Exhaust and Accessory Cooling
The APU breathes through one inlet, but it disposes of heat and spent gas through two completely separate air circuits. One is the bleed and surge air path covered in the load-compressor and surge articles: air the load compressor produces for the aircraft. The other is the accessory cooling path: air a gearbox-driven fan pulls in to cool the lubricating oil and the APU compartment. These two circuits do not share a regulator, a valve, or a control law. The single most useful fact in this article is that turning APU BLEED off does nothing to the cooling fan — the fan is mechanically driven, so it keeps turning and keeps removing heat for as long as the APU runs.
This article covers the accessory cooling system (the cooling fan, the compartment cooling valve and its barometric altitude logic, the oil-cooler air path, and compartment ventilation) and the exhaust system (the muffler, the exhaust coupling, the firewall seal, and the fire-zone thermal management that keeps surface temperatures below the ignition point of fuel and oil). The oil cooler itself and oil-temperature management belong to Oil System; how the surge air is produced belongs to Bleed and Surge Air; the inlet and its silencer belong to Air Intake; the power-section heat shield and exhaust diffuser belong to Power Section; fire detection and extinguishing belong to ATA 26 (Fire Protection).
1. Two independent air circuits
The AMM defines the APU air system in one line, and then states the independence explicitly:
"The air system has two subsystems: - Bleed and Surge Air, - Accessory Cooling."
"The accessory cooling system operates independently of the APU load compressor and the bleed and surge-air system."
These two sentences carry more weight than they look. The bleed and surge air path is air the load compressor produces; its flow is metered by the inlet guide vanes and gated by the bleed valve, and any excess is dumped through the surge control valve into the exhaust cone — the ECB and the APU BLEED pushbutton command all of that. The accessory cooling path is air a separate fan moves, and that fan takes its drive directly from the accessory gearbox. There is no valve in the cooling path that the APU BLEED pushbutton can touch.
The practical consequence is the counterintuitive point a lot of crews miss:
[!warning]- "APU BLEED OFF" does not stop APU cooling
The cooling fan is turned by the accessory drive gearbox, not by bleed air. Selecting
APU BLEEDoff closes the bleed valve and removes the APU's contribution to the aircraft pneumatic system, but it has no effect on the cooling fan: the fan keeps turning, the oil cooler keeps receiving cooling air, and the APU compartment keeps being ventilated for as long as the APU is running. Cooling is a function of the APU running, not of bleed being selected on.
┌────────────────────────────────┐
│ APU inlet plenum chamber │
└────┬───────────────┬─────────┬───┘
│ │ │
BLEED / SURGE │ COOLING │ │ power section
(ATA 49-01/03) │ (this │ │ (combustion air,
│ article) │ │ ATA 49-01)
▼ ▼ ▼
┌──────────────────┐ ┌──────────────┐
│ Load compressor │ │ Cooling fan │
│ (IGV-metered) │ │ 5100KH6 │
└────────┬─────────┘ └──────┬───────┘
ECB + │ │ mechanical drive
APU BLEED │ │ (turns whenever
P/BSW ┌──┴───┐ │ the APU runs)
▼ ▼ ▼ cooling air
bleed surge ┌─────┴──────┐
valve valve ▼ ▼
│ │ compartment oil cooler
▼ ▼ cooling valve (ATA 49-10)
aircraft exhaust 5100KH7 + │
pneumatic cone fixed outlet ▼
(start/AC)(49-80) │ hot air overboard
▼ (door 315AL)
APU compartment
2. The cooling fan (5100KH6)
The cooling fan is a single-stage axial machine driven mechanically off the accessory gearbox:
"The cooling fan assembly 5100KH6 is of the one-stage axial-flow type and the accessory drive gearbox turns the cooling fan rotor."
It draws its air from the same plenum the power section and load compressor use, but it does so by suction rather than by being fed:
"The cooling fan takes the air from the APU inlet plenum chamber by suction."
A V-clamp attaches the fan assembly to the gearbox, and the fan's gearshaft pinion engages the gearbox's cooling-fan idler. Because the drive is a fixed gear train off a constant-speed APU, the fan turns at fixed speeds whenever the APU is running:
"The cooling fan idler turns with 30999 rpm, the cooling fan rotor with 41730 rpm."
Burst containment and bearing design
The fan carries the same defensive engineering you see on the power section and load compressor — it is treated as a rotating part that could burst, so it is wrapped in a containment ring:
"The containment ring is made from aluminum. It collects all the broken pieces if a burst of the cooling fan rotor occurs."
An abradable ring on the inner diameter of the containment ring forms the tip seal against the rotor blades, and the bearing carrier on the rotor side, together with the gearbox attachment area, is deliberately resilient to absorb vibration and the energy of a rotor burst. The gearshaft runs on a roller bearing at the gearbox side and a ball bearing at the rotor side; the APU oil system lubricates the pinion and bearings, with gravity returning the oil to the gearbox. A carbon face seal plus an added buffer-air labyrinth seal seal the rotor side. Air leaves the rotor through single-stage stator vanes; the stator vanes are tipped to the containment ring on the outside and carry the outlet cone (the cover over the fan hub) on the inside.
This is the APU design philosophy showing up again: a constant-speed, directly-driven rotating machine, fully contained against burst, sealed and lubricated off the central oil system — the same pattern seen on the Power Section and the Load Compressor.
3. Cooling-air distribution and the compartment cooling valve
The fan discharges into an outlet duct, and from there the air splits two ways: some cools the APU compartment, the rest cools the oil. The compartment air itself reaches the compartment by two routes — a permanently open outlet and a barometric valve:
"The compartment cooling air flows through an outlet and through the compartment cooling valve into the APU compartment."
The altitude logic
Only the valve-routed portion of the compartment air is modulated, and it is modulated by pressure altitude, not by the ECB:
"The compartment cooling valve decreases the cooling air outflow to the APU compartment when the aircraft has a pressure altitude of 10000 ft. (3047.94 m). The compartment cooling valve is closed at the pressure altitude of 20000 ft. (6095.89 m). Above this pressure altitude the compartment cooling air only flows through the outlet."
Closing the valve does not waste that air — it reroutes it:
"It is in the fully closed position at a pressure altitude of 20000 ft. (6095.89 m). This increases the flow of cooling air to the oil cooler."
The engineering logic, reading those two statements together: at low altitude the ambient air is warm and dense, the compartment needs maximum ventilation, and the valve sits fully open so the compartment gets both routes of air. On the ground this is enough to hold the compartment well within limits:
"On the ground the cooling air is sufficient to keep the temperature in the APU compartment below 100 deg.C (212.00 deg.F) at an ambient air temperature of 55 deg.C (131.00 deg.F)."
As the aircraft climbs, ambient air gets cold and thin; convective cooling of the compartment becomes easier, so the system progressively closes the valve from 10000 ft and diverts that adjustable share to the oil cooler, where heat rejection is harder in thin air. By 20000 ft the valve is shut and the oil cooler is getting the maximum cooling-air share.
[!warning]- "Fully closed at 20000 ft" does not mean the compartment is starved of air
When the compartment cooling valve is fully closed, the AMM says the compartment air "only flows through the outlet." That permanently open outlet keeps supplying a baseline flow to the compartment — closing the valve removes only the adjustable share, which is redirected to the oil cooler. Do not read "fully closed at 20000 ft" as "no compartment cooling at altitude": the compartment keeps its baseline outlet flow and, at altitude, also has cold thin ambient air working in its favour.
How the valve senses altitude
The valve is purely pneumatic — there is no electrical command and no ECB involvement:
"The compartment cooling valve 5100KH7 is a pneumatically operated butterfly valve. It is controlled from a pneumatic actuator with a barometric control-valve function."
Inside the actuator, a diaphragm divides the housing into a pneumatic pressure chamber and an ambient reference chamber. Pressurised reference air feeds the pressure chamber:
"Discharge air from the first stage of the engine compressor (PCD 1 air) is supplied to the pneumatic pressure chamber."
The ambient reference chamber vents to outside air and contains a compensating spring. At low altitude the spring plus the ambient pressure hold the butterfly fully open; as the aircraft climbs and ambient pressure falls, the regulated chamber pressure wins and drives the butterfly toward closed, reaching fully closed at 20000 ft. The whole thing is a self-contained barometric servo — it tracks altitude automatically and needs no pilot or computer input.
The oil-cooler air path
The remaining cooling air flows through the oil-cooler inlet duct, across the oil cooler (a heat-exchanger handled in Oil System), and then overboard. The hot air leaves through the oil-cooler outlet duct, which terminates at the left access door 315AL; bellows connect the duct to the oil cooler's outlet opening and a seal at the duct end forms a flexible joint that absorbs vibration and installation tolerance. This article is concerned only with the cooling-air supply side of the oil cooler; oil temperatures and oil heating belong to the oil system article.
4. Compartment ventilation, drainage, and the overpressure louvers
Two more features close out the cooling-side fire and contamination story, both on the APU compartment itself.
A drain is fitted in the left access door 315AL so that any air and fluid collecting in the compartment runs out overboard. And on the top-left structure of the compartment, between FR99 and FR100, sits a louvered overpressure release door:
"When the APU does not operate, the louvers permit convection to make a flow of air in the compartment."
During operation the cooling air supplied into the compartment exits through these louvers to ambient; when the APU is shut down, the same louvers allow natural convection to keep air moving. Their orientation is deliberate — they keep fluids, flames, and fumes from other aircraft zones out, while letting the compartment's own fumes out to atmosphere:
"The louvers make sure that fumes go out of the APU compartment into the ambient air and do not cause an additional fire hazard."
The drain, the louvers, and the tail-cone fire zone are part of the overall APU installation picture in APU Overview; the inlet-side louvers and silencer are in Air Intake.
5. The exhaust system — what flows, and where it goes
The exhaust system has two jobs: dump the spent gas overboard and quieten it. What flows through it is not only combustion product:
"The exhaust gas contains the combustion gases and the load compressor surge-air."
That second component is the surge air dumped by the surge control valve into the exhaust cone — the same air discussed in Bleed and Surge Air. So the APU's exhaust mass flow is the burned gas plus whatever the surge system is bleeding off to keep the load compressor out of surge.
The hardware lives behind the APU in its own fire compartment:
"The system is installed in the exhaust muffler compartment (Frame 101 thru 103 together with the rear access fairing 317AL)."
The muffler passes through the rear firewall (FR101) of the APU compartment, and an exhaust coupling makes a flexible connection back to the power-section turbine heat shield. The three functional elements are the exhaust coupling, the firewall seal, and the exhaust muffler, and every part of the system is rated for fire:
"All the components of the system are either fireproof or fire resistant."
6. Acoustic attenuation — the muffler
Most of the APU's exhaust noise originates at one specific place:
"The exhaust system also decreases the exhaust noise level, most of the noise comes from the outlet of the APU exhaust diffuser (the flange of the turbine heat shield)."
The muffler attacks that noise with a duct-in-duct construction. The inner wall — the exhaust duct — is made of sound-absorbing feltmetal:
"The exhaust duct is the inner wall of the muffler. It is made of feltmetal, with steel inlet and outlet sleeves."
A steel muffler casing shrouds the inner duct, and the annular space between the two is partitioned into resonant chambers:
"The ring frames divide the space between the exhaust duct and the muffler casing into 8 chambers. This design, together with the capability of the feltmetal to absorb the sound and reduces the APU exhaust noise."
So the muffler is a feltmetal liner plus eight tuned chambers. Together with the silencer on the inlet side (see Air Intake), the APU is acoustically treated at both ends — appropriate for a unit that operates as a ground and auxiliary power source where intake and exhaust noise reach the cabin and ground crew.
7. Fire-zone thermal management
Both the APU compartment and the exhaust muffler compartment are fire zones, so the exhaust system's thermal behaviour is a fire-safety function, not just a comfort one. There are three lines of defence.
Insulation caps the surface temperature. The muffler is blanketed so its skin never reaches a temperature that could ignite a leak:
"The exhaust muffler has a thermal insulation. It keeps the temperature at the external surface of the exhaust muffler assy to a limit of 200 deg.C (392.00 deg.F)."
Compartment ventilation caps the air temperature, and the design target is explicitly below the ignition point:
"the temperature in the compartment is kept down to a maximum of 115 deg.C (239.00 deg.F) at an ambient air temperature of 50 deg.C (122.00 deg.F). The temperature in the compartment is well below the point at which the oil, fuel or vapurized fuel burns."
Read those two limits as a deliberate margin: surfaces held to 200 °C, compartment air held to 115 °C, both chosen so that if oil or fuel ever leaks, no hot metal surface can light it. That is the core of the fire-zone design — keep everything below the ignition point as a matter of construction, not just monitoring.
A seal prevents hot gas from backing up into the compartment. The muffler passes through the FR101 firewall on a fireproof, P-shaped firewall seal, and the rear access fairing carries a second flexible seal inside its titanium end cover. Together they block reverse flow:
"This fireproof seal prevents a reverse flow of exhaust gases into the exhaust muffler compartment."
The exhaust coupling ties this together at the forward end. It is a flexible joint that lets the hot hardware grow and shrink without binding:
"The exhaust coupling makes a flexible connection between the exhaust muffler and the turbine heat shield. It compensates for the expansion and contraction of the APU and the exhaust muffler, due to changes in temperature."
At the lowest point of the coupling there is a drain tube; it removes any fuel, rainwater, and condensed water from the muffler into the drain system (AMM D/O 49-17-00), so liquid cannot pool and become a fire hazard. This dovetails with the compartment drain and the louvers of §4 — multiple, redundant ways to keep flammable liquid and fumes from accumulating.
8. Inside the muffler — structural walk-through
Reading the muffler longitudinal section, the assembly is a sealed, sliding, insulated tube hung so it can grow when hot.
FWD (power section) AFT (overboard)
──────────────────────────────────────────────────────────────────────►
turbine heat shield flange
│ V-coupling
▼
┌──────────────────────────────────────────────────────────────────┐
│ EXHAUST COUPLING │
│ sealing ring (split, piston-ring form) ── sealing ring guide │
│ exhaust deflector (turns gas in, off the ring) │ ──► 49-17
│ drain tube at lowest point · bellows + 2 steel clamps │ drain
└───────────────────────────────┬───────────────────────────────────┘
▼
╔════════════════════════════════════════════════════════════════════╗
║ EXHAUST DUCT (inner wall, feltmetal, steel inlet/outlet sleeves) ║ ◄ combustion
║ ‖ 8 ringframes split the annulus into 8 chambers ‖ (sound absorb) ║ gas +
╠════════════════════════════════════════════════════════════════════╣ surge air
║ MUFFLER CASING (steel shroud) + INSULATION (ext. surface ≤ 200°C) ║
╚════════════════════════════════════════════════════════════════════╝
│ through rear firewall FR101: P-form fireproof firewall seal
│ (prevents reverse flow of exhaust gas into the compartment)
│ suspension: 2 U-rails + 4 mount brackets (bolts at 2 rear)
▼
rear access fairing 317AL (FR103–106A) + flexible end seal ──► ambient
The sliding gas seal. The coupling has to be gas-tight yet free to move as the muffler expands. That is done with a piston-ring style seal:
"The sealing ring seals the space between the exhaust duct and the exhaust coupling. It is a split ring and has the form of a piston ring."
The sealing ring sits in a sealing-ring guide; the forward end of the muffler protrudes into that guide so the seal can slide. An exhaust deflector protects the seal from the gas stream:
"The exhaust deflector guides the exhaust gas into the exhaust muffler. Thus the exhaust gas does not directly impinge onto the sealing ring."
A fire-resistant, gas- and fluid-tight bellows covers the coupling, held by two steel clamps, with its aft end on the muffler casing.
The suspension lets it grow. The muffler is not bolted rigidly — it is slung so it can expand lengthwise:
"The exhaust muffler suspension has two guide rails, which are part of the exhaust muffler compartment-structure. They have the form of an U."
Four mount brackets on the muffler ride in those U-rails; bolts at only the two rear brackets fix the muffler, leaving the front free to slide, so the suspension absorbs heat expansion. The insulation is built from segmented blankets — several layers of insulation material faced with a steel foil.
9. Flight-deck and operational perspective
There is no cockpit control for either the cooling fan or the exhaust system — both are passive, automatic, and mechanical (the cooling fan is gearbox-driven and the compartment cooling valve is barometric, with no ECB involvement). The crew's interaction is limited to understanding the behaviour:
| Scenario | What happens | Why |
|---|---|---|
APU supplying power on the ground with APU BLEED off |
Cooling fan keeps turning; oil and compartment stay cooled | Fan is gearbox-driven, independent of bleed (§1) |
| Climb through 10000 ft, then 20000 ft | Compartment cooling valve closes progressively, then fully; more air goes to the oil cooler | Barometric valve reallocates the adjustable share to the oil cooler at altitude (§3) |
| Long APU run on a hot day on the ground | Compartment held below 100 °C at 55 °C ambient; muffler compartment below 115 °C | Fan ventilation + insulation hold temperatures below the ignition point (§3, §7) |
| Faint oil/fuel smell or a minor leak in the APU bay | Compartment and exhaust temperatures kept below the ignition point; coupling drain and compartment drain remove liquid; louvers vent fumes | Layered fire-zone design (§4, §7) — a significant leak still warrants maintenance investigation |
| Audible APU exhaust noise on the ramp | Attenuated by the feltmetal 8-chamber muffler and the intake silencer | Most noise comes from the diffuser outlet; both ends are silenced (§6) |
Self-test
[!note]- Q1. Where does the APU cooling air come from, what drives the fan, and does it still work with
APU BLEEDoff?The cooling air is drawn by suction from the APU inlet plenum chamber — the same plenum the power section and load compressor use — by the single-stage axial cooling fan 5100KH6, which is turned mechanically by the accessory drive gearbox (idler 30999 rpm, rotor 41730 rpm). It is completely independent of the bleed and surge-air system, so selecting
APU BLEEDoff has no effect: the fan keeps turning and keeps cooling the oil and the compartment for as long as the APU runs. The fan's discharge splits two ways — to the APU compartment and to the oil cooler.
[!note]- Q2. Why is the compartment cooling valve fully closed at 20000 ft, where does that air go, and how is the valve controlled?
At altitude the ambient air is cold and thin, so convection cools the compartment easily; closing the valve diverts its adjustable share of cooling air to the oil cooler, where heat rejection is harder in thin air. The valve decreases compartment outflow from 10000 ft and is fully closed at 20000 ft. Crucially, the compartment is not cut off — the permanently open outlet keeps supplying baseline flow ("the compartment cooling air only flows through the outlet"). The valve is a pneumatically operated butterfly valve with a barometric actuator (PCD 1 air in the pressure chamber, a spring and ambient air in the reference chamber) — purely altitude-driven, with no ECB involvement.
[!note]- Q3. What is in the APU exhaust besides combustion gas, and why?
The load compressor surge air. The surge control valve dumps excess load-compressor air into the exhaust cone to prevent surge (see Bleed and Surge Air), and that air joins the combustion gas to be expelled and silenced through the muffler. So the exhaust mass flow is burned gas plus whatever the surge system is bleeding off.
[!note]- Q4. How are the APU and exhaust compartment temperatures kept below the ignition point, and how is reverse flow of exhaust gas prevented? What is the fire-zone link?
By insulation holding the muffler's external surface to 200 °C, and ventilation holding the compartment to a maximum of 115 °C at 50 °C ambient — both deliberately well below the point at which oil, fuel, or vaporised fuel burns, so a leak cannot be ignited by hot metal. Reverse flow is blocked by the fireproof firewall seal at FR101 (plus the rear-fairing flexible seal), which "prevents a reverse flow of exhaust gases into the exhaust muffler compartment." This is the heart of fire-zone design: temperature limits, sealing, drainage, and louvered venting working together so no leak finds an ignition source.
[!note]- Q5. The APU silences both intake and exhaust. Why so much attention to noise, and where does the exhaust noise mainly come from?
The APU is a ground and auxiliary unit, so its intake and exhaust noise reach the cabin and ground crew — both ends are therefore acoustically treated (intake silencer plus the exhaust muffler's feltmetal 8-chamber design). Most of the exhaust noise comes from the outlet of the APU exhaust diffuser — the flange of the turbine heat shield — which the feltmetal and the eight ring-frame chambers are tuned to absorb.
Key takeaways
| Point | Detail |
|---|---|
| Two independent air circuits | Bleed/surge air (load compressor → aircraft) vs accessory cooling (gearbox-driven fan → oil cooler + compartment); they share no control |
| Cooling survives bleed off | Fan 5100KH6 is mechanically driven; APU BLEED off does not stop cooling |
| Cooling fan | Single-stage axial, idler 30999 rpm / rotor 41730 rpm; aluminium containment ring + abradable tip seal; carbon face + buffer-air seals; APU-oil lubricated |
| Compartment cooling valve | Pneumatic butterfly, barometric actuator (PCD 1 air vs spring + ambient); full open low, closes from 10000 ft, fully closed at 20000 ft → more air to oil cooler; no ECB control |
| "Fully closed" ≠ starved | The permanently open outlet keeps a baseline compartment flow at altitude |
| Ground temperatures | Compartment below 100 °C at 55 °C ambient; muffler compartment below 115 °C at 50 °C ambient |
| Exhaust content | Combustion gas + load compressor surge air |
| Acoustic treatment | Feltmetal exhaust duct + 8 ring-frame chambers; intake silenced separately; main noise source = diffuser outlet |
| Fire-zone thermal management | Insulation ≤ 200 °C surface, compartment ≤ 115 °C, both below ignition point; firewall seal prevents reverse flow; coupling + compartment drains; louvered venting |
| Exhaust mechanical design | Flexible coupling (thermal growth), piston-ring sealing ring + exhaust deflector + bellows, U-rail suspension (4 brackets, 2 rear bolted) |
References
Per AMM D/O 49-50-00 (the APU air system as two subsystems — bleed/surge air and accessory cooling — and the independence of accessory cooling from the load compressor and bleed/surge-air system); AMM D/O 49-52-00 (cooling fan assembly 5100KH6: one-stage axial, gearbox-driven, idler/rotor speeds, suction from the inlet plenum, containment ring and abradable seal, bearings and seals, lubrication; oil-cooler air path; APU compartment cooling air, the compartment cooling valve 5100KH7 barometric butterfly logic at 10000/20000 ft, ground compartment temperature limit, the compartment drain and the FR99–FR100 louvered overpressure release door); AMM D/O 49-80-00 and 49-81-00 (exhaust system: exhaust gas content, the exhaust muffler compartment FR101–103 with rear access fairing 317AL, the muffler through the FR101 rear firewall, feltmetal exhaust duct with 8 ring-frame chambers for noise reduction, thermal insulation to 200 °C surface and 115 °C compartment, the fireproof firewall seal preventing reverse flow, the flexible exhaust coupling with sealing ring/exhaust deflector/bellows/drain tube to AMM D/O 49-17-00, and the U-rail suspension). The low-altitude-protect-compartment / high-altitude-divert-to-oil-cooler rationale, the deliberate sub-ignition-point margin, and the both-ends acoustic-treatment motivation are engineering interpretation built on the cited AMM statements, not separate documented claims.
Independent study material, not an Airbus publication. Refer to current operator FCOM, FCTM, AMM, and QRH for operational use.