Airbus released an extensive report about their investigation and conclusions with respect to contaminated cabin air and its causes in their in house magazine FAST (Flight Airworthiness Support Technology) Issue #52 released in August 2013, pages 4 to 9.
A check amongst the aviation industry showed, that the publication remained largely unknown in the industry.
In general Airbus reports, that residual contaminations following leak events, that have not been cleaned out of the Environmental Control Systems, cause many of the fumes events. “A clean APU means clean cabin air” Airbus labels one of their photos illustrating their report.
In an additional statement to German media Airbus wrote: “It is the operator’s responsibility to ensure their aircraft and its systems are maintained properly.”
Airbus concludes, that failure to follow all maintainence procedures “will lead to ineffective or only partial decontamination and therefore, the risk of future reports of odours in the cabin.”
Airbus opens their report:
Air supplied by the Environmental Conditioning System (ECS) can be contaminated by the unavoidable ingestion of environmental pollutants such as industrial pollution as well as engine exhaust pollution from the many vehicles and aircraft operating in the airport environment.
Since the ECS is supplied with air from the Auxiliary Power Unit (APU), engine bleed system or Ground Power Units (GPU), there is also a possibility of contamination from these.
One source of in-service contamination events reported on the A320 Family fleet is the APU. This results from either internal leakage or re-ingestion of oil following external leakage. It is worth noting that a noticeable cabin odour can be generated from ingesting only a very small amount of oil.
Airbus describes possible symptoms observed on board of aircraft:
The most common reports are those of stale odours or oil smells and occasionally visible smoke, or activation of the smoke detection system in conjunction with a ‘bleed pack’ overheating. From this information alone though, it is not possible to determine whether the APU or the engines contaminated the ECS.
From experience, odours associated with slight APU oil leakage are more often reported shortly after take-off, disappearing in cruise and reoccurring late in the descent. Odours may also be noticeable at the APU start for a few seconds, but quickly disappear.
The odours can disappear after a short time due to changes in the pack configuration (from cooling to heating mode), reoccurring again late during the descent due to the packs returning to the cooling mode.
Operational experience has also shown that intermittent odour reports, as described, can occur after the source of leakage had been identified and rectified. This is due to the residual system contamination and emphasizes the importance of a thorough ECS decontamination.
Airbus investigates various sources of fumes including external fumes, aircraft external fluid leakage into the APU, APU external leakages into the APU as well as APU internal leakages.
With respect to external fumes Airbus writes:
External fumes present in the vicinity of the APU intake (i.e. exhaust gas fumes from main engines of any aircraft nearby, or ground power units (used during ground operation, etc.) may be ingested, leading to smell in cabin. The smell will stop when the fumes are no longer ingested.
Under these circumstances, no further inspections will be required on the APU. However, we recommend performing cabin ventilation to eliminate any residual smell before the next revenue flight.
With respect to aircraft external leakages Airbus writes:
It may happen that hydraulic leaks from the main landing gear bay are directed along outside the aircraft fuselage, up to the APU air intake. Hydraulic fluid can then be ingested while the APU is running, causing an ECS contamination and associated smell in the cabin. Under these circumstances, a visual inspection should be carried out on the aircraft fuselage from the nose landing gear up to the APU inlet, and from the vertical stabilizer up to the APU inlet for presence of hydraulic fluid. Also, check for the presence of hydraulic fluid on the APU inlet duct to confirm ingestion by using ultra-violet/black light. Any hydraulic leak identified must be fixed.
There have also been reports of odour in the cabin following an aircraft de-icing process. Following this operation, inspect the external surfaces of APU doors for presence of de-icing fluid and then check if there is any on the APU inlet duct.
Note that when using de-icing fluid on aircraft, the APU bleed system should be selected ‘off’. Additionally, we recommend waiting three to four minutes after an aircraft de-icing is completed before selecting the APU bleed ‘on’ again.
Then clean the contaminated surfaces (APU doors, intake duct air flow path, etc.).
With respect to APU external leakages Airbus writes:
External oil or fuel leakage from the APU may be re-ingested if the APU inlet seal or APU doors’ seal is degraded, or the air intake diffuser is misaligned.
Additionally, misalignment of APU drain mast and bellows’ seals fitted on the right-hand APU compartment door may not allow fluids to be properly drained outside the APU compartment.
If an APU external fluid leakage re-ingestion by the APU is suspected, inspect the APU compartment and the APU external surfaces to identify the source of the leakage by using ultra-violet/black light.
While inspecting the APU external surfaces, we advise to check fuel and oil tubes and fittings, inspect around the APU oil pump, oil cooler, fuel pump and actuators driven by the fuel as the Inlet Guide Vane Actuator (IGVA), Starter Control Valve (SCV) and Bleed Valve (BV).
Inspect around the oil temperature, oil level and oil chip detectors, and the pressure sensors.
Inspect around the circumference of the APU generator to the gearbox mount, around the APU starter motor flange, and around the load compressor housing to the gearbox split-line.
Check the alignment of the drain tube bellows on the door with the drain tubes on the bottom of the APU by closing the right-hand APU door and checking from the open left door. If they are not aligned, adjust the spring adapter assembly on the door.
If evidence of contaminants is found, identify the contaminants’ path which led to the APU pneumatic system. Inspect the APU intake duct, APU intake plenum and bleed duct inner walls for the presence of contaminants.
Eliminate the source of contaminants as per Aircraft Maintenance Manual (AMM) instructions and if ingestion is confirmed, inspect the condition of the APU doors’ sealing, intake duct sealing, alignment between the APU intake duct and APU inlet plenum, oil cooler discharge duct and restore them if required.
Failure to do so will lead to repeated occurrences of reporting odour in the cabin, whenever an APU external fluid leakage occurs.
With respect to APU internal leakages Airbus writes:
If oil ingestion is suspected and no external leak is evident, inspect the bleed duct downstream of the APU bleed valve with ultra violet/ black light for oil contamination.
If there is evidence of oil, there is the possibility of an internal APU oil leakage or ingested oil.
Check the APU load compressor witness drain for the presence of oil. The load compressor sealing system breaking down would result in oil being present at the seal cavity witness drain.
Inspect the inside of the cooling fan ducts for evidence of oil. If oil is present and the inside of the plenum is clean, further inspection of the oil cooler and cooling fan is required as there might be a cooling fan/oil cooler internal oil leak.
Check the APU oil level for presence of over-servicing. Note that prior to inspecting the oil level, it should be confirmed that a complete de-oiling during normal APU shutdown has been performed. This can be done by checking that the APU air intake flap is fully closed. If the shutdown circumstances are unknown, operate the APU at ‘no-load’ for five minutes, followed by a normal shut down.
For a complete APU de-oil, wait at least 15 minutes after the last APU shutdown prior to checking and eventually completing the oil level.
Eliminate the source of contaminants as per AMM instructions.
Note that it may require an APU removal depending on the source identified.
Airbus concludes the report with the final steps needed to be carried out:
Once the inspections have been carried out, the contaminant source has been eliminated and the APU compartment/duct sealing condition has been restored, the next step will be to clean the contaminated APU compartment, APU intake system, APU inlet plenum and APU Load Compressor as per AMM instructions.
Afterwards, decontaminate the aircraft pneumatic system as per AMM 21-00-00 and SIL 21-029. Failure to do so will lead to ineffective or only partial decontamination and therefore, the risk of future reports of odours in the cabin.
APU overview – a clean APU means clean cabin air (Photo: Airbus):
Read the full article here.