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Once in a while, I get stuck at a gate waiting for a plane to be repaired. It makes me wonder - why do airlines even try to repair at the gate? It takes an unknown amount of time, they never provide a real ETA, and the airline usually doesn’t even admit the problem (or uses vague terms like an advisory or additional maintenance, as if customers are stupid).

Further, it would seem easier, faster, and better service to just swap out the plane.

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    Aircraft generally cost north of US$50 million, up to a few hundred million dollars for larger models. They are not something that airlines generally have lying around spare ready to "swap out" with a broken aircraft.
    – Doc
    Oct 13 '18 at 1:18
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    A very simple answer: I am, for example, Air Canada. My plane has some malfunction in New Delhi. If I want to swap the plane, I need to fly another plane to New Delhi, instead of the one that needs repairs. Even if I actually have a plane that isn't otherwise occupied, surely the time needed to fly it across half the world is considerably longer than the repairs at the gate, even if the repairs take annoyingly long?
    – Galastel supports GoFundMonica
    Oct 13 '18 at 22:00
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    I worked for a major computer company providing technical sales support for expert systems. One demo was for dealing with the maintenance schedule distribution caused by swapping planes. The gist is that the routes that a plane will fly is planned far in advance so that the plane ends up at an airport that can provide the required maintenance at the required time. Obviously the schedule for the maintenance facility has to be considered also. Thus "just swapping" planes is a lot more complex of a problem than it would seem.
    – MaxW
    Oct 14 '18 at 0:19
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    I was on a flight where they replaced the engine at the gate. So we all watched while the old engine was taken out and a new engine put in. That was pretty nerve-racking. In those cases, I really wish they would take the plane for a quick flight around the corner.
    – RoboKaren
    Oct 14 '18 at 1:35
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    @DavidRicherby - I did post it as an answer. Some moderator must have turned it into a comment...
    – MaxW
    Oct 14 '18 at 14:43
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  • A cabin crew member to her/his chief: The securing strap for the oxygen bottle at 3R is loose.
  • Chief to pilot: The 3R oxygen bottle cannot be secured, something is wrong with the strap.
  • Pilot to maintenance over the radio: We need someone onboard, we have an issue with the oxygen bottle strap at 3R.
  • Maintenance guy arrives, after checking it, he goes to the pilot: it will take 5 minutes to repair it and another 5 to get it, I just radioed the guys to bring the spare part.
  • Captain over PA: Dear passengers, we have a 10 minutes advisory due to a small technical problem. However, the weather is blah blah..
  • 10 minutes later, the maintenance guy goes to the pilot: Captain, the guys couldn't find the spare part, we need to get it from the main spare parts shop at the far side of the airport, it will take 10 more minutes.
  • Captain over the PA again: Dear passengers, it seems that we need 15 more minutes, blah blah..
  • etc. etc.

The above is a real-world scenario, which I faced hundreds of times during my years working as a cabin crew.

Even if a similar thing happened before passenger boarding had started, the pilots would give a green light for it to start because they would expect the plane to be fixed and they would have better chances for the plane to take-off on-time. It’s better than delaying the boarding while they fix the problem, which virtually guarantees to cause a delay. This is the general logic if they think there is a good chance of fixing the plane on-gate.

Airlines do not lie, they just like to think about the "best possible scenario" when passing information about the delays.

As for "swapping airplanes", this is really not something airlines do easily:

  • They wouldn't have an aircraft that has nothing to do and waiting for "swapping".
  • Even if they wanted to, usually this only happens at "bases" or main hubs for the airline.
  • Also, when swapping the planes, you will need to swap the catering and other stuff which is specific for that flight, that's not easy and it's costly.
  • What about luggage and cargo? what if they already started loading that?
  • Lastly, swapping the plane will usually cause two delays instead of one, the first delay is for the originally scheduled flight, and a second delay for the flight in which the plane was pulled from. so why create two problems instead of one?

If there's some big technical problem or the pilots decided that the plane is grounded, the airline then will consider swapping the plane. New airlines' systems are smart enough to suggest the best scenario: should the flight be canceled or should the airline swap the plane with another flight (For example, one that has a scheduled, non-mandatory check-ups that can be re-scheduled)? This will usually cause a chain of delays, which can be neutralized after a few flights for the same plane (making up a few minutes from each flight until finally it catches up to its original schedule).

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    Also, the amount of fuel on the plane may need to be adjusted. This can require finding a fuel truck with a large enough tank to remove the fuel from the plane. This happened to me a few months ago when my flight from Atlanta to New York was served by a plane that had been prepared to fly to Munich.
    – phoog
    Oct 13 '18 at 2:52
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    Think about how long it takes between arrival of a plane and departure. Take it times two, that's what it would take to swap planes, if one is near
    – PlasmaHH
    Oct 13 '18 at 8:45
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    There’s also the problem that even if there was another plane around, it might not have the same size (think A321 replaces by A320), or not the same layout, or not the same equipment/certification (think ETOPS for flights to Hawaii). But it DOES happen. You usually only know it if there’s this kind of issue (well, the first two, as the last one is just a no-no).
    – jcaron
    Oct 13 '18 at 9:05
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    AI is just a magic word for “it does stuff we pretend is so complex”. Everybody and their cousin pretend they do AI these days...
    – jcaron
    Oct 13 '18 at 11:26
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    Great answer thanks! (Men, there's no point having an argument about the term "AI". It's totally normal that systems like TensorFlow etc (which is at root of the systems under discussion) is referred to as "AI". (And/or "ML" or some other trendy term.) It's utterly OK to refer to Siri as "AI". At the same time, those in the software industry know that AI is crap :)
    – Fattie
    Oct 13 '18 at 18:42
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When the airline chooses to delay a flight to "repair" a plane at the gate, it is usually because:

  1. The problem is relatively common and minor, and...
  2. The solution is either already known, or there is a good expectation of a common and timely solution

Delays from that point are usually just in terms of

  1. Confirming the problem and solution
  2. Getting the right people to the plane
  3. Getting the right parts to the plane
  4. Performing the fix
  5. Testing the fix
  6. Paperwork

In other words, most of the "problems" and "repairs" at gate are relatively "routine". If there was a big, unexpected problem without a clear solution, then the airline might think of canceling the flight or swapping out the plane, and getting the problematic plane to a hanger where the problem can really be investigated.

Also remember that most modern planes have very extensive sensors and diagnostic software that can not only tell you there is a problem, but where the problem is and what is causing it, and what needs to be replaced. This makes diagnostics and repair a much more straightforward and predictable process in terms of time to fix.

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    I'd add paperwork to that. Aircraft maintenance is carefully documented. You don't just change a light bulb; you have to record which bulb you changed, who changed it, which replacement bulb you used, etc... That all makes it take a bit longer.
    – Zach Lipton
    Oct 13 '18 at 21:27
  • You're right. I added paperwork.
    – Daniel
    Oct 14 '18 at 14:49
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Airlines are required to follow FAA rules, which includes ground maintenance. Be glad they’re not running repairs mid-flight. Also, realise a plane is one massive object: timing and directing on board passengers on or off a plane is not as simple as it looks. Rather than waste more time getting people off a jet, it’s more time saving for the airline to spend that time making the repairs.

It helps when your old man is a retired aerospace engineer. Safety and reliability is every engineer’s major concern, from what I understand.

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    “Airlines are required to follow FAA rules”, or ICAO ones in some small, backwards parts of the world.
    – DaG
    Oct 13 '18 at 20:39
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    Airlines are required to follow the local CAA's rules (civil aviation authority), be it FAA in the US, or whatever local authority anywhere else in the world.
    – Nean Der Thal
    Oct 13 '18 at 21:54
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    @DaG most airlines don't follow FAA rules (because they don't operate in the US), and there are no airlines that follows ICAO rules (though local CAA usually base most of their local rules on ICAO).
    – Lie Ryan
    Oct 14 '18 at 10:08
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    @LieRyan: “most airlines don't follow FAA rules”: That was kind of somehow my point, wasn't it.
    – DaG
    Oct 14 '18 at 11:33
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    @NeanDerThal To be nitpicky, in the EU, national CAA authority has been largely taken away and integrated into EASA. It's also true, back to the original point, that nations tend to copy EASA, FAA rules, the two bodies that have the resources to make rules. (ex-Soviet/CIS and China are slowly aligning theirs too, i.e. phase out of metric system)
    – user71659
    Oct 14 '18 at 21:22

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