How to take care of Cabin Air conditioning Odours originating from an Aircraft APU during ground operations ?

Find out the Source of  Air to the Aircraft Environmental Conditioning System (ECS) !

Environmental Conditioning System (ECS)  or  Air conditioning system can be contaminated by the non-aircraft source ingestions, i.e. industrial pollution as well as engine exhaust pollution from the many vehicles and aircraft operating in the airport.

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 aircraft sources .

• Ambient  fumes ingested by the APU.
• Aircraft external fluid leakages ingested by the APU.
• APU external fuel or oil leakages ingested by the APU
• APU internal oil leakages .

 

The APU Highlight !

One of the contamination sources is the APU, which can result  from either internal leakage or re-ingestion of oil following external leakage.

It is always important to find out the exact source of the contamination, so that a corrective action can be initiated.

►  Ambient fumes ingested by the APU.

External fumes or ambient 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.

The Action :

When this aspect is confirmed, no further inspections is needed on the APU. Performing a cabin ventilation helps !

►  Aircraft external fluid leakages ingested by the APU.

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• Hydraulic Fluid !

Sometimes, leaked aircraft fluids from the main landing gear (MLG) bay may get directed up to the APU air intake. Hydraulic fluid / fumes can get  ingested to a running APU, causing an ECS contamination and smell in the cabin.

Under these circumstances, a 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.

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• Aircraft de-icing Fluid !

Many times odour in the cabin can happen following an aircraft de-icing process. If , that is the case, 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.

Be vigilant that, when using de-icing fluid on aircraft, the APU bleed system should be selected ‘off’. Also avoid APU bleed immediately after an aircraft de-icing is completed.

• The Action :

When this aspect is confirmed, no further inspections is needed on the APU. Performing a cabin ventilation helps !

► APU external fuel or oil leakages ingested by the APU.

Sometimes, External oil  or  fuel leakage  from the  APU may get  re-ingested if,

• APU inlet seal or APU doors’ seal is damaged / or aged,
• The air intake diffuser is misaligned.
• APU drain mast and bellows’ seals misaligned and fails to darin the fluids outside the APU compartment.

• The Action :

If an APU external fluid leakage re-ingestion by the APU is suspected, a thorough check up of APU system is required using AMM , some examples are as below !

• Inspect the APU compartment and the APU external surfaces to identify the exact source of the leakage by checking the fuel and oil tubes and fittings.
• inspect around the APU oil pump, oil cooler, fuel pump and actuators driven by the fuel .
• Inspect around the oil temperature, oil level and oil chip detectors.
• Inspect around the circumference of the APU generator to the gearbox mount.
• Check the alignment of the drain tube bellows on the door with the drain tubes on the bottom of the APU .
• If evidence of contaminants is found, identify the contaminants’ path which led to the APU pneumatic system. 
• Eliminate the source of contaminants as per Aircraft Maintenance Manual (AMM) instructions.
• Failure to do a thorough inspection will lead to repeated occurrences of reporting odour in the cabin.

► APU internal oil leakages .

• If external leak is not the cause of odour, inspect the bleed duct downstream of the APU bleed valve thoroughly for oil contamination.
• If there is evidence of oil, possibility indicates 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 in cooling fan ducts and the plenum is clean, the oil cooler and cooling fan is next suspect for the oil leak.
• Check the APU oil level for presence of Over-servicing.  
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• The Action :

Eliminate the source of contaminants as per AMM instructions. Sometimes an  APU removal is needed depending on the source of contamination.

► Proper Cleaning of  APU compartment, intake system and ECS :

Only elimination is not enough, after all the contamination sources are eliminated, cleaning the contaminated APU compartment is imporatant !

Clean the APU intake system, APU inlet plenum and  APU  Load Compressor as per AMM guidelines. Aircraft pneumatic system decontamination is also required.

Failure to do so will lead to recurring reports of odours in the cabin.

Note : Use OEM / Aircraft manufacturer recommended practices and manuals for any maintenance attempt on the aircraft. Above write up is for the purpose of general understaning only !