APU contamination and cabin air quality

On the ground, A320 family aircraft often take their cabin air from the APU: bleed air from its load compressor feeds the air conditioning packs. Airbus identifies the APU as a source of cabin contamination events on the family, through oil, hydraulic fluid, de-icing fluid and exhaust reaching the APU inlet. Keeping the inlet plenum and compartment clean removes residue from that air path. It does not replace finding and fixing a leak.

How APU air reaches the cabin

With the main engines shut down, the cabin still needs conditioned air, and it comes either from ground equipment or from the APU. EASA's 2017 cabin air quality study notes that on the ground, bleed air comes from the APU.^[2]

Airbus traces the route on the A320 family. Outside air comes in through the APU air intake and the inlet plenum to the APU's load compressor. From there it leaves through the bleed duct downstream of the APU bleed valve, into the aircraft pneumatic system and on to the air conditioning packs.^[1] Anything that reaches the inlet has a path to the cabin.

Two APU types are fitted across the family today, the Pratt & Whitney Canada APS3200 and the Honeywell 131-9A, and early aircraft carried the GTCP36-300.^{[1, 5, 6]} Honeywell describes the 131-9A as running the air conditioning while the aircraft is on the ground.^[5] Pratt & Whitney rates the APS3200 at 1.8 kg/s of bleed air at 42 psig.^[6]

What gets into the APU inlet

Airbus is direct about it: a source of in-service contamination events reported on the A320 Family fleet is the APU.^[1] The routes it describes:

• Oil. Internal leakage in the APU, or oil drawn back into the intake. A very small amount of oil can be enough to cause a noticeable odor in the cabin.^[1]
• Hydraulic fluid. Leaks from the main landing gear bay can run aft along the outside of the fuselage to the APU air intake.^[1]
• Exhaust. The intake can draw in exhaust from nearby engines or ground power units.^[1]
• De-icing fluid. Airbus recommends selecting APU bleed off during de-icing and waiting three to four minutes afterward before selecting it back on.^[1]
• Fluids that cannot drain. A misaligned APU drain mast, or the bellows seals on the right hand compartment door, can stop fluid from draining.^[1]

The FAA's directive on the APU air inlet plenum of certain A320 family aircraft lists what builds up there: dirt, debris, dust, sand, oil, combustible residues and grease.^[7] That directive is about fire risk rather than cabin air, but the list is the same material.

Investigations bear this out. In a 2020 event on an A319, the UK Air Accidents Investigation Branch found the likely source was oil leaking from APU line replaceable units, including the drain mast and the oil cooler. Nine environmental control system components were later replaced.^[8]

How often it happens

Not often, measured against the number of flights. The FAA puts fume events, from all causes, at fewer than 33 per million departures, and lists APU contamination alongside engine oil seal failures, hydraulic leaks and de-icing fluid as known causes.^[10]

Where the APU sits among those causes is less settled. A 2025 peer-reviewed analysis of 2,845 fume-related Service Difficulty Reports filed with the FAA in 2018, 2019, 2022 and 2023 found the APU was the most often identified source, and that A320 family aircraft, about a fifth of the US fleet, accounted for four in five of the reports.^[11] The author works for a flight attendants' union and discloses expert work for injured crew, and the data is US only, so the proportions deserve care.

Not every event is the APU, either. In a 2020 bulletin the AAIB examined a run of odor events on an A320 that occurred in descent with the APU off, and concluded the APU was unlikely to be the source.^[9]

What regulators and standards bodies say

ICAO Circular 344, published in 2015, gives States guidance on training flight crew, cabin crew and maintenance technicians to prevent, recognize and respond to fumes, particularly fumes from the air supply system, and on how to report them.^[12]

In Europe, EASA published two studies in March 2017. The cabin air quality measurement campaign found air quality in the cabin similar to or better than that of normal indoor environments. A companion study, AVOIL, carried out by TNO and RIVM, examined aviation engine oils.^{[2, 3, 4]}

In the United States, section 362 of the FAA Reauthorization Act of 2024 deals with cabin air safety.^[13] The FAA says it has completed the studies that section called for and signed an agreement with the National Academies.^[10] The earlier 2018 Act led to FAA educational material on cabin air for crews and technicians.^[14]

For measurement, SAE ARP4418B sets out a procedure for sampling and measuring engine and APU generated contaminants in bleed air.^[15]

What cleaning does, and what it does not

Cleaning the plenum and the compartment removes residue that has collected in the air path and around the APU. It does not fix the source. Airbus's recovery sequence starts with inspection: the intake duct, the intake plenum and the bleed duct walls. Then the compartment, intake system, inlet plenum and load compressor are cleaned in line with the AMM, and the pneumatic system is decontaminated. DASH Aero cleans the compartment, the intake system and the inlet plenum. Washing the load compressor and decontaminating the pneumatic system are separate tasks, and DASH Aero does not perform either of them.^[1]

That sequence is written for after an event. DASH Aero's view is that the cleaning part of it belongs on a schedule as well, so that what has built up since the last clean is removed before anyone smells it. That is an operational judgment, not a regulatory requirement, and it sits alongside your approved maintenance program rather than replacing anything in it.

How DASH Aero does it