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My flight was delayed and then canceled because there were problems with the plane.

Why don't airlines have backup planes just in case there're mechanical problems with the plane? Or they do have them, but they just don't use them in these cases for some reason?

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    Google for "airline spare planes" turns up a lot of information. Summary seems to be that idle planes are extremely expensive, yet there usually are some spare planes, but they might not be where you are, and usually it's more efficient to rebook passengers than to move an aircraft. – Nate Eldredge Aug 24 '16 at 3:17
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    For the same reason most people don't have a spare car to use in case their main one breaks down. – fkraiem Aug 24 '16 at 3:32
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    Many airlines do have 1 or 2 "spare" or "spare-ish" planes at their main hub(s). They don't want too many sat around un-used, as planes are expensive! There's also the issue that they might have one widebody spare, then a shorthaul narrowbody goes tech, or they had 1 spare narrowbody and two fail. At an outstation there won't be anything spare – Gagravarr Aug 24 '16 at 6:32
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    @fkraiem: Actually, it's more like having a whole fleet of spare cars, one stationed at every destination. Say, you drive your kid to school, then drive to work, afterwards you drive to the doctor, go grocery shopping, clothes shopping, have coffee with a friend, then drive to the school and drive your kid to soccer practice. You would need 9 cars, one each stationed at home, school, work, the doctor, grocery shop, mall, coffee place, soccer field, and one for driving around. And you would need drivers for repositioning those cars for the next day. And a fleet of cars to get the drivers … – Jörg W Mittag Aug 24 '16 at 8:07
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    @SantiBailors A bus company has spare buses at their home depot, just like an airline does, but it also doesn't have a spare bus at each stop - if a bus breaks down in the middle of the line, the people waiting at the next stop will experience exactly the same situation, as the bus scheduled to be there at xx:yy simply won't arrive and will not take passengers today, they will have to wait for the next scheduled bus, or (for routes with long gaps) when a replacement will arrive (late) from the depot. – Peteris Aug 24 '16 at 9:57

10 Answers 10

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Because it would be ridiculously expensive.

Not only would it require a massive increase in the number of planes an airline has to purchase and maintain, it would not even garantee that there would always be a spare since there are many different types of airplanes that serve different purposes. It would be impossible to have one of each on standby in every airport.

Airlines therefore chose to live with the cost (and reputation hit) of not having immediate spares, even if that means compensating passengers or even losing customers.

Small caveat: Airlines will usually have some aircraft on standby for a variety of reasons. For example: at their main hub, for maintenance, etc. My answer is targeted towards airlines not having enough spares to cover for every flight delay around the world.

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    Not to mention that flight crews are apparently trained on specific models and not allowed to work models they haven't been trained for. I got this from talking to a flight attendant who was a fellow passenger on a delayed flight while we waited for crew to show up. I asked her if she couldn't step in and work the flight and she said she couldn't because she hadn't been trained on the aircraft. So to make this spare plane thing work, you'd have to have one of every model! – G. Ann - SonarSource Team Aug 24 '16 at 16:07
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    There is also the matter of cascading effects. For example, once one flight is cancelled, the crew and plane are out of position for their next flight, and so on. Or, a plane might be available but there isn't a crew that can fly according to FAA regulations for on-duty hours, etc. – GalacticCowboy Aug 24 '16 at 18:00
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    "Airlines therefore chose to live with the cost (and reputation hit) of not having immediate spares, even if that means compensating passengers or even losing customers." The alternative is not having customers. Not many customers would be willing to pay the premium of flying an airline with a large fleet of spares. – Steven Rumbalski Aug 24 '16 at 20:24
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    While I agree with the answer in principle, it's worth noting that airlines would usually have (almost) readily-available replacement aircraft in their home airport. For example, BA does have spares in Heathrow as does KLM in Amsterdam. The problem usually arises when something goes wrong far from home - worse still, in the air and an emergency landing results in the need for a replacement somewhere where the airline doesn't fly at all. (to be continued) – Aleks G Aug 24 '16 at 20:38
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    I once was on a BA flight from USA to UK, which made an emergency landing due to fire on board - in Goose Bay in Canada - on a military base - as that was the closest available runway that could accept Boeing 747. We ended up spending over 24 hours in Goose Bay while they were repairing the aircraft - it was cheaper to fly their mechanics and parts to Goose Bay from Toronto on a chartered plane than it was to fly an empty spare 747 there (and then the damaged/repaired 747 back to London). – Aleks G Aug 24 '16 at 20:41
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Look at a typical airline company on Wikipedia. Let's take KLM for example:

  • Fleet size: 117
  • Destinations: 138

As you can see, in order to have a spare plane on every flight, KLM would need to buy more spare planes than they have in the actual fleet, meaning their ticket prices would have to double in order to cover the expenses. Since people are not ready to pay twice the price for the comfort of saving a couple of hours in rare cases of cancelled flights, this is not happening.

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    Probably not double, as the cost of the plane (amortisation, maintenance) is actually just a fraction of the total cost of the flight (which also includes crew, fuel, etc.), but still a significant cost. – jcaron Aug 24 '16 at 10:29
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    And all the space needed at each airport to hold all those extra jets ! – Max Aug 24 '16 at 17:19
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    KLM almost certainly does have several spare aircraft at Schiphol, though, and will fly them to outstations as required when another aircraft breaks down. There will be a delay in getting the replacement there, of course, but that's usually better than a cancellation, depending on the number of passengers that would be stranded and for how long, etc. Cancellation might be chosen if they can accommodate the passengers on other flights (including on other airlines,) though. – reirab Aug 24 '16 at 19:27
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    @reirab: and even if a particular airline, perhaps one smaller than KLM, doesn't have a suitable spare, planes can be leased. After all, when a plane is grounded due to a fault or an incident, the airline doesn't want to have to cancel every flight that plane would have flown until it's back in action. I don't know how big the global pool is of aircraft available to lease on short notice, but they do provide spares of a sort. – Steve Jessop Aug 25 '16 at 9:03
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    @SteveJessop Yes, smaller airlines can lease a plane to fill a longer-term gap when one of their aircraft goes down unexpectedly, however it's not generally something they can do on short order, so cancellations are much more likely with those airlines in the event of unforeseen maintenance issues. – reirab Aug 25 '16 at 14:51
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They usually do? We just don't notice when backup gets used.

They have some planes to spare and they can wet lease planes (including crew) from other companies. They can also redirect passengers to another airlines.

Cancelling flight costs money (and reputation losses) and getting spare plane also costs money. They just have to choose what's cheaper in a given situation.

From my personal experience, I had my tickets transferred to another (more expensive) airline, had older/unusual plane for boarding instead of a regular one.

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    I too have been rerouted to a backup plane on occasion. There's still a significant delay when that happens, because they usually don't "give up" on the original plane until it's become obvious it's a goner (a critical part is broken and can't be replaced for a day, for example), so even when they do swap for a spare it takes a couple hours or more. – thanby Aug 24 '16 at 13:11
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    @thanby in my experience they managed in around an hour – alamar Aug 24 '16 at 13:30
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    In my case the plane had an unexpected re-routing in Spain, and could not come back to the UK in time for the next scheduled flight, so they pulled a plane out of the line waiting for general maintenance. It was visible in small things not really right, like a toilet out of use. Still gave a 3 hour delay. – Willeke Aug 24 '16 at 16:57
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    Yes. This is the correct answer. The large airlines do have spares and they do indeed use them when possible when a plane breaks down. They will normally be based at the hubs and, if a plane breaks down elsewhere, a replacement will be flown out from the hub if possible/feasible. I've had this happen quite a few times in my travels. For example, last year I was ready to board a flight when the pilot found in his pre-flight walkaround that the aircraft had a bird strike on its previous flight, so it was sent to maintenance and we were switched to another aircraft. – reirab Aug 24 '16 at 19:07
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    @AnderBiguri On U.S. domestic flights, most of the maintenance equipment swaps I've encountered only resulted in delays of an hour or two. Certainly, longer delays can happen, though, especially on larger aircraft and/or at far-away out stations. – reirab Aug 25 '16 at 15:06
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Most larger airlines do have aircraft in reserve for just such situations. Most often, they are aircraft that just completed a maintenance cycle and are not scheduled to begin a normal flight sequence for 1 or 2 days.

The problem from the passenger perspective is that these reserve aircraft are at a hub or maintenance facility, not the station with the out or service aircraft. So, even if they did use a spare, it has to get there first.

Another option the airline has at larger stations is swapping in another active aircraft that's not scheduled to fly until much later.

It would be prohibitively expensive to have spare aircraft at every station. Prohibitive for the passengers.

  • Do we really use the term "station" for airports? – Lightness Races with Monica Aug 28 '16 at 11:51
  • In this case, yes, because it's not the size of the airport that matters, it's the size of the airlines operation that's the factor. – Johns-305 Aug 28 '16 at 13:21
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Yes, airlines do have spare planes.

Many airlines have one or two spare planes parked somewhere to prepare for unexpected technical issues. The more flights you have, the more likely there will be one aircraft stuck on the ground. Airlines and aircraft manufacturers call this Dispatch Rate. It is the rate which measures how often a flight is delayed or cancelled due to technical problems of the aircraft.

But wait! A spare plane is incredibly expensive!

Yes, it is. But one also has to take into account the opposite: not having a spare plane.

It is not really a problem if the flight is only partly booked. It is really a problem if it's holiday season - every flight is nearly full, and annoyed passengers hate waiting for any minute longer. You either schedule the delayed passengers to a much later flight, which almost guarantees anger and negatively impacts the airline's image, or you move them to the next flight, then move those in the next flight to the second next flight, causing a cascade delay. Neither is satisfactory.

Not to mention, pilots and flight attendants need to be scheduled as well. There is only a limited time slot which a pilot is legally allowed to fly before the company must relieve the pilot from duty. If the crew is forced to stay, the accommodation cost is paid by the company. The company also has to find another crew to replace them as well. Needless to say, this gets costly very soon.

The larger the fleet size, the cheaper a spare plane

Obviously, if the company only has one aircraft, then the opportunity cost of the wasted spare plane is really high, since the fleet utilization is only 50%. Now, if the company has 100 aircraft, then the chance of at least one of those planes having a technical issue today is higher. If we assume a dispatch rate of 99%, we can expect one aircraft to be stuck on the ground, so we can keep a spare plane, making the total fleet size 101 planes: 100 scheduled planes plus 1 extra plane.

You don't keep one spare plane for every flight. You keep one for the entire fleet.

One advantage of booking with a large airline is that they have spare planes around. If there is a technical delay, the delay is usually shorter, since the airline can easily rotate its aircraft with only minor impact to its schedule. If it is a small airline, say with only 7 planes, and all 7 are dispatched on routes, the company has no choice but to wait until maintenance is completed. For example, SouthWest has ~700 planes; they're all even of the same model. This allows the airline to rotate a flight to a different plane without re-assigning the flight crew and without re-seating everyone in the cabin.

Spare planes usually only exists at the major hub

The major hub is where the airline is based. It is where most of its flight crews are based, and where major maintenance work is carried out. Maintenance tasks are not fixed in duration- some finish early, some late. Therefore, airlines do allow some flexibility in maintenance schedules. This flexibility can be utilized to resolve unexpected delays.

On the contrary, it does not make much sense to station an extra plane at a remote destination. Some destinations may not even be equipped to service that type of aircraft. If a plane breaks down far away, the only choices are to fix it asap, or fly in an empty plane to get the passengers out. These are referred to ferry flights by pilots, and they are not uncommon.

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    You claim the other answers are wrong but most or all mention there being spare planes, most of them at the hub of the airline. Your answer is not adding much if anything, as the things you bring up have been covered by several answers at least. – Willeke Aug 25 '16 at 19:21
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    @Willeke He said that most, not all, of the existing answers were wrong. I would say this is the most thorough and correct answer so far. A couple of the existing answers were correct, but most of them (including the current top 2 answers) were indeed wrong, claiming that airlines don't keep spare planes because it's too expensive. – reirab Aug 25 '16 at 21:04
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    There is a point where a “spare” plane isn't really a spare plane anymore. In your 100-aircraft example, they are all used to the maximum of their capacity. Most of the time, the aircraft that's on the ground is one that happened to have a technical problem and it's not available for anything. By contrast, the question clearly hinted towards dozens of planes sitting unused and ready to be dispatched. – Relaxed Aug 27 '16 at 6:39
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    @kevin Well I misunderstood your example then. With 99% dispatch rate and 100 planes needed to cover the regular schedule, you can expect to be short of one plane at any given moment and the 101st airplane will fly a lot. Even if you add a second one, there will still be many times with two planes unavailable and the "spares" in the air. In effect, you are rotating planes and have a bit of spare capacity rather than having a specific aircraft sitting there more of the time. Which makes a lot of sense. – Relaxed Aug 27 '16 at 8:59
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    Keep in mind, there is a difference between an actual spare aircraft and otherwise available aircraft. IIRC, the only examples of real spare aircraft lately have been some 747's at Delta and United. For most situations, a 'spare' is whatever a/c is nearby with a workable gap in its schedule because it's just not scheduled to fly, came out of the hangar early, it's destination airport is closed....etc. – Johns-305 Aug 27 '16 at 21:43
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The reason air travel is as cheap as it is, is due to the planes being used for as many hours a day as possible. If an airline had one plane spare that would 100,000,000's of pounds sitting doing nothing when it could be earning them money by flying, if that plane could earn them £10,000 a day if used but if used as a spare it would save them £100,000 when used but is only used once a month then using it is the cheaper option.

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    A wide body jet like a 747 costs about £10,000 per hour to run so hopefully they are earning more than £10,000 per day ;) – Calchas Aug 24 '16 at 15:10
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It depends where you are in the airline's route network. If it's a hub airport then odds are better that there will be one or more comparable spare planes available at the beginning/end of a maintenance cycle. This probably means a delay of 3-6 hours though still because it's often not quick to bring something into service and takes a while for that decision to be made to begin with.

If you're at an airport where there's one (or fewer) daily flights to/from a hub that turnaround and head straight back on landing (a spoke in the hub/spoke model) then it makes no sense to have spares available, instead the airline will have minimal facilities available and the backup plan will almost certainly be "fly a spare aircraft out there" on major failure. Delays are likely to be much longer in that scenario.

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Airline scheduling is a horrendously difficult task. Airlines have to strike a balance between multiple priorities, such as minimizing cost, maximizing revenue, having just enough passenger capacity, satisfying crew requirements, and not exceeding the allotted airport space. Airlines do have some spare capacity to account for scheduled maintenance, but there is no way to swap aircraft at a moment's notice for unforeseen breakdowns while meeting all of the other economic demands.

Keep in mind that airlines try hard to minimize turnaround time at the gate, because every minute a plane spends on the ground costs them money. To maintain a spare idle plane at every airport is simply not realistic. On top of that, aircraft differ in capabilities, so they are not freely interchangeable. Obviously, larger planes are required for high-traffic routes, and smaller planes are required for flying to regional airports. Less obviously, there are subtle differences as well: in August 2015, American Airlines mistakenly used a non-ETOPS-certified plane to fly to Hawaii, and got in trouble for it.

Not only do you have different types of aircraft, you also have to have suitably matched crews available for them. In particular, the pilots have to have the appropriate type rating — a 767 pilot is not allowed to fly a 737.

Given all those considerations, it is generally much more economical to rebook passengers on alternate flights than to keep spare planes around.

  • "but there is no way to swap aircraft at a moment's notice for unforeseen breakdowns while meeting all of the other economic demands" Of course there is. All major airlines do this. They don't keep them at all airports, though, usually just at hubs or maybe focus cities. They'll be flown to out stations as required when another aircraft goes down for maintenance. – reirab Aug 24 '16 at 19:23
  • @reirab Is there really a way to "swap aircraft at a moment's notice for unforeseen breakdowns"? How is this done? – Steve Ives Aug 25 '16 at 8:39
  • Perhaps it's not clear whether the flight time from the hub to wherever you are constitutes "at a moment's notice" within the meaning used in this answer. I suspect not. Literally, I suppose it's "at a moment's notice" in the sense that as soon as the airline knows there's a problem, it's able to start doing something about it. But if the spare is 12 hours away this response is not "at a moment's notice" in any sense of the idiom that any of the affected passengers would recognise :-) – Steve Jessop Aug 25 '16 at 9:11
  • @SteveIves If they're already at a hub (which, statistically, around half of flights will be, depending on the airline's network) they simply switch equipment to one of the aircraft kept on reserve, ideally one close in size to the originally-scheduled aircraft. If it's of the same certification type or if the originally-scheduled crew is certified to fly it, they'll usually fly it. Otherwise, they'd swap crew. If it's at an outstation, they'll have to fly the replacement there, which will increase the delay depending on flight time. – reirab Aug 25 '16 at 15:00
  • @reirab - that still means several 'spare' aircraft, for every airline, ready to fly, at every hub. – Steve Ives Aug 25 '16 at 18:56
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Previous answers cover "spare" aircraft which are in active service but have been rotated out for repairs. In addition, there are desert storage "boneyards" for aircraft which are being taken out of service longer-term. Many (most?) of the aircraft in boneyards have been permanently decommissioned, and are being held there to be broken for spares. Sometimes though these aircraft are simply being stored whilst demand for flights is low, and they will be returned to service in the future when demand increases.

It should be clear though that getting one of these out of storage is not simply a matter of fuelling it and away you go!

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    This is correct, but, as you mentioned, it's not just a matter of fueling. Lots of checks are required. These will take much longer than necessary to operate usefully as spares. The actual spares are kept ready to go at hubs. However, if enough aircraft go down that the reserves start to get low, they will indeed activate aircraft out of boneyards to get the reserves back up to normal. Delta had to do this with one of their retired 747s last year, for example, when one of the ones that was in service was seriously damaged by hail. – reirab Aug 24 '16 at 19:21
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There is not enough aircraft parking for all aircraft to park: in disasters which close large numbers of aircraft, they have to use runways and such to store the planes.

Keeping an idle aircraft for every one aloft would be infeasible because there'd be nowhere to store them.

But the airlines could get together and have a shared plane or two fueled at each major airport, using something like collective risk management/insurance schemes.

But then... who'd fly them? They'd also need to keep an airline crew on call at those airports. There'd be a call-in time, so an immediate swap-out would not be feasible, unless the on-call crew were in the airport at all times.

Something like this appears feasible, at least at first glance. That it's not done suggests that other types of damage control are considered more effective.

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    If a plane is taken out of service for a certain flight and replaced with a spare, the crew assigned to the flight is still available to operate the spare plane. – phoog Aug 24 '16 at 17:05
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    Good call. If the flight is cancelled at the same place that the replacement would come from, and if the pilots have had the correct training to fly the replacement plane, they could be used. Even if the pilots are untrained, the remainder of the crew could be used anyway. Though I guess standby crews aren't a bad idea anyway, for the same reason as standby planes. – Dewi Morgan Aug 24 '16 at 17:17
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    @DewiMorgan Large airlines have standby crews and they also have standby aircraft. Both the standby aircraft and the standby crews are usually based at their hubs, so they are in the same place already. There's a delay in getting the standby aircraft from storage to the gate, activating the standby crew, and in running the preflight checks on the new equipment (and, if the broken-down aircraft is not at a hub, flying the replacement to outstation,) but the delay usually isn't too bad. – reirab Aug 24 '16 at 19:18

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