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With the Covid-19 crisis many thousands of passenger aircraft have been put in storage. In this Bloomberg article from July 2019 shows several aircraft in what appears to be storage with covers on engines and cockpit windows. They also appear to be connected to an electrical supply and the engines to some sort of compressor.

Here is a screen shot of the image: Screen shot of stored aircraft

And a link to a large size version of the full image. Another link to a different photo of stored 737MAX with similar connections.

The Airbus website describes requirements for long term storage, but mentions no connections to air or electricity.

Why are these aircraft provided with air and power?

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    $\begingroup$ The cables may be ground connections. $\endgroup$ – Uwe Jun 29 at 11:22
  • $\begingroup$ Here's a Forbes story that adds detail to what is done to store 737MAXs but doesn't describe connection details in the above photos. forbes.com/sites/jeremybogaisky/2019/08/12/… $\endgroup$ – D Duck Jun 29 at 12:30
  • $\begingroup$ howdy @DDuck , whoever threw together that page at Bloomberg, perhaps just added a "representative photo". It's unlikely that person specifically had detailed knowledge about whether these particular planes are in "long term" storage. The planes may be in - just a random example - three-week storage while waiting for a decision to be made. Jörg's answer seems to clarify the situation. $\endgroup$ – Fattie Jun 29 at 12:30
  • $\begingroup$ @Fattie True, but 6 months later you'd expect Flight Global to get it right. Here is a different set of aircraft stored in a similar way flightglobal.com/air-transport/… $\endgroup$ – D Duck Jun 29 at 12:51
  • $\begingroup$ This link shows how Boeing is storing their 737MAXs and they don't have the extra connections. businessinsider.com/boeing-737-max-fills-storage-lots-2019-4 $\endgroup$ – D Duck Jun 29 at 12:53
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Why do these aircraft in long term storage have air and power connections?

The photo is of 737MAX being stored in July 2019. Back then, airlines might still have hoped that this wouldn't be "long-term storage" at all!

Usually, airplanes being mothballed are stripped of corrosive / humidity-sensitive parts (e.g. the interior) as much as possible, stored in the dry desert, and it is considered acceptable if getting them back out of storage requires weeks if not months of refurbishment.

None of these three things seems to apply here. The photo doesn't look like it was taken in the desert, the airplanes are probably fully intact, and the airlines were hoping that with a simple software patch, they might be able to get them off the ground quickly again.

Therefore, they would have opted to, instead of preparing the airplanes for long-term storage, have them partially powered up, or use external air conditioning units. I would be willing to bet that those exact some planes are stored differently in July of 2020!

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    $\begingroup$ It turns out that they're not actually even owned by airlines, yet. According to airfleets.net, that aircraft in AC livery hasn't actually been delivered yet (as of summer 2020,) so I'd bet that's the case with all of them. This appears to be some of the MAXs stored by Boeing waiting for delivery pending the recertification of the MAX. $\endgroup$ – reirab Jun 29 at 16:00
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    $\begingroup$ How long can you store an aircraft before it loses it's "new 'plane smell?" $\endgroup$ – D Duck Jun 29 at 16:29
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    $\begingroup$ @DDuck: Maybe that's what those air connections are for? $\endgroup$ – Jörg W Mittag Jun 30 at 5:13
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They probably have dehumidifiers inside to keep the air absolutely dry in there. This prevents any corrosion from occurring due to condensation or excess humidity.

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  • $\begingroup$ I understand the requirement to keep things dry. A dehumidifier in the cabin (and engine) would make sense - but this requires someone to periodically drop by and empty the tank. The engines seem to be connected via pneumatic hose to some sort of unit - a compressor or heater - this video for the CFM56 doesn't connect any unit to the engine. $\endgroup$ – D Duck Jun 28 at 21:44
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    $\begingroup$ @DDuck: They can just dump the water outside. $\endgroup$ – Joshua Jun 29 at 4:22
  • $\begingroup$ Let's see. You could use the aircraft's own air conditioning, you could use some sort of desiccant (silica gel etc) or you could flow dry air from somewhere - perhaps compressed air, that is cooled and dewatered or you could just use a chiller. Or you could use 100s of domestic dehumidifiers. You could also hang out in a region with dry air - perhaps somewhere really cold all the time. So I guess that there are two - cooling & chemical drying. (Some might argue compression, but there's cooling involved there too). Could heat everything too, but that's sometimes problematic. $\endgroup$ – D Duck Jun 29 at 16:09
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    $\begingroup$ @DDuck Industrial dehumidifiers exist, and you could just pipe a couple into the aircraft. $\endgroup$ – Dan Jun 30 at 17:49
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To cool and dehumidify the interiors

Except they don’t need to empty the tank every 6 hours.

As you probably know, when an aircraft gates, it shuts off all engines including the APU. That means it relies on external power for electricity and HVAC.

The problem of hot interiors

Ever get in your car after it’s sitting in the hot sun, and it’s freakishly hot and you immediately start the A/C? Try not doing that, close the door, leave the windows up and sit there for 5 minutes. You can’t. It’s too darn hot. That’s why it’s so lethal when a baby is left in a car. I’ve had PVC pipe, left in the hot car, actually bend into a pretzel.

That’s caused by the sun bearing down on the car. It’s called “solar gain”. The same thing happens to railroad cars; that’s why railroad museums only have 2 kinds of cars: open-window, and air-conditioned.

For the auto industry to design interiors that can endure that for 20 years and not melt and bend like my PVC pipe, *is actually a really hard problem, and they spend billions on it. Obviously it’s the top priority in interior design.

Now turn the page to an airliner. Exactly the same thing will happen due to solar load - except aviation interiors have a different top priority (safety and weight). So they must compromise away automotive traits to get what they need.

Mind you, the best places to store airliners are dry and very hot - like the Mojave Desert or Tucson. This makes the solar-load problem even worse.

So we have a) a vulnerable interior, b) very hot storage locations, and c) you can’t open the windows on an airplane.

That means you must run HVAC, or remove the interior, or lose it!

Damp interiors are just as much of a nightmare, as they will breed mold. An unmaintained interior can accumulate moisture due to condensation: hot humid air leaks in during the day, then the humidity condenses in the cold night. Now you have liquid water where you did not expect.

Cooling (and dehumidifying) an airplane

Aircraft are already set up to take external sources for a few things: direct HVAC, compressed air and electricity. That’s so they can power down entirely at the gate for smog and fuel savings, and rely on shore-side equipment to keep the aircraft comfortable.

If they can hook up HVAC hoses (conditioned air) directly, that’s grand; however the airplane can make its own HVAC out of compressed air, thusly

The HVAC packs are designed to work on “bleed air”, compressed air drawn off the engines. Shore-side compressed air works just as well. The same thing happens whether shore-side or from engines: compressed air is very hot, and water-laden. It has the same air-mass:water-mass ratio as it did before it was compressed, so much more water per CC, and the hot air can hold that.

But when you cool compressed air, you reduce its capacity to hold moisture. The moisture condenses at this point, and every compressor and intercooler has gear to contend with this (cheap home compressors have a drain on the bottom of the tank).

Now that the compressed air is cool, if you allow it to decompress, it becomes cooler still. (This is just the reverse of the air heating up when compressed; it’s the poor man’s version of the freon cycle, same basic physics but not benefiting from the vapor-liquid enthalpy).

When you use an air-powered hand tool vigorously, it gets quite cold.

Since we’ve already wrung most of the water out of this air, when we decompress it, it comes out exceedingly dry compared to the air we started with at that same pressure.

Thus, we both air-condition and de-humidify the airplane in one stroke.

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  • $\begingroup$ in a selection of articles found at the bottom of this page, none mentions connecting the aircraft to dry air. The suggestion is to wrap them up somewhere hot and dry. $\endgroup$ – D Duck Jun 29 at 20:34
  • $\begingroup$ Barton says keeping an aircraft's cabin free of humidity is key. "[That way] it won't start to smell. That's what we worry about the most." https://www.cnet.com/features/parking-a-375-million-airplane-takes-more-than-just-locking-the-door/ $\endgroup$ – D Duck Jun 29 at 20:37
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    $\begingroup$ @DDuck Because you don’t need to specify dry air; if you’re feeding conditioned air, it’s dry by nature because of the factors I discussed. Nobody says “Hey, make sure that shore-side air is dry”. As far as parking it at places like Mojave, it’s not quite that simple. There are big temperature swings, and now you learn all about relative humidity. That 110F air might be 20% RH, but when it cools at night to 50F, now it’s over 100% RH and condensing. $\endgroup$ – Harper - Reinstate Monica Jun 30 at 0:44
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I lack the details, but its something to do with minimising the refurbishment time. Its cheaper and easier to leave them on "low power" than to completely cold-store the aircraft.

You might find that periodically the engines are cycled in some way, the APU is started and run for a while, and the whole aircraft gets moved forward a few feet to roll the tyres over and minimise flat spots.

Cold-storing an aeroplane means about a hundred hours of ground-crew time, to change out all the oils, hydraulic fluids and coolant. And its roughly the same level of effort required to reactivate a stored craft again. Contrast that to maybe an hour a week of effort to "warm-store" the aircraft, and this "hedging" becomes quite a good strategy for now.

It would be a fair bet that if air travel remains at current levels, some of these aircraft will be prepped for long-term storage, which may mean relocating them to one of the boneyard areas of the world, like Arizona, or the Australian outback. At the moment, the jury is out on whether air travel will ever rebound to pre-pandemic levels.


Its the difference between winter-storing your petrol lawnmower, vs keeping it inside over the winter, if you want a more at-home similarity.

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    $\begingroup$ "this "hedging" becomes quite a good strategy for now" – Indeed. After all, the photo is of 737MAX in July 2019, back then airlines might still have hoped to have them back in the air within weeks. $\endgroup$ – Jörg W Mittag Jun 29 at 6:12
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I'd guess the white cart with the black hoses coming out provides high pressure air to start the engines periodically without having to start the APU. Its my understanding that the engines are started briefly once a week. The skinny light-colored cables look like ground cables, usually connected to a lug on the landing gear. I don't see anything resembling ground power cables, which would be much thicker and connect near the nose of the airplane.

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