Bleed and Surge Air

The previous article showed how the load compressor uses its variable inlet guide vanes (IGVs) to set the quantity of bleed air. This article covers what happens to that air next: how it reaches the aircraft pneumatic system through one valve, how the compressor is protected against surge through a second valve, and how the system protects itself when a bleed duct leaks.

The whole subsystem is just two valves plus one flow sensor, but the two valves have opposite characters — and that contrast is the single most useful thing to carry away from this article. The APU bleed valve is a simple shutoff: pneumatically opened, spring closed, fail-safe closed. The surge control valve is a modulating valve: fuel-driven, fail-safe open. One says stop the air to be safe, the other says dump the air to be safe — both lean towards protecting the hardware, just in opposite directions.

"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 from the APU to the pneumatic system of the aircraft. The surge control valve 59KH18 prevents a load compressor surge. The excess air passes through the valve to the exhaust cone. The ECB 59KD controls and monitors the APU bleed air supply and the surge air flow."

Per AMM 49-50-00 / 49-51-00. Three roles are split cleanly: quantity belongs to the IGVs (see Load Compressor), on/off belongs to the bleed valve, surge protection belongs to the surge control valve. Throughout, the ECB 59KD controls and monitors both valves.

1. The bleed-air tee-duct — two valves, two destinations

Bleed air leaves the load-compressor scroll and enters a tee-duct mounted on the right side of the APU. The tee has two outlet orifices: the bleed valve at the forward end sends air forward to the aircraft, and the surge control valve at the aft end dumps surplus air to the exhaust cone.

"The bleed air flows from the scroll of the APU load compressor to the tee-duct which is installed on the right side of the APU. The APU bleed valve 59KH7 is installed at the forward end and the surge control valve 59KH18 at the aft end of the tee-duct."

The forward end of the bleed valve connects to the bleed air duct-elbow — itself part of the pneumatic system (ATA 36). When the bleed valve is open, APU bleed air flows through that elbow to the aircraft.

        APU load compressor scroll (ATA-49-02)
                      │
                      │ bleed air
                      ▼
        ┌──────── bleed air T-duct ────────┐
        │   (mounted right side of APU)     │
        │                                   │
        │  forward end          aft end     │
        │  ┌───────────┐    ┌────────────┐  │
        │  │ APU BLEED │    │ SURGE CTL  │  │
        │  │ VALVE     │    │ VALVE      │  │
        │  │ 59KH7     │    │ 59KH18     │  │
        │  └─────┬─────┘    └─────┬──────┘  │
        └────────┼────────────────┼─────────┘
                 │                 │
                 ▼                 ▼
        bleed air duct-elbow   APU exhaust cone
        (ATA 36)               (surge air dumped)
                 │
                 ▼
        pneumatic system (ATA 36):
        air-conditioning packs / main engine start

The bleed-and-surge subsystem has five primary components per the AMM: a total pressure sensor, a bleed flow sensor, the bleed air duct, the APU bleed valve, and the surge control valve. The first two feed the surge logic (Section 5); the last two are the muscle.

2. The two valves contrasted

This table is the article in one frame. Read it down the fail-safe row last — that is where the design intent lives.

[!warning]- Both valves fail safe, but in opposite directions — why both are correct

It looks contradictory: one valve fails closed, the other fails open. Both are right because "safe" means a different thing for each. The bleed valve's worst case is no bleed air — annoying, never dangerous — so on any loss of power or air it springs closed. The surge valve's worst case is a compressor surge — violent reverse flow that can damage the load compressor — so on a torque-motor failure it drives open and keeps dumping air. The rule to remember: the shutoff valve fails shut (conservative), the anti-surge valve fails open (relieving). Both lean towards protecting the hardware.

3. The APU bleed valve — shutoff only, pneumatic open, spring close

"The APU bleed valve 59KH7 permits or stops the bleed air flow from the APU to the pneumatic system of the aircraft. 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."

[!warning]- Misconception: "opening the bleed valve more sends more bleed air"

A common mental model is that the bleed valve throttles the supply — open it part-way for a little air, fully for a lot. That is wrong. The bleed valve has two states only, open or closed, and the AMM states explicitly that it does not control the quantity. Quantity is always the IGVs' job (see Load Compressor). Think of the bleed valve as the main isolation cock — it lets the air through or shuts it off, nothing in between. Quantity = IGVs; on/off = bleed valve; do not conflate the two.

How it opens — the APU's own compressor air

The actuator is pneumatic. A diaphragm splits the housing into a pressure chamber and a reference chamber. First-stage compressor discharge air (PCD 1 air) drives the pressure chamber to open the butterfly; a spring in the reference chamber closes it when that air is removed.

"Discharge air from the first stage of the engine compressor (PCD 1 air) supplies the pneumatic power to the pressure chamber. The PCD 1 air supply is controlled by the solenoid valve. There is a spring in the reference chamber. It moves the butterfly plate to the closed position and holds it there when no PCD 1 air is supplied to the pressure chamber."

A solenoid valve gates the PCD 1 air. When the ECB commands open, the solenoid admits PCD 1 air and the butterfly opens. When the ECB drops the command, the solenoid both cuts the PCD 1 supply and opens a bleed outlet, venting the trapped air to ambient:

"When the ECB 59KD stops the open command, the solenoid valve closes. It stops the flow of PCD 1 air to the opening chamber and opens a bleed outlet. The remaining pressurized air in the opening chamber flows through the bleed outlet into the ambient air."

Because the chamber depressurises through that small outlet, the valve closes smoothly but not quickly — there is no slam. This is also exactly why the bleed valve is fail-safe closed: any loss of electrical power or PCD 1 air lets the reference-chamber spring drive it shut, and the worst outcome is simply no bleed air.

Position feedback and the IGV interlock

A position indicator switch (a single microswitch, operated by the crank lever) sends the ECB discrete open / not-open signals — note it reports two states, not a continuous angle, because the valve only has two states. The ECB forwards the position to the EIS System Display. A visual position indicator (OPEN/CLOSED marks and a slot on top of the butterfly shaft) lets maintenance read the position directly.

One linkage worth carrying forward from the previous article: when the bleed valve is shut, the IGVs shut too —

"When the APU bleed valve 59KH7 is in the closed position, the IGVs are also in the closed position."

With no air being delivered, the inlet guide vanes close fully and the load compressor sheds its pneumatic load entirely. (The flow body has a 4.5 in inner diameter and is held by V-clamps — identification detail only.)

4. The APU BLEED command path — the BMC gatekeeper

Pressing the APU BLEED pushbutton does not wire straight to the ECB. The signal first passes through the Bleed Air Monitoring Computers (BMCs), which act as a gatekeeper and can cancel the request.

"The APU bleed signal comes from the APU BLEED P/BSW 7HV on the AIR overhead panel 225VU in the cockpit. The signal goes through the Bleed Air Monitoring-Computers (BMCs) 1 1HA1 and 2 1HA2. 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."

So selecting APU BLEED ON does not guarantee the valve opens. The BMCs first check whether the bleed ducts are leaking and whether the leak-sensing elements are healthy. If they detect a leak (an overheat signal) or a sensing-element fault, they cancel the request and the ECB never gets the open command — preventing hot bleed air from feeding a leak and damaging surrounding structure. This is the system's automatic leak protection.

There is one counter-intuitive exception that must be remembered:

"NOTE: If the BMCs finds a leak in the APU bleed air ducts, and there is a Main Engine Start (MES) signal from the engines, they also transmit these information to the ECB to open the APU bleed valve."

[!warning]- A detected leak does not always close the valve — engine start overrides it

The natural assumption is "leak detected → valve always closed." Not so. If there is a main engine start (MES) signal at the same time, the BMCs open the bleed valve despite the leak. The reasoning is a deliberate trade-off: an in-flight engine start can be the more urgent need (for example, recovering a failed engine), so the system accepts the leak risk to get the engine running. The exam trap is "does a detected leak always close the bleed valve?" — answer: normally yes, but with an MES signal it opens even with a leak.

The detection mechanism itself — the overheat sensing elements / loops and their temperature thresholds — belongs to the pneumatic-system leak monitoring in ATA 36. This article uses only the interface: overheat → BMC → close valve, with the MES override above.

5. The surge control valve and surge prevention

What surge is, and why a dedicated valve

A centrifugal compressor needs enough flow passing through it to run stably. If downstream demand falls away and the outlet is effectively choked while the compressor keeps spinning at speed, the flow inside the compressor can stall and even reverse violently — that is surge, accompanied by heavy vibration and a coughing of the airflow, and it can damage the compressor. The picture: like blowing hard into a blocked tube — the air cannot escape, so it surges back at you.

The defence is to always keep a minimum-flow path open for the compressor — dumping surplus air overboard (to the exhaust cone) even when the user does not want it — so the flow through the compressor stays above the surge line. That is exactly the surge control valve's job:

"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."

"For each opening angle of the IGVs there is a calculated bleed air flow. This bleed air flow is the permitted minimum to prevent a load compressor surge."

How the ECB knows it is approaching the surge line — a pressure ratio that reads as flow

The ECB does not measure flow with a flow meter. It infers flow from a pressure ratio:

"The ratio of the allowed differential pressure to the total pressure of the bleed air refers directly to the bleed air flow."

The bleed flow sensor carries two transducers — a total pressure transducer and a differential pressure transducer — and the ratio of differential to total pressure tracks flow directly (the dynamic share of the pressure rises with flow, a standard way to read flow from pressure).

"The ECB 59KD receives the signals about the differential and the total pressure of the bleed air from the bleed flow sensor-assy. The bleed flow sensor-assy is installed on the left side of the inlet plenum chamber. It has two transducers: - the total pressure transducer, - the differential pressure transducer."

The ECB computes the ratio in real time, compares it against the minimum flow it has stored for the current IGV angle, and modulates the surge valve to keep flow above that minimum:

"The ECB 59KD calculates the ratio of the differential and the total pressure during the APU operation. It compares this value to the value for the calculated bleed air flow which it has stored of the opening angle of the IGVs. Each difference between the momentarily allowed value stored in the ECB 59KD and the calculated value causes a control signal from the ECB 59KD to the surge control valve. The surge control valve 59KH18 corrects the surge airflow."

This is a continuous closed loop, which is why the valve is modulating rather than a simple on/off device. (The total pressure transducer is fed by the total pressure sensor on the load-compressor scroll, adjacent to the outlet; the differential transducer also reads static pressure through a channel in the scroll. The surge control law itself is developed further in ECB Control and Monitoring.)

How it is driven — fuel hydraulics, EHSV and LVDT

The surge valve shares its power source with the IGV actuator: high-pressure fuel from the Fuel Control Unit (FCU).

"The surge control valve 59KH18 is a hydraulically operated modulating butterfly valve. It uses high pressure fuel supplied from the Fuel Control Unit (FCU)"

An EHSV drives it — a first stage with a torque motor (TM) (spool plus flapper nozzle) and a second stage with a four-way sleeve servo valve — and an LVDT closes the position loop back to the ECB. The ECB supplies 28 V DC to the TM spool; with the servo valve in neutral, the piston is hydraulically locked. Movement is deliberately faster opening than closing:

"During the normal operation the surge control valve 59KH18 moves from the fully closed to the fully open position in a minimum time of 0.25 seconds and from the fully open to the fully closed position in a minimum time of 0.45 seconds."

Faster opening (0.25 s) than closing (0.45 s) because relieving surge is the urgent direction.

Fail-safe open

"The flapper nozzle remains in a mechanically biased position if the TM spool receives no power. With the flapper nozzle in this position the hydraulic actuator extends fully and moves the butterfly plate to the open position. The butterfly plate moves from the fully closed to the fully open position in maximum of 3 seconds. This prevents a load compressor surge in the case if a TM control failure occurs."

The safe direction here is open and venting: on a control failure the valve drives fully open within a maximum of 3 seconds and keeps dumping air — accepting a loss of bleed efficiency rather than risking surge damage. This is the mirror image of the bleed valve's fail-closed behaviour, but the logic is identical: both fail towards protecting the hardware. And because it opens so quickly, the AMM notes it needs no extra quick-dump function:

"The hydraulically operated surge control valve 59KH18 needs no extra quick dump function because it opens quickly."

Anti-freeze and drain details

When the APU is shut down in a cold environment, residual fuel in the valve could thicken or freeze and jam the torque motor. A strap-on heater prevents this:

"The TM from the EHSV is heated by a Strap On Heater when the APU Master SW is in the OFF position and the Service Bus 113XP is supplied with 115 V AC."

And because the actuator runs on fuel, a leak path is provided so escaping fuel cannot build up internally:

"There are two hydraulic-actuator output-shaft seals with a drain port between them. If the inner seal has a leak, the fuel leakage flows through the drain port to the APU drain system"

   ┌─ EHSV (electrohydraulic servo valve) ──┐   strap-on heater
   │  1st stage: torque motor (TM)            │   (anti-freeze: 115 V AC
   │  2nd stage: four-way sleeve servo valve  │    when MASTER SW OFF)
   └─────────────────────┬────────────────────┘
                         │ high-pressure fuel (from FCU)
                         ▼
   ┌──────────────────────────────────────────┐ ◄── fuel supply / return
   │  hydraulic actuator (double-action piston) │
   │  LVDT ──► ECB (position feedback)           │ ◄── drain port
   └─────────────────────┬──────────────────────┘     (inner-seal leak → 49-17)
                         │ linear output
                         ▼
   ┌──────────────────────────────────────────┐
   │  butterfly plate (flow body 4.5 in)        │
   │  between bleed air duct and surge air duct  │
   │  ────► APU exhaust cone                      │
   └──────────────────────────────────────────┘
   visual position indicator: OPEN / CLOSED slot

The valve is, in one line, a fuel-driven servo head on top of a venting butterfly. The fuel source is covered in Fuel and Control; the exhaust cone destination in Exhaust and Cooling.

6. Bringing up bleed air — the sequence and the 95 % / 88 % thresholds

Two speed thresholds bracket bleed-air availability, and they are not the same number. The APU must reach 95 % before it is available to supply bleed air; once it is supplying, the ECB tolerates a decay to 88 % before it stops the supply automatically.

"The APU start sequence is completed when the APU reaches 95 % speed" ... "Above 95 % speed the APU is available to supply bleed air and electrical power."

"During a normal APU operation a ECB 59KD logic permits an APU speed decrease to 88 % before the bleed air supply stops automatically."

[!warning]- 95 % to come available, 88 % to stay supplying — and why the gap

Pilots often remember a single "APU available" speed. There are two thresholds. Start completes and the APU becomes available to bleed at 95 %; but once it is already feeding bleed air, the supply is allowed to continue down to 88 % before the ECB cuts it. The 95 % → 88 % gap is hysteresis — it stops the bleed supply chattering on and off if speed hovers near the boundary while the APU is loaded.

The full bring-up sequence ties the previous sections together:

  APU reaches 95 % ──► AVAIL (EWD / SD / START pb show green AVAIL)
        │
        ▼
  Select APU BLEED pb ON (blue ON legend lights)
        │
        ▼
  BMCs test the leak-sensing elements on the APU bleed duct
  and the left (engine 1 & 2) bleed ducts
        │  no leak + elements serviceable
        ▼
  BMCs tell the ECB to open the bleed valve
        │
        ▼
  ECB checks: speed > 88 % (95 % at start) / no auto-shutdown / can supply
        │  power to the bleed-valve solenoid
        ▼
  Bleed valve opens ──► position switch reports OPEN to ECB and BMCs
        │
        ▼
  SD BLEED page + SD APU page show green open legend + bleed pressure

Per AMM 49-51-00, when the ECB confirms speed above 88 % (95 % in the start condition), no automatic-shutdown condition, and that the APU can supply, it powers the bleed-valve solenoid and the valve opens; the position switch then reports OPEN to both the ECB and the four BMCs. Selecting APU BLEED OFF removes the ground signal from the BMCs, they send an OFF to the ECB, the solenoid is de-powered, and the valve closes.

One hard cut-off applies regardless of the pushbutton:

"NOTE: If an APU emergency shutdown or an APU automatic shutdown occurs, the APU bleed air supply stops."

Any emergency shutdown or automatic shutdown stops the bleed supply.

7. Indications and two ECAM traps

The SD APU page shows bleed air pressure as a green digital value:

"green digital indication for bleed air pressure, with 2 PSI resolution. It is the difference between APU bleed air pressure and corrected average static pressure" ... "The digital indication can vary from 0 to 98 PSI."

It is replaced by amber XX when a pressure datum is missing. The valve position itself is shown per FCOM DSC-49-20:

"In line – Green : The APU valve is fully open. Crossline – Green : The APU valve is not fully open, and APU BLEED pb is OFF. Crossline – Amber : The APU valve is not fully open, if APU BLEED pb is ON."

Two display traps catch pilots out when bleed is OFF:

"When the APU operates in the APU BLEED OFF condition: - a random BLEED pressure figure is shown on the ECAM APU page. - the indication on the ECAM BLEED page must not be more than 12 psi."

So with bleed OFF, the number on the ECAM APU page is a random, meaningless figure — do not read anything into it — while the ECAM BLEED page should not exceed 12 psi (more than that suggests the valve is not fully shut and warrants investigation).

When a leak is detected, the cockpit presentation is unambiguous. The bleed valve closes and:

"the FAULT light in the APU BLEED P/BSW comes on amber, - the MASTER CAUT P/BSW comes on amber, - a single chime is heard"

The EWD shows AIR / APU BLEED LEAK / APU BLEED ... OFF; after the pilot selects APU BLEED OFF, the EWD STATUS shows INOP SYS: APU LEAK (per AMM 49-51-00). Full ECAM and indication detail is in Indicating.

8. When APU bleed becomes the lifeline — BLEED 1+2 FAULT

The highest-value use of APU bleed is the loss of both engine bleeds. With both engine bleed sources gone, cabin pressurisation depends on the APU, and the QRH branches on whether APU bleed is available:

"BLEED 1+2 FAULT (APU BLEED ON)" ... "MAX FL: 220"

"BLEED 1+2 FAULT (APU BLEED NOT AVAILABLE)" ... "MAX FL: 100 / MEA-MORA"

The presence of a serviceable APU bleed supply is the difference between being held at FL 220 and being held at FL 100 / MEA-MORA — the latter can crowd terrain on high airway segments, drive fuel burn up sharply, and even threaten reach to a diversion field. This puts a concrete number on the idea, from the APU Overview, that the APU is an airborne backup: in a dual engine-bleed failure, APU bleed is the pneumatic source that preserves high-altitude cruise capability. The full procedure is run from the QRH / ATA 36.

Self-test

[!note]- Q1. What does each valve do, and which one sets the bleed-air quantity?

The bleed valve 59KH7 stops or permits bleed air to the aircraft — it is shutoff only, fully open or fully closed, and does not control quantity. The surge control valve 59KH18 prevents load-compressor surge by dumping surplus air to the exhaust cone, and it is modulating. The quantity of bleed air is set by the IGVs (ATA-49-02), not by either of these valves.

[!note]- Q2. How does the bleed valve open and close, and which way does it fail?

It opens pneumatically — first-stage compressor discharge (PCD 1) air, gated by a solenoid, fills the actuator pressure chamber and drives the butterfly open. It closes by a spring in the reference chamber when the solenoid cuts the PCD 1 air and vents the chamber to ambient (closing smoothly, not quickly). It is fail-safe closed: any loss of power or PCD 1 air lets the spring shut it, and the worst case is merely no bleed air.

[!note]- Q3. How does the surge control valve move, and why does it fail open rather than closed?

It is driven by high-pressure fuel from the FCU through an EHSV (torque motor + sleeve servo valve), with an LVDT closing the position loop. It is fail-safe open: if the torque-motor spool loses power, the flapper nozzle sits in a mechanically biased position that drives the actuator fully open within a maximum of 3 seconds, so it keeps venting and prevents surge. It fails the opposite way to the bleed valve because the safe direction for surge protection is open and relieving — both valves still lean towards protecting the hardware.

[!note]- Q4. What is compressor surge, how does the ECB sense it coming, and how is it prevented?

Surge is the flow stall / violent reversal that occurs when too little air passes through a centrifugal compressor running at speed — it can damage the compressor. The ECB infers actual flow from the ratio of differential to total bleed-air pressure (which "refers directly to the bleed air flow"), compares it against the minimum flow stored for the current IGV angle, and modulates the surge valve to dump air to the exhaust cone whenever flow approaches that minimum — keeping flow through the compressor above the surge line.

[!note]- Q5. After pressing APU BLEED, what does the signal pass through, when is the request cancelled, what is the exception, and what do 95 % / 88 % mean?

The signal passes through the BMCs before reaching the ECB. The BMCs cancel the request if they detect a leak (overheat signal) or a sensing-element fault — automatic leak protection. The counter-intuitive exception: if there is a main engine start (MES) signal, they open the valve even with a leak (engine start takes priority). The thresholds: 95 % is the speed at which the APU becomes available to supply bleed after start; 88 % is the speed it is allowed to decay to while supplying before the ECB stops the supply automatically (95 → 88 is hysteresis against chatter).

Key takeaways

References

Per AMM 49-50-00 and 49-51-00 Description and Operation (the two valves and their location on the bleed-air tee-duct, the bleed valve's pneumatic actuator / PCD 1 / spring / solenoid and discrete position switch, the IGV-closed interlock, the BMC command path and leak cancellation with the MES exception, the surge control valve's modulating hydraulic operation / EHSV / LVDT / 0.25 s–0.45 s timings / fail-open behaviour / strap-on heater / drain, the bleed flow sensor and ΔP-to-total surge logic, the 95 % / 88 % bleed thresholds, the start / stop / leak-detection sequences, and the BLEED-OFF display notes); FCOM DSC-49-10-20 (air bleed system, ECB control of the bleed valve) and DSC-49-20 (ECAM bleed-valve position in line / crossline green / amber and the bleed-air pressure box); QRH 11.01A / 11.02A (BLEED 1+2 FAULT maximum flight levels with APU bleed available versus not available). The detailed surge control law and IGV–surge coordination are developed in ATA-49-07; the overheat sensing-element architecture and thresholds for leak detection belong to ATA 36.

Independent study material, not an Airbus publication. Refer to current operator FCOM, FCTM, AMM, and QRH for operational use.