What kind of maintenance has to be performed on an aircraft if it has been stored for 6 months?
For example, do fluids need to be changed, have the tires become flat, etc.
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Bringing any plane back into service will involve more or less the same things but vary in scope and scale depending on the size of the aircraft. It looks like Southwest is moving their 737's to their long term storage facility. Since its in the desert they may wrap/plug the engine inlets to prevent any debris/dust from getting in. This will need to be removed before the aircraft are brought back into service.
Aside from consumables like fuel, oil and hydraulic fluid, etc which are checked before every flight they will need to check all calendar-related maintenance items. Most aircraft maintenance items are tied to hours or cycles which don't really matter when the airplane sits, but some things are tied to the calendar and a sitting plane may lapse on checks and updates it needs. Off the cuff a plane sitting for a while may lapse or need replacement on:
From a non-hardware standpoint, crew currency or type proficiency due to lack of flying hours on the airframe may land a chunk of pilots and FO's in the sim before the planes come back into service.
Jet fuel can grow bacteria in the tanks and a tank flush/purge may be done on aircraft that have been sitting for a long time depending on what they find when they get to the aircraft.
Since more information has been released since the original answer was posted. It now appears most airlines are expecting 100 - 150 hours of work to bring the current resting 737's back in to service after the software update gets approved. This only applies to the mechanical aspect, pilot training is still under debate:
each aircraft will likely require between 100 and 150 hours of preparation before flying
The preparations were discussed at a meeting between Boeing and MAX customers in Miami earlier this week, and include a list of items ranging from fluid changes and engine checks to uploading new 737 MAX software. The estimated time frame does not include pilot training, they said.
In addition to the modifications that will be listed by the FAA specifically for the 737max, generally speaking before bringing back Into service any aircraft, you need to know what has been done to store the aircraft: the engines have been removed or not? the hydraulic fluid has been exchanged with special storage fluid, or the hydraulics have been periodically recycled using external powering? The list of possibilities is long, therefore to bring back into service you must know ahead what has been done before storage and you must know the storage conditions.