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This article details the recent (March 2020) grounding of 90% of Ryanair's fleet due to the COVID-19 outbreak.

Flight tracking data showed that all bar one of the jets had been flown in recent days: 35 had made loops of the airport, while the remaining 11 aircraft had operated a service at least once every four days.

Why are these flights required? The article states this is to keep the aircraft 'serviceable', but the airport I'm learning at has planes that haven't been in the air for months... if they are booked for a lesson, they get a service and away we go.

Why would a budget airline want to spend so much money on fuel and flight crew for these flights, when this could continue for months, it seems like it would make more sense to keep them grounded, and then service them all when this epidemic is over.

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    $\begingroup$ Passenger airplanes transport cargo as well and since so many of them are grounded, there are not enough cargo airplanes to satisfy urgent transportation needs. Some airlines therefore operate ghost flights without passengers for freight only. I don't know whether this applies to Ryanair though. $\endgroup$ – svoop Mar 31 at 11:03
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    $\begingroup$ In Europe, an airline looses a slot if they don't use 80 % of it in a certain timeframe ("use it or lose it" rule). So some planes are operated just to keep the slots. This rule is, however, currently suspended (or at least being discussed) to avoid such ghost flights due to the Corona virus. See also businessinsider.de/international/… $\endgroup$ – PerlDuck Mar 31 at 13:31
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    $\begingroup$ @svoop Then that wouldn't be a ghost flight, it'd be a cargo flight. You can run cargo in passenger jets, in fact it's a good plan for heavier pallets. 90% of the time a semitrailer "cubes out" before it hits weight limits; that number is way lower for aviation. So you put the heavy cargo on passenger liners in their limited cargo holds. Now all the planes "cube out". $\endgroup$ – Harper - Reinstate Monica Mar 31 at 21:18
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Airliners are both larger and more complex than the small aircraft you're familiar with at your local flight school. For an example of some of the items to consider, see: What do you need to do to bring a 737 Max back in service after 6 months in storage?

A lot of those things won't be improved just by flying the plane periodically, but some items are. Tires can develop flat spots from sitting for too long. Engines and systems need to have their fluids circulated. Getting things warmed up and moving helps to prevent corrosion. Even with proper storage procedures, an aircraft would have to be thoroughly checked for any foreign debris or damage that could have accumulated.

As the article you linked states:

Planes that have been grounded for a significant period have to be checked over before they are cleared to fly again, a process that keeps them from flying for even longer and costs the airline money.

There is of course a tradeoff between paying to put the aircraft in long-term storage, and later to bring them back into service, versus keeping them in working order. Also, as mentioned in the answer to the linked question, the flight crew need to fly a minimum amount to be considered "current" and allowed to fly passengers. Even aside from regulations, having periodic practice will help to maintain skills for everyone involved.

Having crews do a few laps around the airport helps to keep both the airplane and crew in working order, and avoids having to do more rigorous checks to get them back up to speed when needed, which can save both time and money.

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    $\begingroup$ "Airliners are both larger and more complex than the small aircraft you're familiar with" -- that's true but, frankly, all of the same things still apply to the smaller airplanes. There is a noticeable increase in net maintenance costs even for small planes flown monthly vs. those flown at least once a week. I don't know where the OP is flying or what "they get a service" entails, but I wouldn't trust an airplane that's been sitting on the ground "for months" to be ready for me to just hop in and go. ... $\endgroup$ – Peter Duniho Mar 31 at 18:34
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    $\begingroup$ ... Same thing applies to wheeled vehicles and boats, for that matter. Sitting idle is very bad, and if one is going to leave them idle for extended periods of time, it's cheaper in the long run to put them into storage properly. Conversely, if you want to make sure you're ready to resume operations immediately, it's better to just keep operating them. $\endgroup$ – Peter Duniho Mar 31 at 18:34
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    $\begingroup$ @Darrel: "you can start the engines up and run them for a while without actually taking off to keep them in decent condition" -- not really. Most small airplanes use air-cooled engines. To operate them long enough to warm up the oil, evaporating accumulated moisture, keeping the engine warm for long enough to really do it properly, it's safest and easiest to just fly the plane, which keeps adequate cooling air moving over the engine. And while simulators light plane pilots have access to are great for procedures, they aren't all that good for basic operational skills. $\endgroup$ – Peter Duniho Mar 31 at 19:24
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    $\begingroup$ @DarrelHoffman Once you get a Turbofan engine (like a commercial airliner) up to temperatures to properly warm up the oil, burn off moisture, et cetera, it will want air going through it at at least 300mph, for at least half an hour - good luck managing that on the ground! This is similar to how a diesel car requires a fairly long journey (30 mins at 50mph) for the DPF to be able to get hot enough to "regenerate" successfully. (Making them bad for "city drivers" doing lots of short or slow trips instead) $\endgroup$ – Chronocidal Mar 31 at 20:09
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    $\begingroup$ You can do that on the ground, but your neighbors will hate your guts. $\endgroup$ – Harper - Reinstate Monica Mar 31 at 21:11
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Perhaps this adds little to the answer already posted, but I feel it needs to be emphasized that

airliners are designed to fly.

Designed to fly a lot, in fact. Like, spend a significant portion of their working lives in the air. Southwest Airlines, for example, has each of its 737s in the sky for 9 hours a day. I imagine Ryanair's number is similar. And most of that time on the ground is spent moving as quickly as possible to do the tasks required to get back into the air again. Basically, having your fleet sit around is an off-design condition.

Not only is it an off-design condition for the aircraft itself, it's an off-design condition for the entire maintenance concept and infrastructure of the airline. An airline knows how to put airplanes in the air and keep them there as much as possible, because that's how to make the most money. (If you're an airline, of course; aviation is generally not the way to make the most money.) Airlines are not experts at parking large numbers of big airplanes for a long period of time and then returning them all to service. In fact, no one is because, in general, this situation just doesn't happen.

So, your choice is either go through the process of mothballing your airplanes for an indeterminate period, which involves a bunch of maintenance tasks you're not equipped for (not to mention finding a place to put them) or that you'll have to pay someone else to do (likely at a premium right now), or try to keep the fleet operating as close to the design condition as is reasonable. The former option has the added drawback that the airplanes are likely to be a bit cranky for a while (i.e., more unexpected problems) after coming out of storage. The latter option has the added benefit of keeping flight and maintenance crews more current.

We can even run some rough numbers. Based on the article, let's say each airplane currently flies about an hour a day rather than nine. That means it might be due for what amounts to an A check every year or so, which at the high end could take 100 maintenance hours performing tasks that the airline does all the time and has the people and materials in place for. Compare to the process of mothballing and then unmothballing an airplane, which might take at the low end 200 hours of tasks that are not often performed. Of course in the interim the cost of flying is greater than the cost of sitting, but still it could be cheaper for an airline to run these "ghost" flights for quite a while than put their fleet in storage.

Overall, it is simply a less financially risky proposition at this point to give the airplanes a little bit of time in the air every once in a while than ground them and expect to resume service quickly. Flying and maintaining them "as usual" presents a known situation for an airline, especially when the horizon for returning to normal is closer to months than to years. Storing the aircraft for what could potentially be a relatively short period (as far as these things go) is fraught with unknowns.

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    $\begingroup$ Hey, I enjoyed the million dollar blog post. Thanks for that and the answer! $\endgroup$ – Cloud Apr 1 at 8:46
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    $\begingroup$ I'm pretty sure the ryanair number for plane utilization is higher than nine hours a day. Couldn't find a recent source though. $\endgroup$ – bobsburner Apr 1 at 9:09
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    $\begingroup$ @bobsburner: Could the difference be between "in service" (including turn-around time on the ground) and "in the sky"? 9 hours per day in the sky sounds reasonable enough for a 737 on domestic routes. That could easily translate to 16+ hours in service, as you have a lot of loading/unloading time when you fly many short hops. $\endgroup$ – MSalters Apr 2 at 10:29
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In addition to all these great breakdowns of the technical aspects of planes airlines will lose their precious airport-slots if they fail to actually use them.

I.e. if an airline pays for a (lucrative, useful) time-slot & terminal at a certain airport, but then doesn't actually use it, the airport will give away the slot to someone else.

Therefore there are occasional empty flights just to keep up the slot at the airport.

Source:

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    $\begingroup$ As far as I'm aware, just about everyone has waived the requirement for using timeslots right now. $\endgroup$ – Mark Apr 1 at 20:10
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    $\begingroup$ Oh neat, I didn't know that! That'd mean the current ghost-flights at least are less motivated by airport-slots :) $\endgroup$ – IcarusTyler Apr 2 at 7:04
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The "use it or lose it" rules in Europe are basically trying to compel airlines to keep flying their aircraft. There is a 80/20 rule, which means airlines need to operate 80% of their allocated slots, or a competing airline can take them during the annual assessments by officials. And just like IcarusTyler said, they won't just give away their expensive spot by not using it.

Source: https://nypost.com/2020/03/09/why-airlines-are-running-ghost-flights-amid-coronavirus-panic/

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