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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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    $\begingroup$ No airplane experience but I've gotten generators, cars, weed whackers, etc. all going after sitting for extended time. In all of the cases the biggest thing was stale/old fuel in the system. Doing nice things like clogging the jets on carbs, breaking down gaskets, etc. Brake lines swell, etc. Tires get flat spots. Sitting is the worst thing that can happen for something mechanical like that. Powering up the engines, getting 'em up to "working RPMs" and taxing up and down the run-way and then parking again on a weekly basis would probably be better than static stagnant storage. $\endgroup$
    – ivanivan
    Commented Apr 18, 2019 at 19:56
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    $\begingroup$ @ivanivan Aircraft are much more complex machines than your day to day vehicle/tool. Not just because there are many more systems and subsystems aboard, but the overall tolerances for failure and wear and whatnot are waaaay tighter than you can believe, all things considered. If an aircraft is left to sit for 6 months, there are many more procedures and check lists to go over than just occasionally starting the systems and wheeling it around here and there. $\endgroup$
    – Jihyun
    Commented May 24, 2019 at 15:22
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    $\begingroup$ I really like the question as it mays also apply to other airplanes $\endgroup$
    – Manu H
    Commented May 25, 2019 at 19:58

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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:

  • Bi-annual Transponder Checks
  • Navigation/Terrain Database updates
  • Fire extinguishers past check/expiration
  • Expired Oxygen generation canisters
  • Expired slides/lifejackets (mostly the chemical inflator systems)
  • Batteries may need tending/replacement if run dead (or allowed to be run dead)

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.

-----EDIT------

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.

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    $\begingroup$ Interesting choice to call a test for microbial contamination a 'pregnancy-style test'... Congratulations, you have fungus in your wings... $\endgroup$
    – Sanchises
    Commented Apr 17, 2019 at 9:37
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    $\begingroup$ @Sanchises It's a boy!!! $\endgroup$
    – FreeMan
    Commented Apr 17, 2019 at 17:40
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    $\begingroup$ Or a yeast infection. $\endgroup$
    – Mike Brass
    Commented Apr 21, 2019 at 5:00
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    $\begingroup$ Considering it takes about 80 hours to put a plane into storage (from the Reuters article linked above), 100-150 to get it out does seem to be reasonable. $\endgroup$
    – FreeMan
    Commented May 24, 2019 at 15:26
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    $\begingroup$ Just clarifying - those are person-hours? So a team of 20 could do approximately one plane a day. $\endgroup$
    – Criggie
    Commented Apr 1, 2020 at 2:39
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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.

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