Do you have an unexplained odor in the cabin? Ever consider it may be Cicadas?
Cicadas are insects, with large transparent well-veined wings, that in many geographical areas emerge in limited numbers every year. However in the southern states of North America they emerge in large numbers every 13 years and every 17 years in the northern states. Although these insects are harmless to people, when they emerge in large swarms they can be easily ingested by an aircraft’s APU and pushed through its environmental system causing a foul odor in the cabin. Although not often, if enough cicadas have been ingested it may cause damage to an aircraft’s APU.
If you are operating in a region where cicadas are out, it has been suggested using your engine in lieu of your APU. The engine can better cope with ingesting these insects and grinding them up without harm to components. If you are not sure if Cicadas are in the area, stop and listen. They can be heard as they chirp at an ear piercing 120 DB.
The process of cleaning your environmental system from cicada ingestion can be as simple as replacing or cleaning water socks to the removal and flushing of precooolers and intercoolers and duct work.