Airbus does not often write about smells. In August 2013 it did, in issue 52 of FAST, its technical magazine, under the title “A clean APU means clean cabin air.” It is still the clearest public account of how the auxiliary power unit on the A320 family ends up putting odors into the cabin, and it is worth reading in full if you own the problem.
What follows is a summary, with our notes on where cleaning fits.
The air path
On the ground, with the engines off, the APU supplies the bleed air that feeds the air conditioning packs. Airbus traces the path: APU load compressor, the bleed duct downstream of the APU bleed valve, the aircraft pneumatic system, then the packs. Anything that gets into the APU inlet has a route to the cabin.
Airbus puts it without hedging: a source of in-service contamination events reported on the A320 Family fleet is the APU.
How contamination gets there
- Oil. Internal leakage in the APU, or oil drawn back into the intake. Airbus notes that a very small amount of oil is enough to produce a noticeable odor in the cabin.
- Hydraulic fluid. Leaks from the main landing gear bay can run aft along the outside of the fuselage and reach the APU air intake.
- Exhaust. The intake can ingest exhaust from nearby engines or from ground power units.
- De-icing fluid. Airbus recommends selecting APU bleed off during de-icing, then waiting three to four minutes before selecting it back on.
- Fluids that cannot drain. A misaligned APU drain mast, or the bellows seals on the right hand compartment door, can stop fluid from draining out of the compartment.
What Airbus says to do
The recovery steps in the article run in a sensible order. Inspect the intake duct, the intake plenum and the walls of the bleed duct. Clean the compartment, the intake system, the inlet plenum and the load compressor in line with the AMM. Then decontaminate the pneumatic system, with Airbus pointing to AMM 21-00-00 and SIL 21-029.
The article also lists the AMM tasks that existed at the time for checking the APU compartment and air intake duct for oil contamination, one for each of the three APU types then fitted to the family (GTCP36-300, APS3200 and 131-9A). Task references move with manual revisions, so check the current AMM for your aircraft rather than relying on a 2013 list.
Where cleaning fits
Two things are true at once. Cleaning is not a substitute for finding the leak, and Airbus’s own sequence puts inspection first. And the steps Airbus describes are troubleshooting steps: they start after someone has reported a smell.
That second point is why DASH Aero exists. Residue in the plenum and the compartment builds up between events. Cleaning it out on a schedule, rather than after a complaint, is the difference between maintenance and recovery. It also means the aircraft is not taken out of service at short notice to do the work.
We have written a longer explainer, with every source linked, on APU contamination and cabin air quality.
Source: Airbus, FAST 52, August 2013, “A clean APU means clean cabin air,” pages 4 to 9. Available from the Airbus website.
