Bleed & Surge Air — APU Bleed Valve, Surge Control Valve, 25000 ft Logic

The load compressor makes the bleed; this article is the bleed delivery and protection — two valves on the T-duct that switch the bleed on/off and protect the compressor from surge, plus the altitude logic the crew sees.

The APU load compressor supplies the APU bleed air... An APU bleed valve 59KH7 and a surge control valve 59KH18 are installed on the outlet orifices of the bleed air T-duct. The APU bleed valve 59KH7 stops or permits the bleed air flow... The surge control valve 59KH18 prevents a load compressor surge. The excess air passes through the valve to the exhaust cone. — AMM 49-51-00

1. Two valves, two jobs — and the division of labour with the IGV

Per AMM 49-51-00, the bleed air leaves the load-compressor scroll into a T-duct carrying two valves: the APU bleed valve 59KH7 (forward, on/off) and the surge control valve 59KH18 (aft, surge protection). The ECB controls both.

[!warning]- Three things control APU bleed — and they do different jobs Don't conflate them: the IGV (02) meters the quantity; the bleed valve switches the flow on/off (it does not control quantity); the surge control valve dumps excess air to prevent surge. Quantity / on-off / surge-protection — three separate functions.

2. APU bleed valve 59KH7 — on/off, normally closed

The APU bleed valve 59KH7 permits or stops the bleed air flow... It does not control the quantity of the bleed air flow. The APU bleed valve 59KH7 is a shutoff (butterfly) valve which is normally spring loaded closed. The valve opens pneumatically.

Per AMM 49-51-00, it is a butterfly shutoff valve, spring-loaded closed, opening pneumatically, ECB-controlled and monitored (a position switch → ECB → ECAM SD + a visual indicator on the shaft). The command path is the key detail:

The APU bleed signal comes from the APU BLEED P/BSW... The signal goes through the Bleed Air Monitoring-Computers (BMCs)... The BMCs cancel the APU bleed signal when they: ‐ receive an overheat signal from the overheat sensing elements (the signal shows that there is a leak in the bleed air ducts), ‐ find a fault in the sensing elements... When the APU bleed valve 59KH7 is in the closed position, the IGVs are also in the closed position.

[!warning]- The BMC cancels APU bleed on a duct overheat (leak) — a built-in leak protection The APU BLEED pb signal routes through the Bleed Air Monitoring Computers (BMC 1 & 2), which cancel it on a bleed-duct overheat (= leak) or a sensing fault (AMM 49-51-00) — so a leaking bleed duct automatically shuts APU bleed (ATA 36 leak protection). And when the bleed valve closes, the IGVs close too — no point flowing the load compressor if the bleed is shut off.

3. Surge control valve 59KH18 — fuel-powered, modulating

The surge control-valve bleeds the surge air from the bleed air to avoid a load compressor surge. The surge air flows to the APU exhaust cone... The surge control valve 59KH18 is a hydraulically operated modulating butterfly valve. It uses high pressure fuel supplied from the Fuel Control Unit (FCU) as the hydraulic power source. The ECB 59KD controls and monitors the position of the surge control valve.

Per AMM 49-51-00, the surge control valve is a modulating butterfly, fuel-pressure-powered (FCU), ECB-controlled; it dumps excess (surge) air to the exhaust cone to keep the load compressor out of surge, moving full-closed to full-open in a minimum ~0.2 s (fast).

[!warning]- A third fuel-powered actuator — and surge air rejoins the exhaust Like the IGV (02), the surge control valve runs on high-pressure fuel from the FCU, not air or electrics. The dumped surge air goes to the exhaust cone (01 heat shield / 09) — which is why the exhaust carries combustion gas plus load-compressor surge air. When bleed demand drops fast (e.g. MES ends), the surge valve opens to bleed off the now-excess air before the compressor surges.

Sensors: a total pressure sensor + a bleed flow sensor (59KH23) feed the ECB for bleed/surge control.

4. The 25000 ft / 23000 ft bleed logic

The ECB controls the APU BLEED valves. 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. (DSC-49-10-20)

Per DSC-49-10-20, with APU BLEED ON the bleed valve closes at 25000 ft climbing, reopens at 23000 ft descending — a 2000 ft hysteresis so it doesn't chatter around the threshold.

[!note]- Why APU bleed cuts out at altitude (integrative synthesis) APU bleed pressure/flow falls with altitude, and high-altitude bleed offers little while stressing the load compressor; the ECB therefore closes the bleed valve above 25000 ft (DSC-49-10-20). The 25000-up / 23000-down split is deliberate hysteresis. This is the altitude side of the APU-bleed availability the crew plans around.

5. Counterintuitive points

[!warning]- On/off, quantity, surge — three separate controls Bleed valve = on/off (normally closed, pneumatic open); IGV = quantity; surge control valve = surge protection (dumps excess to exhaust). Confusing them misreads the APU bleed system.

[!warning]- A bleed-duct leak auto-cancels APU bleed via the BMC The BMCs drop the APU bleed signal on a duct overheat/leak (AMM 49-51-00) — the bleed valve closes itself, and the IGVs with it.

Self-test

[!note]- Q1. The two T-duct valves and their jobs? Bleed valve 59KH7 (on/off, normally closed, pneumatic open — not quantity) + surge control valve 59KH18 (fuel-powered modulating, dumps surge air to exhaust).

[!note]- Q2. What three things separately control the bleed? IGV (quantity) / bleed valve (on-off) / surge control valve (surge protection).

[!note]- Q3. What cancels APU bleed automatically, and what closes with it? The BMC cancels it on a bleed-duct overheat/leak or sensing fault; when the bleed valve closes, the IGVs close too.

[!note]- Q4. The altitude bleed logic? APU BLEED ON: bleed valve closes at 25000 ft climbing, reopens at 23000 ft descending (2000 ft hysteresis).

Key takeaways

References

• AMM 49-51-00 (Bleed and Surge Air — Description and Operation) — bleed valve 59KH7 (butterfly, spring-closed, pneumatic open, on/off, BMC path, IGV closes with it); surge control valve 59KH18 (fuel-powered modulating, dumps surge air to exhaust, ~0.2 s); total-pressure + bleed-flow (59KH23) sensors.
• FCOM DSC-49-10-20 — APU BLEED ON: bleed valve closes 25000 ft climb / reopens 23000 ft descent.

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.