Exhaust — Combustion Gas Plus Surge Air, Thermal Management, the FR101 Firewall Seal
The exhaust system does more than vent gas. It carries both the combustion gases and the load-compressor surge air, silences the loudest part of the APU, and forms the aft thermal/fire boundary of the tailcone compartment at the rear firewall FR101.
The exhaust system releases the APU exhaust gas to the ambient air. The exhaust gas contains the combustion gases and the load compressor surge-air. The exhaust system also decreases the exhaust noise level... The exhaust muffler has a thermal insulation to protect the structure of the exhaust muffler-compartment... A firewall seal seals the space between the exhaust and the rear firewall (FR 101). — AMM 49-81-00
1. Two flows down one pipe — combustion gas + surge air
Per AMM 49-81-00, the exhaust gas is combustion gas plus load-compressor surge air.
[!note]- This is where the surge valve's dump goes (integrative synthesis) Recall 03 bleed & surge: the surge control valve dumps surge air to the exhaust to protect the load compressor. Here is its destination — the surge air joins the combustion gas in the muffler and leaves through the same outlet (AMM 49-81-00). So the exhaust is the common sink for both the turbine's gas and the compressor's relieved air; they are not separate pipes.
The exhaust muffler is the duct from the APU exhaust diffuser (the rear flange of the turbine heat shield) to ambient. Most APU noise originates at that diffuser outlet, so the muffler is sized to attenuate it: a feltmetal inner duct with sound-absorbing chambers reduces the exhaust noise.
2. Thermal management — kept below where fuel burns
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... The compartment also has a ventilation system... Thus the temperature in the compartment is kept down to a maximum of 115 deg.C at an ambient air temperature of 50 deg.C. The temperature in the compartment is well below the point at which the oil, fuel or vapurized fuel burns.
Per AMM 49-81-00, the muffler's thermal insulation holds its external surface to ≤ 200 °C, and the compartment's louvre ventilation holds the compartment to ≤ 115 °C (at 50 °C ambient) — deliberately below the temperature at which oil, fuel or fuel vapour ignites.
[!warning]- The exhaust compartment is engineered to stay below fuel auto-ignition (fire prevention) This is the teaching point: the hottest part of the APU is wrapped and ventilated specifically so that leaking oil/fuel cannot auto-ignite on a hot surface (AMM 49-81-00). It is passive fire prevention in the fire zone — complementing the active APU fire detection/extinguishing. A pilot reading "APU compartment overheat" should understand the design margin that condition is eating into.
3. The FR101 firewall seal — no reverse flow
Per AMM 49-81-00, a fireproof firewall seal closes the gap where the muffler passes through the rear firewall FR101, with a second flexible fireproof seal in the rear access fairing. Together they prevent a reverse flow of exhaust gases back into the exhaust muffler compartment.
[!note]- FR101 is the aft edge of the fire zone (integrative synthesis) The APU fire zone runs FR95 (forward firewall, behind the air intake) to FR101 (rear firewall, here). The firewall seal makes FR101 a real boundary — hot exhaust goes out, and nothing flows back in (AMM 49-81-00). This closes the loop on the compartment that fire protection monitors.
The exhaust coupling makes a flexible connection between the turbine heat shield and the muffler, absorbing thermal expansion; its drain tube sends any fuel, rainwater or condensed water to the drain system (49-17) rather than letting it pool in the muffler. An exhaust deflector keeps the hot gas off the coupling's sealing ring.
4. Counterintuitive points
[!warning]- The exhaust carries the surge air too — not just combustion gas The surge control valve's dump (03) ends up here; combustion gas and relieved compressor air share the muffler and outlet (AMM 49-81-00).
[!warning]- The hot end is held below fuel ignition on purpose Insulation ≤ 200 °C surface, ventilation ≤ 115 °C compartment — by design below where oil/fuel/vapour burns (AMM 49-81-00). It is fire prevention, not just heat tolerance.
Self-test
[!note]- Q1. What two flows does the exhaust carry? Combustion gas + load-compressor surge air — the surge valve's dump (03) joins the turbine gas here.
[!note]- Q2. What temperatures does the thermal design hold, and why? External muffler surface ≤ 200 °C; compartment ≤ 115 °C (at 50 °C ambient) — below where oil/fuel/vapour ignites (fire prevention).
[!note]- Q3. What does the FR101 firewall seal do? Seals the muffler through the rear firewall FR101 and prevents reverse flow of exhaust into the compartment — the aft edge of the fire zone.
[!note]- Q4. Where does the coupling's drain tube send fluids? To the drain system (49-17) — fuel/rainwater/condensate, so it doesn't pool in the muffler.
Key takeaways
| Point | Detail |
|---|---|
| Carries | combustion gas + load-compressor surge air (surge-valve dump destination) |
| Noise | feltmetal duct + chambers attenuate the diffuser-outlet noise (loudest source) |
| Thermal | insulation surface ≤ 200 °C; louvre-ventilated compartment ≤ 115 °C — below fuel ignition |
| Firewall | fireproof seal at FR101; prevents reverse flow into compartment (aft fire-zone boundary) |
| Coupling | flexible (thermal expansion); drain tube → drain system (49-17); deflector shields sealing ring |
References
- AMM 49-81-00 (Exhaust System — Description and Operation) — exhaust gas = combustion gas + load-compressor surge air; muffler attenuates diffuser-outlet noise (feltmetal duct + chambers); thermal insulation holds external surface ≤ 200 °C, louvre ventilation holds compartment ≤ 115 °C (at 50 °C ambient), below oil/fuel/vapour ignition; fireproof firewall seal at rear firewall FR101 prevents reverse flow into compartment; exhaust coupling flexible for thermal expansion with drain tube to drain system 49-17 and exhaust deflector shielding the sealing ring.
- FCOM DSC-49 — APU exhaust and compartment (overview).
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.