ENG Fire Pushbutton. Pressing the fire pushbutton does not close the HPV through this electrical path — deliberately. The fire-protection chain is designed never to depend on a computer: the fire pushbutton closes the PRV directly (section 4), the FADEC is being de-powered anyway, and the HPV then closes itself pneumatically when the dead-ended duct back-pressures it. Every link in the fire chain is either hardwired or physics.

For the maintenance-minded: the butterfly has a mechanical position indicator with a threaded hole and an air-vent knurled screw that can lock it closed — the physical act behind the MEL's "HPV deactivated in the closed position" (article 11) — plus a dual microswitch reporting fully-closed/not-fully-closed to the EEC.

3. The IP check valve — dumbest part, hardest guarantee

Per AMM 36-11-00:

The IP bleed check valve is a 5,5 in. dia. valve of the split flapper type. This valve, located on the bleed IP stage, prevents recirculation of the HP air towards the IP stage when the HP bleed valve is open.

Rotation in the opening direction is limited by a stop pin which prevents the two flaps from being positioned on the same side of the shaft and which therefore ensures the check valve function in all cases.

No control, no indication, no electrics — the only component in the chain that is never on any bus. The stop pin is the detail worth teaching: if the two half-flaps could ever swing past the shaft centreline together, the valve would become a permanently open door. One pin makes that geometrically impossible. The most critical protections in this chapter keep landing on the simplest mechanisms — a pattern that repeats with thermal fuses (section 4) and eutectic-salt sensing elements (article 06).

4. The PRV — shutoff and regulator in one body

Per FCOM DSC-36-10-20:

Downstream of the junction of HP and IP ducting, air is admitted into the bleed valve. This bleed valve acts as a shut-off, and as a Pressure Regulating Valve (PRV). Delivery pressure is regulated between 44 and 52 PSI , depending on the flow.

The AMM adds the nominal figure — the sub-system keeps delivery at or below 48 psig (cruise normal flow). A useful mnemonic: 44–48–52 — cruise sits near 48, the band runs 44 to 52 (higher flow, lower pressure and vice versa; any green "4x" on the BLEED page is healthy — article 05).

Opening threshold and closure paths, per AMM 36-11-00:

The valve is normally spring-loaded closed in the absence of upstream pressure. A minimum upstream pressure of 8 psig is necessary to open the valve. The valve is pneumatically controlled to the closed position by the valve thermal fuse in case of fire.

That 8 psig is the component-level truth behind the FCOM's "closed pneumatically if upstream pressure less than 8 PSI": with the engine shut down or barely turning, the PRV physically cannot lift — no command needed. The FCOM's complete closure inventory is worth quoting whole, because half the abnormal procedures in article 09 trace back to it:

The bleed valve is fully closed : ‐ Pneumatically in case of : • Upstream pressure less than 8 PSI, or • Engine fire ‐ Electrically through the: • BLEED pushbutton, when switched OFF • ENG FIRE pushbutton when pushed • BMC, in the following cases : ‐ Overtemperature ‐ Overpressure ‐ Leak detection ‐ APU bleed ON (for Engine 2, provided crossbleed valve is not closed). ‐ Starting sequence ‐ Engine shutdown

[!warning]- Fire closes this valve twice, by two unrelated mechanisms "Engine fire" appears under pneumatically (a thermal fuse on the valve melts and dumps the actuating pressure) and the ENG FIRE pushbutton appears under electrically. Even with the aircraft fully dark, flame itself will close the bleed valve. Same philosophy as the HPV's fire exception: no single wire, no single computer, stands between a fire and a closed bleed.

[!warning]- The APU-bleed line item has a precise bracket BMC closure for "APU bleed ON" applies to engine 2 provided crossbleed valve is not closed. The AMM spells out both cases: X-BLEED selector in AUTO with APU bleed on closes both engine bleeds; selector in CLOSE closes only system 1 — the right side stays on engine 2's own air because the APU's air cannot reach it. A deliberate half-and-half configuration that reappears in one MEL procedure (article 11).

Also in the AMM inventory: the BMC treats HP FUEL shut off valve indicated closed as its engine-shutdown cue — pull the master lever and the bleed valve follows the fuel valve immediately, without waiting for the spool to run down.

Three pressure tappings live on the PRV body, and their names matter for the rest of the chapter: upstream pressure feeds the transferred-pressure transducer, the downstream annular chamber feeds the regulated-pressure transducer, and the third connects the regulator to its control solenoid. Transferred before the PRV, regulated after it — the pair returns in force in article 05.

[!warning]- The PRV does NOT protect against reverse flow Per AMM 36-11-00: The bleed valve does not ensure the reverse flow protection. It is a butterfly; reverse pressure can push it open. Blocking backflow is the IPC's and HPV's job upstream. This is why, during a crossbleed start, the receiving engine's bleed pushbutton is selected OFF — the electrical closure does what the valve's own mechanics cannot (article 07).

The control solenoid — one hand, one brain. Mounted in the duct downstream of the precooler exchanger and connected to the PRV by a pneumatic sense line, it has two functions (AMM 36-11-00). As a hand: energised by the BMC, the BLEED pushbutton or the FIRE pushbutton, it vents the regulator's pressure signal and the spring closes the valve — it starves the valve rather than pushing it. As a brain:

The bleed valve control solenoid reduces the bleed valve regulated pressure when the temperature exceeds 235°C.

This is the component behind the FCOM's cryptic "The pressure can be reduced, in case of over temperature at the precooler inlet." Before anyone kills a whole bleed system for overtemperature, the solenoid quietly reduces the pressure setting — less flow through the precooler, easier cooling, temperature recovers. Only if that fails does the BMC escalate to closure on the 257/270/290 °C ladder (article 03 and article 05). Pressure cut-back first, shutdown second.

5. The OPV — an all-pneumatic last gate

Per AMM 36-11-00:

The overpressure valve (6 in. dia) protects the downstream pneumatic system if the bleed valve does not operate correctly (overpressure). The overpressure valve is entirely pneumatic and normally open.

- the valve is normally fully open when the upstream pressure is lower than 75 psig - the valve is controlled to the closed position when the upstream pressure is higher than 85 psig plus 0, minus 10 psig. After closure due to overpressure, the valve opens again as soon as the upstream pressure is 52 psig plus or minus 3 psig.

The FCOM gives only the 85 PSI closure; the AMM reveals the hysteresis loop — close above 85, reopen only back at 52 ± 3. A 33-psi dead band so the valve cannot chatter at the threshold, and 52 is no accident: it is the top of the PRV's normal band, so the gate reopens exactly when pressure is provably normal again. Note whom it protects: the downstream network (packs, anti-ice ducting), not the engine. If a runaway PRV defeats both its own regulation and the BMC's electrical closure, this computer-free valve is what stands between 85-plus psi and the manifold. The MEL's remarkably relaxed treatment of a failed OPV — dispatchable with no history of overpressure on the previous flight — is priced on exactly this layering (article 11).

6. The family portrait

The crew's entire authority over this chain is one pushbutton per side: ENG BLEED. ON delegates to the automatics; OFF closes PRV and HPV together and — critically — resets the FAULT light and the autoclosure signal (FCOM DSC-36-20). That reset is the mechanism that makes "OFF THEN ON" a legitimate recovery attempt in the ENG BLEED FAULT procedure, and the whole story of article 09.

Self-test

[!note]- Q1. State the three pneumatic cases with their boundary numbers. Which case is an idle descent?

Case 1: HP stage <40 psig — HPV fully open, IPC closed. Case 2: HP 40–185 psig and IP <40 — HPV regulates at 40 psig, IPC closed (idle descent lives here). Case 3: IP >40 psig — HPV closed, IPC open, transfer purely pneumatic.

[!note]- Q2. Give the EEC's three automatic HPV-closure conditions and the one exception.

BMC-commanded PRV closure via the relay (except when the ENG FIRE pushbutton is pressed); P30 >85 psig with wing anti-ice off above 26 000 ft; P30 >75 psig with T30 >430 °C. The fire-pushbutton case is excepted because the fire chain must not depend on the FADEC — the HPV then closes pneumatically on back-pressure.

[!note]- Q3. By what two independent mechanisms does an engine fire close the PRV?

A thermal fuse on the valve melts and vents the actuating pressure (pure pneumatics — works with the aircraft unpowered), and the ENG FIRE pushbutton commands electrical closure through the control solenoid.

[!note]- Q4. What are the PRV's opening threshold and regulation band, and why doesn't it block reverse flow? Who does?

Opens at 8 psig upstream minimum; regulates 44–52 psig (nominal ≤48 in cruise). It is a butterfly valve that reverse pressure can push open — backflow protection belongs to the IP check valve and the HPV, both of which close when downstream pressure meets or exceeds upstream.

[!note]- Q5. What happens at 235 °C, and how does it relate to the 257/270/290 °C ladder?

The control solenoid reduces the PRV's regulated-pressure setting — a flow cut-back that helps the precooler recover. The 257/270/290 °C ladder is the next tier: BMC-commanded closure of the valve if temperature keeps rising. Cut-back first, shutdown second.

[!note]- Q6. Quote the OPV's closure and reopening values. Why the 33-psi gap, and why reopen at 52?

Closes above 85 +0/−10 psig; reopens at 52 ± 3 psig. The dead band prevents chattering at the threshold, and 52 is the top of the PRV's normal band — the gate reopens only when pressure is demonstrably normal again.

Key takeaways

References

Design tasks, IP/HP offtake with 40 ± 4 PSI limitation, PRV dual role with the 44–52 PSI band, the OPV 85 PSI closure and the complete bleed-valve closure inventory per FCOM DSC-36-10-20. Three-case pneumatic operation, pneumatic transfer and double reverse-flow protection, HPV construction and thresholds (12 / 40±2 / 40±4 / 185±5 psig) with EEC dual-coil closure logic and the wing-anti-ice inhibit, IP check-valve construction, PRV construction (8 psig, thermal fuse, three pressure tappings, no reverse-flow duty), control-solenoid functions with the 235 °C cut-back, and OPV characteristics (75 / 85 +0/−10 / 52±3 psig) per AMM 36-11-00 (Description and Operation). Pushbutton reset behaviour per FCOM DSC-36-20. The three-case table, "family portrait" summary and the personality framings are integrative syntheses of the referenced text. MEL implications reflect some operators' MEL practice and are developed in article 11.

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.