Cabin air smells like a wet dog? Clean your socks!

Effectivity: Challenger 600 aircraft

ATA 21

All Challenger 600 series aircraft are equipped two cooling packs each containing a water separator located downstream of the cooling pack. Cold air from the turbine section of the air cycle machine passes through the water separator before it gets distributed into the cabin and cockpit areas. When hot bleed air converts to cold during the air expansion process, any air moisture will condense to form a fine mist and the purpose of the water separator is to remove this misty air before distribution. As cold air passes through the coalescer bag (aka: SOCK), the mist is converted to larger droplets of water. The mixture of air/water droplets is then forced through louvered openings on the coalescer bag support cone where the heavy water droplets get separated from the air. The water droplets accumulate at the bottom of the collector sump where it then gets routed to the water aspirator at the bottom of the cooling pack plenum. The aspirator sprays water directly to the core of the dual heat exchanger to cool it down.

One of the inherent problems with the coalescer bag, which is made of a fabric material, is that it gets dirty as air passes through it. This is normal! And when it does get dirty, its efficiency to separate the water droplets is reduced and air flow is impaired.

A dirty coalescer bag is often caused by pollutants in the ambient air during ground cooling operations. Of course, the more time spent on the ground with the cooling packs running, the greater the risk of getting the coalescer bag dirty. In some cases, the issue can be exacerbated. This is especially true if oil migrates into the bleed air circuit from a failed APU compressor seal or a failed seal on engine compressor shaft bearing. 

What happens when the coalescer bag gets dirty?

A dirty coalescer bag will be more restrictive causing the differential pressure between the inlet side and the outlet side of the water separator to increase. When this happens, the normally-closed bypass valve at the inlet side of the coalescer bag support cone will begin to open (less than five psid) allowing moisture-laden air to pass right through the water separator without moisture separation.

Over the passage of time, heavy levels of moisture entering the pressurized vessel area can cause electronic component malfunction, corrosion, mould on surfaces and water accumulation in the distribution ducting leading to cabin odors (often reported as a dirty sock or wet dog smell). Also, under the right environmental conditions, you might witness fog manifesting inside the cabin through the cold air gasper vents.

Section 3 of the Maintenance Planning Document (MPD) describes optional scheduled maintenance intervals and recommends cleaning of the coalescer bag at a service interval of 800H for the Challenger 604/Challenger 605/Challenger 650 aircraft and 600H for the Challenger 600/Challenger 601 aircraft. Although the cleaning intervals are suggestions, you can customize the cleaning interval depending on the duration of ground operation and the environment cleanliness. The coalescer bag can be cleaned as much as three times before discarding it, however, if it is heavily soiled or appears to have traces of oil contamination (soot) then it is best to replace it. 

The coalescer bag can be removed for cleaning by unlatching the quick-disconnect band coupling first then separating the two parts of the water separator housing. The coalescer bag cleaning procedures can be found in the following AMM references: For the Challenger 604/Challenger 605/Challenger 650 aircraft, refer to TASK 21−51−13−160−801. For the Challenger 600/Challenger 601 aircraft, refer to Chapter 21-70-11, Water Separator - Maintenance Practices. The coalescer bag, Part No. 180849-28, is a line replaceable unit residing in Chapter 21 of each respective Illustrated Parts Catalog.

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Challenger 600 Advisory Committee Meeting September 21-22

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