There are currently many aircraft of several types being transitioned into operation after prolonged storage.
The storage activities are well described in Maintenance Manuals and SL's
AMM 10-11 for parking
AMM 10-12 for prolonged parking (more than 7 days, a month, two months, a year)
These Aircraft Maintenance Manual sections attempt to address different climate conditions and create, as far as possible, standard natural deterioration controllable conditions such as;
Corrosion
Aging of paint, composites, interior parts by elevated temperatures and UV radiation.
Fuel tank contamination by algae and fungus growth
Environmental Control System (ECS) duct contamination
On a number of past transition projects and recent reactivation, we found heavy contamination of gasper ducting in the cabin on well maintained aircraft of about 10 years of age. Picture below is consistent with findings over multiple projects.
Needless to say this is a perfect breeding ground for bacterial growth.
In a recent project, this condition had not been sufficiently addressed after a storage period, and resulted in persistent smell of mould in the aircraft cabin, even after a week of operation.
As the aircraft had prior been subject to a 10 year check where the complete interior had been stripped, consequently, the only source of this smell could be contamination of the ECS ducting (which had not been removed during the check).
Please note that this contamination had been accumulating over 10 years of service WITH periodic HEPA (High Effectivity Particlulate Air) filter replacements.
The AMM contains a duct cleaning task (AMM 21-23-01). It contains light check cleaning and heavy check cleaning. These tasks are NOT part of the Maintenance Planning Document (MPD) repetitive tasks.
Please note that HEPA filters filter recirculated air only. Air bled from the engines to pressurise the cabin in flight after being conditioned to the desired temperature (and pressure in flight) is not filtered. Only recirculated air is filtered. However, before recirculated air passes by the HEPA filters is can collect contamination and dust elsewhere; on carpet, dado panels (which are often very dusty) and pick up dust and contamination on the way. Dust is also picked up from upholstery materials, passenger and crew clothing and personal belongings.
Hence dust can collect throughout the airframe and air distribution system due to static charge, sticky surfaces, surface roughness already collected contamination, etc. .
Also note that, during ground stops in moderate climate, many operator and airport policies stipulate to minimise APU use and without running the ECS packs, cabin air is recirculated only. Please find attached graphic below.
We highly recommend to inspect the overhead cabin ducting for contamination during heavy checks when ceiling panels and overhead luggage bins are removed as part of a work pack. That way the duct cleaning procedure has as little impact on the man hour count of a project as possible.
A low impact inspection can be carried out by feeding a flexible borescope into the ducting for a visual verification of the contamination.
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