APU Bleed, Crossbleed and Ground Air

Article 01 drew the H-shaped network; articles 02–03 covered the two vertical strokes. This one covers the crossbar: how APU air enters the network, how the crossbleed valve partitions it, and the etiquette of ground carts. Three questions organise everything — why the APU needs no regulation hardware at all, why the crossbleed valve carries two motors, and when a crew is actually supposed to turn that rotary selector out of AUTO.

1. APU bleed: why "no regulation required" is a feature

Per FCOM DSC-36-10-30:

Air supplied by the APU load compressor is both available on ground and in flight. APU bleed air is controlled by the APU bleed valve, which operates as shutoff valve. It is electrically-controlled and pneumatically-operated.

Set that against the engine side's regulation arsenal — HPV, PRV, precooler, thermostat — and the asymmetry begs a question the AMM answers in one line. Per AMM 36-12-00:

Temperature, pressure and flow regulation of the APU bleed air is not necessary. The pressure and flow regulation agrees with the user demand.

The reason is what the compressor is for. An engine compressor's day job is thrust; bleed is a tax skimmed off the side, so the offtake arrives at whatever state the throttle dictates and must be tamed downstream. The APU's load compressor exists solely to make service air — it produces exactly what the users ask for. Production volume is set by the inlet guide vanes; per FCOM DSC-49-10-20:

The IGVs control bleed air flow, and a fuel-pressure-powered actuator positions the IGVs. The ECB controls the actuator in response to aircraft demand.

There is one supporting act. Per AMM 36-12-00: The APU has a bleed valve and a surge control valve. — and the surge valve's job: The surge control valve keeps the pressure of the APU bleed air in this set tolerance. When demand collapses suddenly (a start finishing, a pack closing) the load compressor would otherwise surge into a dead-ended duct; the surge valve dumps the excess. IGVs decide how much to make; the surge valve handles the leftovers; the ECB runs both — the pneumatic chapter only borrows the result.

2. Pressing the pushbutton: a handshake across three computers

The pushbutton-level conditions, per FCOM DSC-36-20:

ON : APU valve opens provided: ‐ N > 95 % ‐ Altitude < 25 000 ft climbing or < 23 000 ft descending ‐ No leak detected on APU or LH bleed (Should a leak occur on the RH side, the X-bleed would close).

The AMM turns "opens" into a relay race. First checkpoint — per AMM 36-12-00:

the BMC 1 start a test of the sensing elements on the APU bleed-air duct and the left wing bleed-air ducts.

Only if that pre-opening loop test passes does BMC 1 tell the ECB to proceed. Second checkpoint, inside the ECB:

the ECB makes sure that APU is available to supply the bleed air (APU speed is more than 88 % (95% in the start condition) and there is no automatic shutdown-condition), - then the ECB supplies electrical power to the solenoid of the APU bleed valve, - the APU bleed valve opens

So: pushbutton → BMC 1 checks the pipes (leak loops on the APU duct and the left wing — the corridor its air will travel) → ECB checks the machine (speed, shutdown conditions) → solenoid → valve. Neither computer can open the valve alone.

[!warning]- 95 % to start supplying, 88 % to keep supplying Per AMM 36-12-00: During normal APU operation an ECB 59KD logic permits an APU speed decrease to 88 % before it stops the bleed air supply automaticly. The asymmetry is anti-chatter engineering: a heavy load (both packs plus a main-engine start) can drag the shaft momentarily below 95 %; without the 7 % cushion the supply would cut out at exactly the wrong moment.

[!warning]- During a main engine start, the leak veto is overridden Per AMM 36-12-00: If the BMC 1 finds a leak in the APU bleed-air ducts, while there is a Main Engine Start (MES) signal from the engines, it ignores the leak signal and tells the ECB to open the APU bleed valve. Mid-start, a rotor parked hot at half speed is judged the greater hazard than a few seconds of leaking supply — the start gets its air. The same start-window philosophy recurs throughout the leak matrix in article 06.

Automatic closures come in three families: leaks (APU duct, left wing, or pylon 1 — engine-start window excepted; per FCOM DSC-36-10-30 the valve automatically closes in the case of APU leak, left wing leak, or pylon 1 leak (except during engine start)); the altitude envelope (per FCOM DSC-49-10-20, If APU BLEED is set to ON, the bleed valve closes at 25 000 ft when climbing, and reopens at 23 000 ft when descending.); and the APU's own emergency or automatic shutdown. Notice the leak list is all left-sided — the APU duct runs forward along the left half of the network, so left-side fires sit on its delivery corridor. A right-wing leak doesn't touch the APU valve; closing the crossbleed is enough to wall it off.

One post-flight habit explained — per FCOM DSC-49-20, on a manual APU shutdown: If the APU BLEED was selected, the APU performs a cooling period of 85 s at 82 % speed, — an APU that has been supplying bleed runs hotter, and it takes a cool-down lap before stopping. The MASTER SW light staying on for a minute and a half is the machine showering, not a fault.

3. The APU check valve: a door that costs nothing to open

Per FCOM DSC-36-10-30: A non-return valve, located near the crossbleed duct, protects the APU when air is bled from another supply source. The AMM gives its almost comical sensitivity — per AMM 36-12-00:

The low-forced spring keeps the valve flaps in the closed position. The valve flaps open at approx. 0.002 bar (0.029 psi) differential pressure.

0.029 psi — essentially zero. This valve is not a restrictor; it is a direction. Forward flow costs nothing; reverse flow (engine or ground-cart air at 40-plus psi trying to drive back into a stopped APU) meets a closed flapper. Same family philosophy as the IP check valve in article 02. Hold onto this component: a stuck-open APU check valve is the villain of the APU LEAK FED BY ENG scenario (article 06 and article 08), and the MEL will demand it be physically dealt with — removed and blanked, or inspected before every flight (article 11).

4. The crossbleed valve: one disc, two motors, three personalities

Per FCOM DSC-36-10-50:

There is a crossbleed valve on the crossbleed duct. This valve enables the isolation or interconnection of LH (ENG1) and RH (ENG2) air supply system. The crossbleed valve is electrically controlled from a rotary selector that is located on the overhead AIR panel. Two electric motors control the crossbleed valve : One is for automatic mode, the other is for manual mode.

Component detail, per AMM 36-12-00:

The double motor actuator has two electric motors (primary and secondary), which work independently. The motors open and close the valve. One motor controls the automatic operation, the other motor controls the manual operation. Each motor has a brake system which locks the butterfly disk in position when the electrical power supply stops. Limit switches installed in the actuator automatically stop the electrical power supply to the motors when the valve is in the fully open or the fully closed position.

And the exam-grade detail — the power split. Per AMM 36-12-00:

The essential bus 401PP supplies 28VDC through the circuit breaker 2HV and the selector switch 3HV to the secondary motor of the crossbleed valve. The normal bus 206PP supplies 28VDC through the circuit breaker 1HV, the selector switch 3HV and the X-feed valve auto-ctl relay 4HV to the primary motor of the crossbleed valve.

[!warning]- The MANUAL motor rides the essential bus Design intent in one sentence: in an electrical emergency the automatic logic may die, but the crew's rotary selector must stay alive — so the manual (secondary) motor is fed from the essential bus while the automatic (primary) motor sits on a normal bus behind a relay. Add the brake-locks-in-place behaviour and the worst case is graceful: power fails, the disc freezes where it is, and a human hand finishes the job. This power split is also why, on batteries only, the valve will not open by itself — a fact with a hard operational consequence in the battery-start procedure (article 07).

Mode logic: AUTO slaves the valve to the APU bleed valve — per FCOM DSC-36-10-50, In automatic mode, the crossbleed valve is normally closed. It opens when APU bleed air is used. It closes, if the system detects an air leak (except during engine start). OPEN/CLOSE drive it directly. And the hierarchy is settled in a one-line AMM note: NOTE: The manual control prevails over the automatic control. — the selector outranks the software.

When is manual OPEN actually legitimate? The AMM names exactly four uses — per AMM 36-12-00:

You should only use this procedure in the subsequent cases: - the cross supply of the air-conditioning packs (the left engine supplies air to the right pack or the right engine supplies air to the left pack), - the start of one engine with bleed air from the other (but not during flight. Start by self-rotation is possible), - an engine bleed-air failure and WAI condition, - start of the right engine on the ground through the ground connectors or with the APU bleed-air supply.

Four scenarios, each traceable: cross-feeding packs; the ground crossbleed start (note the parenthesis — not in flight, where windmill relight makes borrowed air unnecessary, article 10); a failed bleed with anti-ice needed (article 09); and the geometry lesson from article 01 — ground connectors join left of the valve, so feeding engine 2's starter means opening the door by hand.

If the valve stops obeying, the AIR X BLEED FAULT alert hands the whole job to the manual motor — three-scenario management developed in article 09.

5. Ground air: self-catering guests at the left-hand table

Per FCOM DSC-36-10-40:

Air is supplied to the aircraft's pneumatic system via two HP ground connectors. The crossbleed valve has to be opened manually to provide air for both sides.

Per AMM 36-13-00, each connector is 3 in diameter, mounted in the bottom of the belly fairing, and Each connector contains a nonreturn valve and a built-in nipple. The flapper etiquette is self-managing: The HP compressed air pushes the spring-loaded flaps of the nonreturn valves open and the air flows into the aircraft pneumatic system. When the external air supply stops, a spring closes the valve flaps automatically. Thus no air flows out of the system when the engines or APU supply the bleed air. No caps to fit, no valves to select — the connector seals itself the moment the cart stops pushing.

One line of the limitations chapter belongs to this article, and it is the only ATA-36 limitation there is. Per FCOM LIM-AIR:

The flight crew must not use bleed air from the APU BLEED and from the HP Air Start Unit at the same time, to prevent any adverse effect on the Bleed Air System.

Two self-regulating sources shoving against each other in the same duct — pressure oscillation, flapping check valves, unpredictable flow reversals. The SOP-level implementation (check the ground crew and the BLEED page for cart pressure before switching APU bleed on) is in article 07, along with the crossbleed-start CAUTION that prohibits mixing engine bleed with external carts.

A last cockpit-indication footnote that saves confusion: the APU bleed valve symbol appears on the BLEED and APU pages only when the APU MASTER SW is on, and the APU page's bleed-pressure readout — per FCOM DSC-49-20 — shows an amber XX, when ADIRS 1 is not available or selected OFF. Relative pressure needs a static reference, and that reference happens to come from ADIRS 1. A dashed-out APU bleed pressure after a certain air-data switching problem is a plumbing-irrelevant display artefact (article 05).

Self-test

[!note]- Q1. Why does APU bleed need no pressure/temperature regulation hardware?

The load compressor exists only to make service air: the ECB positions the IGVs to match aircraft demand, so output follows the users; a surge control valve absorbs sudden demand collapses. Engine bleed, by contrast, is a byproduct of thrust-setting and must be regulated downstream.

[!note]- Q2. Walk the handshake from APU BLEED pushbutton to valve-open, naming both checkpoints.

Pushbutton signal → BMC 1 runs a sensing-element test on the APU duct and left-wing ducts (the delivery corridor) → if clean, BMC tells the ECB → ECB verifies APU speed (>88 %, 95 % in the start condition) and no auto-shutdown condition → energises the valve solenoid → valve opens and its position switch reports back to both.

[!note]- Q3. Which three leaks auto-close the APU bleed valve, and why are they all on the left?

APU duct leak, left-wing leak, pylon-1 leak (engine-start window excepted). The APU delivery duct runs along the left half of the network — those three zones are its corridor. A right-side leak is handled by closing the crossbleed instead.

[!note]- Q4. Crossbleed valve: which motor is on which bus, and what happens to the disc when power fails?

Manual (secondary) motor on essential bus 401PP; automatic (primary) motor on normal bus 206PP via the auto-control relay. Each motor's brake locks the disc in its current position on power loss — it neither drifts open nor slams shut, and waits for the hand-driven motor.

[!note]- Q5. Recite the four legitimate uses of manual OPEN. Which one is forbidden in flight, and why does one of them exist at all?

Cross-supplying packs; starting one engine from the other's bleed (ground only — in flight, windmill start is available); an engine bleed failure with wing anti-ice required; starting the right engine from ground connectors or APU supply. The last exists because the ground connectors feed the crossbleed duct left of the valve.

[!note]- Q6. What is the single ATA-36 limitation, and what does it prevent?

Never use APU bleed and an HP air start unit simultaneously — two independently regulated sources fighting in one manifold produce pressure oscillation and check-valve instability.

Key takeaways

References

APU bleed valve function and leak auto-closures per FCOM DSC-36-10-30; crossbleed valve, rotary selector and dual-motor statement per FCOM DSC-36-10-50; HP ground connectors and the manual-open requirement per FCOM DSC-36-10-40; pushbutton conditions and selector modes per FCOM DSC-36-20; the APU-bleed/HP-start-unit prohibition per FCOM LIM-AIR. Load-compressor demand-following, surge control valve, opening sequence with BMC-1 loop pre-test and ECB verification (95/88 %), MES leak override, check-valve construction (0.002 bar), crossbleed actuator construction with motor brakes and limit switches, bus assignments (401PP manual / 206PP automatic), the manual-prevails note and the four legitimate manual uses per AMM 36-12-00 (Description and Operation); connector construction and flapper behaviour per AMM 36-13-00. IGV control and the 25 000/23 000 ft envelope per FCOM DSC-49-10-20; the 85-second cooling period and APU-page indication notes per FCOM DSC-49-20. The "handshake" and "self-catering guest" framings 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.