Engine Air, Cooling & Surge Protection

Not all the air the compressors work so hard to pressurise goes to combustion or thrust. The engine taxes its own product first. Per the FCOM:

"The air bleed system is used for: ‐ The pneumatic system. ‐ Cooling the engine compartment and the turbine. ‐ Cooling the engine and IDG oil. ‐ Engine stability."

The first item — bleed air handed over to the aircraft for packs and anti-ice — is ATA-36's story and ends, for our purposes, at the engine bleed valve. The remaining three are this article's subject: the air the engine keeps for itself. And of the three, "engine stability" is the largest safety topic on the list. A compressor is a machine that passes air rearward through queues of aerofoils; give those aerofoils the wrong flow angle or the wrong flow quantity and they stall, and the whole compressor chokes at once — a stall/surge. The Trent 700 defends against this with two sets of hardware — variable geometry (VIGVs/VSVs that adjust flow angle) and bleed valves (that relieve flow quantity at low speed) — and those two systems take up half this article, because they are also the hardware foundation for the stall article (27).

1. The bleed map

                    PRECOOLER  (→ ATA-36 aircraft pneumatics)
                        ▲
                   ENGINE BLEED VALVE
                        │                              FAN AIR VALVE
   ┌────────────────────┼───────────────────────┐          │
   │  6TH HP ◄──┐   3RD HP      8TH IP          │          ▼
   │            │      │           │       NACELLE ANTI-ICE valve (→ ATA-30)
   │  COMPRESSOR┴──────┼───────────┼────────────│───────────────┐
   │                   │           │            │               │
   │                   ▼           ▼            │            f  │
   │              ┌─ ACAC ──┐  ┌─────────┐      │            a  │
   │              │HP3 cooled│ │IP8 / HP3 │     │            n  │
   │              │by fan air│ │bleed     │     │               │
   │              │→dual vlv │ │valves 4+3│────►│            a  │
   │              │→internal │ └─────────┘      │            i  │
   │              │ gearbox  │   discharge      │            r  │
   │              └──────────┘   into bypass    │               │
   │  LP+IP TURBINE ◄── TCC VALVE ◄─────────────│◄── fan air ───┤
   └────────────────────────────────────────────┘               │
                                       TO IDG AIR/OIL COOLER ◄──┤
                                       TO ENGINE AIR/OIL COOLER ◄┘ (control valve)

Reading points from the system schematic: three tapping stages are named on the diagram — HP6 (to the aircraft, via the bleed valve and precooler), HP3 and IP8 (the surge bleed valves and the servo/cooling supplies). The ACAC sits in the HP3 path to the internal gearbox — cool the hot air first, then send it to cool the bearings. The TCC valve has its own dedicated line, and its destination is written plainly: the LP and IP turbine cases together. And fan air is the universal cold source — nacelle anti-ice, the ACAC cold side, the IDG oil cooler and the engine oil cooler (AOHE) all draw on the bypass stream's huge mass of cold air.

The full tap-by-tap allocation:

2. Surge protection, hand one: VIGVs and VSVs

The AMM's functional statement:

"The IP and HP compressor airflow control system has one stage of Variable Inlet Guide Vanes (VIGVs) and two stages of IP compressor Variable Stator Vanes (VSVs). The VIGVs and VSVs control the angle at which the airflow is supplied to the first three stages of the IP compressor. … This helps to prevent a stall/surge condition in the IP and HP compressors."

An analogy that earns its keep: a compressor blade is a cyclist riding into wind — a headwind it can lean into, but a gust slicing in from the side knocks it over (the aerofoil stalls). The VIGVs and VSVs are adjustable louvres at the entry to the first three IP stages: at low speed, when the natural flow angle turns hostile, they close down and bend the flow back to an angle the blades can tolerate.

The control chain. FADEC → the VIGV/VSV control unit → two actuators → LVDT position feedback to the FADEC, closed-loop. Note the working fluid: fuel. Wherever this engine needs hydraulic muscle — VSVs, the TCC actuator, the fuel metering valve itself — it uses high-pressure fuel, not aircraft hydraulics.

Inside the control unit sit four components that together form a textbook fuel-hydraulic servo (the same philosophy reappears in the metering valve, article 09):

1. a constant-pressure valve holding the torque-motor supply at LP servo pressure + 180 psi;
2. a torque motor with flapper valve — the EEC's electrical signal (one coil per channel, electrically isolated, either coil alone has full authority) deflects the flapper toward one nozzle, creating a hydraulic imbalance; the EEC begins issuing control signals from 8 % N3;
3. a pressure-drop regulator holding a constant 50 psi across the control servo valve — with the pressure drop fixed, orifice area alone determines flow (the constant-ΔP metering philosophy again);
4. the control servo valve (spool): centred, it hydraulically locks the actuators; direction of offset selects extend/retract, magnitude selects rate.

Actuation. Two actuators sit diametrically opposed on the case at the horizontal centreline, each with an internal LVDT (both energised, one used for control). Their linear stroke turns unison rings through bellcranks; every vane carries a lever pinned to the ring — one ring turns, all vanes turn. The stage-1 and stage-2 VSV rings rotate in opposite directions. Each ring rides on 18 centralising screws, with clearances set cold by feeler gauge such that case thermal growth in flight consumes the clearance to zero — thermal expansion deliberately designed into the fit (a theme that returns with turbine case cooling, §6).

The failure direction — worth committing to memory:

"If a failure of the electrical supply occurs the two springs move the spool valve away from the neutral position. … The pistons then move at five to ten per cent of the usual piston speed to the low speed stop (fully extended position). This causes the VIGV/VSV actuating mechanism to close the VIGVs and VSVs."

Loss of signal = a slow drift (at 5–10 % of normal rate, so the running flow field is never slammed) to the closed, low-speed position. Why is closed the safe side? Because the low-speed position is the start/low-RPM configuration — the most surge-conservative geometry the system knows.

3. Surge protection, hand two: the 4 + 3 bleed valves

The FCOM skeleton:

"Two air bleed systems provide greater compressor stability in different flight phases. The volume of airflow through the intermediate pressure and low pressure compressors is regulated by four intermediate pressure stage 8, and three high pressure stage 3, bleed valves controlled by the FADEC. At low engine speed, the bleed valves are open to prevent engine stall."

The mechanism in plain language first: a compressor's stage-matching is designed for high speed. At low speed the rear stages cannot swallow what the front stages deliver; flow backs up mid-compressor, and enough congestion reverses it — surge. The bleed valves are relief floodgates in the middle of the queue: they dump IP8/HP3 air straight into the bypass duct so the front stages always have somewhere to send their flow. Hence open at low speed, closed at high speed. Each discharge passes through a silencer crossing the C-duct into the bypass stream — high-pressure air dumped into a duct would otherwise scream, which is why you never hear this system working.

Control hardware. Five solenoid valves share one bleed-valve controller:

"Two of the solenoid valves are used to operate the four IP bleed valves in pairs. One of these solenoid valves operates the IP bleed valves installed at the top right and the bottom left … The other … the bottom right and top left. The remaining three solenoid valves each operate one of the three HP bleed valves."

Note the diagonal pairing of the IP valves: if one solenoid fails, the surviving pair is still symmetrically disposed around the case — no one-sided discharge, no asymmetric flow field. Each solenoid carries two coils, one per EEC channel, either sufficient alone — the same channel redundancy as the torque motors.

Pneumatic logic and failure direction:

"When the solenoid valve is not energized, HP3 servo air is supplied from the solenoid valve to the bleed valve(s). The HP3 servo air pressure added to the spring pressure holds the bleed valve in the open position." — "If a failure of the electrical supply occurs the bleed valves are moved to their open position." — "When the engine is not in operation the spring pressure holds the bleed valve in the open position. This gives the correct airflow through the IP compressor for engine start."

Put the two failure directions side by side and a design language emerges:

Opposite directions, identical purpose — each system retreats to its own most-anti-surge end. De-energised state = engine-start state = most conservative state: three identities, one configuration. The price is performance, not safety: valves open and vanes closed cost thrust and fuel — which is exactly the "thrust limited after a protection activates" experience described in the stall and thrust-degradation articles (27, 21).

4. The sequence across one flight

"During an engine start the VIGVs and VSVs are held in the closed position until 8 percent N3. The four IP and three HP bleed valves are held in the open position during engine start." — "As the engine speed increases the VIGVs and VSVs start to move to the open position (high speed position). During engine acceleration the EEC controls the sequence in which the bleed valves close." — "During engine deceleration the EEC controls the sequence in which the bleed valves open."

(One transcription note for the careful reader: the deceleration paragraph in the source opens with "As the engine speed increases the VIGVs and VSVs start to move to their closed position" — in context this is evidently decreases; the original is quoted as printed.)

The phrase "controls the sequence" matters: the seven valves do not bang open or shut together. The EEC works them one at a time against engine speed, so the flow field transitions smoothly. The control signals also tell a redundancy story: VSVs and IP valves are scheduled on N2 plus IP temperature; HP valves on N3 plus IP temperature; on signal loss the scheduling degrades to pressure-ratio signals, and the EEC can also set the valves from thrust-lever angle — each valve set listens to its own spool, with fallback strategies layered behind (article 05 shows this degradation pattern engine-wide).

5. The internal air system — four flows of "domestic policy"

Behind the visible valves runs an unseen circuit:

"Air which is supplied by the IP compressor is bled off at stages IP5 and IP8. And air which is supplied by the HP compressor is bled off at stages HP3 and HP6. Parts of the engine which are at different pressures are isolated from each other by labyrinth seals."

That last line deserves a pause. The 92 HP turbine blades of article 01 are hollow: HP6 air flows through their cores and bleeds out of surface holes, wrapping each blade in a film of cooler air. The blades live immersed in gas hotter than their own melting point, kept alive by built-in air conditioning plus a film-air coat. This is why the EGT red lines of article 00 are hard limits: an exceedance means the margin between cooling air and gas temperature — the entire survival budget of those blades — has been spent.

Three seal types close the section: stepped seals (deliberately metered leakage — the escaping air itself forms the curtain that keeps exhaust gas out), labyrinth air seals (isolating different pressure zones), and the cleverest of all, oil-filled hydraulic labyrinth seals between co-rotating shafts: centrifugal force slings oil into an annular groove and the seal fin runs immersed in the oil — air cannot pass, oil cannot leave. The fluid being protected is itself the sealing medium.

6. Turbine case cooling: TCC (FCOM) = TIC (AMM)

First, the name. The FCOM calls it TCC — Turbine Case Cooling; the AMM chapter calls it TIC — Turbine Impingement Cooling. Same valve, same actuator, same manifold: one system, two names — the FCOM names the purpose, the AMM names the method. Do not book them as two systems when reading maintenance records.

"The TCC system is controlled by the FADEC to improve engine performance by controlling the intermediate pressure turbine blade tip clearance, and cooling intermediate pressure and low pressure turbine cases. The TCC valve is controlled by the FADEC to modulate air flow depending on flight conditions. The valve is fully open in cruise for optimal engine performance."

The mechanism: LP (bypass) cool air passes a butterfly valve into a box-section manifold wrapped around the IP turbine case; two rows of holes impinge the cool air directly onto the case; the case cools and shrinks; blade-tip clearance closes; less gas leaks over the tips doing no work; cruise fuel burn improves. The flow then washes rearward over shroud and liner to cool the LP turbine case as well. Think of a loose ring on a finger — except here you chill the ring (the case) to make it grip. Cruise is the steady, efficiency-critical condition, so the valve is fully open in cruise; in manoeuvring flight the valve closes, leaving clearances generous so that a cold-shrunk case never bites into hot-grown blades.

Three details complete it: the valve's closed position is not zero flow — two rows of small holes in the butterfly plate plus edge clearance guarantee a baseline trickle, so the case temperature is never wholly unmanaged. The actuator is driven by HP3 air against a return spring — loss of power closes the valve, leaving clearances large: the safe side costs fuel, never blade tips. And the manifold's rear shroud profile is precisely where the EGT thermocouples and the turbine overheat detector mount — the same bolts hold both (article 14 picks up the sensing side).

7. ACAC and IDG cooling — two oil conditioners and one air conditioner

"The internal gearbox cooling system is controlled by the FADEC to regulate air pressure and temperature within the engine center bearing. The HP 3 air to gearbox is cooled in the Air Cooled Air Cooler (ACAC) using fan air and modulated by a dual valve controlled by the FADEC. The valve is fully open during hot day take-off and climb conditions."

HP3 air seals the internal gearbox — home of all three location bearings — but HP3 air is hot (compression is not free). Sending it straight to a bearing chamber would be cooling with a hair-dryer, so it first passes the ACAC, an air-cooled air cooler using fan air as the cold side. Hot-day takeoff and climb combine the hottest HP3 with the hottest ambient, hence dual valve fully open — one of the few explicit valve-position-versus-condition statements in the FCOM, worth memorising as stated.

[!warning]- Configuration note: the ACAC has been deleted on most engines Per the AMM FADEC description (post-SB): "The ACAC is not used on the engine. The ACAC has been deleted from most engines. If the ACAC is installed, its function has been mechanically and electrically disconnected." The FCOM paragraph above describes the original configuration; the actual state of any given engine follows its service-bulletin status, and on modified engines the HP3 supply to the internal gearbox no longer passes through an ACAC. The related alert (ENG AIR EXCHANGER FAULT) concerns the oil temperature-control system, not the ACAC — see article 20.

"Fan air is used to cool the IDG oil through the air/oil cooler. A bypass valve opens when oil is too cold and cannot flow easily through the cooler."

The bypass clause is the counter-intuitive part: a cooler can work against you. Cold-soaked oil is too viscous to squeeze through the cooler matrix, so the bypass lets it circulate uncooled until it warms — a cooling system that knows when not to cool. The engine's own oil uses the same fan-air principle through the AOHE with its control valve; that story belongs to article 10.

8. Zone ventilation and the electronics box

"The power plant is divided into three primary fire-resistant zones isolated from each other by fireproof diaphragms and seals. Calibrated airflows are supplied to the zones … decrease the temperature of the main fuel and oil accessory units and prevent the collection of flammable fumes."

Ventilation has a double identity: it cools the accessories and it sweeps fuel vapours away — the first line of fire protection is never letting a flammable mixture reach flammable concentration. The zone walls are the fire bulkheads of article 02, and they are the skeleton on which the fire-detection loops of article 17 are strung.

And a fourth "zone", small but central:

"An equally important fire-resistant zone gives protection to the items which follow: An EEC (Engine Electronic Controller) with its Data Entry Plug. A PCU (Power Control Unit). An OPU (Overspeed Protection Unit)."

A titanium box with a hinged lid secured by 43 Camloc fasteners, every cable entering through fire/smoke seals; cooling air is taken from the intake duct, ducted through the box — with a dedicated feed straight to the OPU — and returned to the intake. This is the first joint appearance of the FADEC trio: the EEC (control computer), the OPU (independent overspeed protection unit) and the PCU (power conditioning unit) share one titanium fire enclosure, cooled by a through-draught borrowed from the intake. What happens when that draught weakens is the FADEC HI TEMP / OVHT story of article 20; who the three residents are and how they divide the work is article 04.

9. Where this hardware meets the failure chapters

Self-test

[!note]- Q1. How many bleed valves, on which stages, and when are they open? Four on IP stage 8 and three on HP stage 3. Open at low engine speed — including throughout the start — to relieve the mid-compressor congestion that causes stall; closed in sequence by the EEC as speed rises.

[!note]- Q2. VSVs fail closed; bleed valves fail open. Opposite directions — how are both "safe"? Each retreats to its own most-anti-surge configuration: the VSV low-speed (closed) position is the start geometry, maximally conservative; open bleed valves are permanent floodgates. De-energised = start state = most surge-protected state. The cost is performance, never stability.

[!note]- Q3. Where is the TCC valve in cruise, and where does it go on loss of power? Fully open in cruise — impingement air shrinks the IP turbine case, closing tip clearance for best efficiency. On loss of power the return spring closes it, leaving clearances generous: pay fuel, never grind blade tips. The closed butterfly still passes a baseline flow through its drilled holes.

[!note]- Q4. Which air cools the HP turbine blades, and by what route? HP6 air: around the HP shaft to the disc front face → through pre-swirl nozzles (matching the air's tangential speed to the disc) → into the hollow blades → out through surface film holes into the gas stream. Internal convection plus film cooling — the budget the EGT limits protect.

[!note]- Q5. What is the EEC's "room", who shares it, and what cools it? The titanium electronics protection box (its own fire zone within Zone 1), shared with the OPU and PCU; cooled by a through-flow of intake-duct air (with a dedicated feed to the OPU), exhausted back to the intake.

Key takeaways

References

• FCOM DSC-70 (engine air system) — four uses of bleed air, 4+3 bleed valves, TCC description and cruise-open statement, ACAC and IDG cooler descriptions (quoted verbatim above).
• AMM 75-33 (IP/HP compressor airflow control, D/O) — VIGV/VSV function, control-unit internals (180 psi, 50 psi, dual-coil torque motor, 8 % N3), actuators and unison rings, failure directions, solenoid pairing, silencers, start/acceleration/deceleration sequencing and scheduling signals.
• AMM 75-22 (engine cooling and sealing, D/O) — IP5/IP8/HP3/HP6 duties, pre-swirl and hollow-blade cooling, the three seal types.
• AMM 75-24 (turbine impingement cooling, D/O) — manifold and impingement mechanism, baseline flow through the closed butterfly, spring-closed failure mode, thermocouple co-location.
• AMM 75-21 (accessory/zone cooling, D/O) — three zones, calibrated flows, the electronics protection box and its 43 Camloc fasteners and OPU feed.
• AMM 73-21 (FADEC, D/O) — ACAC deletion service-bulletin status (configuration note).
• Integrative synthesis (marked in text): the traffic/floodgate framing of surge; the safe-side comparison table; the ring-chilling analogy for TCC; the "cooling with a hair-dryer" reading of the ACAC.

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.