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Why is there a weight limit for carry-on luggage on some airlines (e.g., 7 kg for China Airlines) that is enforced even when the plane is almost empty?

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  • Comments are not for extended discussion; this conversation has been moved to chat. The comments are locked for a while, please go to the chatroom. – Willeke Mar 13 at 18:21
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At what point should they not enforce it? Half full? Quarter full? Can you imagine the arguments!

And then - when you are packing at home, most passengers don't know how full the plane might or might not be. So it's a bit impractical.

Logically it's more sensible to enforce a limit, and then gate agents might just have an unofficial policy of some leniency where they see fit.

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    I just meant that strictly enforcing, at the check-in counter and at the gate (as China Airlines is currently doing on some flight), a harsh weight limit when the plane is 80% empty doesn't seem obviously useful to me, unless the point is milking customers. So I was wondering whether I was missing some other reasons :-) – Franck Dernoncourt Mar 11 at 23:03
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    @FranckDernoncourt well if the plane is 80% empty then the flight is probably operating at a loss. they will want to get every dime they can get to reduce loss. – hjf Mar 12 at 14:44
  • @hjf fair enough, but that's a different reason than what Mark Mayo's answer is raising. – Franck Dernoncourt Mar 12 at 14:50
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    @FranckDernoncourt, it can also be "company policy". It's often easier to strictly enforce a rule always than "sometimes", because passengers have long memories and online reviews have even longer memories. As an airline, you don't want to spend time arguing with people why they could bring an overweight bag to their vacation, but not back from it, as an example. – computercarguy Mar 12 at 21:36
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The weight of the luggage affects the fuel consumption, and the airline might very well adapt how much fuel the put on the plane to the number of passengers. If they were to allow all the passengers to bring extra luggage, just because it fits, they would use extra fuel, and have to pay for that.

It's probably also easier to enforce the limit at all times, rather than taking countless discussions with passengers on whether slightly overweight luggage is acceptable on a particular flight (and people would start such discussions if the airline did loosen those rules).

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    @NicoHaase Think about it. If the plane is half full (say, 100 people), and all passengers do this, that could be, say, 5000 extra pounds. If the airline only put in enough fuel for 100 people with normal luggage weights, this would be a big problem. 2+ tons is nothing to sneeze at. – user91988 Mar 11 at 16:20
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    @NicoHaase You can argue with me all you want, but that's the reason. Fuel is expensive. Airlines fly hundreds of flights a day. It adds up. Airlines want to save every penny they can, and you should want them to, too, since ultimately the cost gets passed to you. – user91988 Mar 11 at 16:56
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    @NicoHaase I worked for an airline. They even made the cutlery smaller to save weight. Literally everything counts. – Mark Mayo Mar 11 at 19:49
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    @NicoHaase You may be ignoring the "multiplier effect" in the total weight. As a simple first approximation, for the same speed in flight the fuel burn will be proportional to weight. But 1 ton of extra cargo needs more than 1 ton of extra fuel, because you need to burn even more fuel to carry that 1 ton of extra fuel. – alephzero Mar 11 at 22:31
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    @NicoHaase please see this answer at Aviation for an explanation of how much extra fuel is burnt. That specifically addresses carrying extra fuel, but fuel is weight, much as baggage is weight. The difference is that the weight of fuel decreases during the flight, while the weight of baggage doesn't (unless some is lost which is Bad™), so the added fuel burn will actually be higher than shown in that answer. – FreeMan Mar 12 at 14:38
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The main reason for the weight limit (and its enforcement) is that the airline wants you to pay for the extra weight. It is a major income stream.

Once you pay the fee, the plane can easily fly with the extra weight. Unless you bring an extra ton, it is just noise for the needed-gas calculation - they wouldn't add more gas just because a passenger brings an extra heavy carry-on (or is overweight himself).

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    They may not add more fuel for that extra luggage weight, but they will certainly burn more fuel. The laws of physics don't care whether your added weight is paid for or not, or accounted for in your calculations or not. Something is doing the work of lifting your luggage, and that work comes from extra fuel consumed. – Ken Williams Mar 11 at 20:50
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    That's not true - it's rare to dump fuel before landing, it only happens in emergency situations. Sources: science.howstuffworks.com/transport/flight/modern/… and en.wikipedia.org/wiki/Fuel_dumping – Ken Williams Mar 11 at 22:38
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    Cue the baseline: For The Love of Money – Mazura Mar 12 at 0:11
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    (for clarification - my second comment was responding to a comment that's now deleted.) I was curious how much impact a 25-pound suitcase really has; using nationalgeographic.com/environment/urban-expeditions/… I infer that the cost per pound of luggage is about 7.4¢, so a 20 pound suitcase should be about $1.48 in extra cost to the airline. I think Aganju's original point about this being mainly an income stream, rather than cost recovery, is essentially correct. – Ken Williams Mar 12 at 15:31
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    Two things can be true at once. The engineers in the company probably look at it as a fuel requirement issue, where the business side looks at is as a revenue stream. One of those reasons is an actual requirement. The flight costs more to operate the heavier it is. The other is simply a factor of "what can we get away with." The fact that there ARE several good reasons, which have been articulated on this question already is why tolerance for the policy is fairly high and people just don't go to other airlines. The actual dollar amount is likely to be heavily influenced by marketing. – AHamilton Mar 13 at 9:20
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I don't think these limits exist for one reason only. Each airline analyses the pros and cons of their options and set a limit.

Besides the already mentioned reasons I am presenting a few other:

The weight limit in carry-on luggage also relates to the maximum weight the overhead compartments can hold. Although sturdy there are limits. I could not find an official source mentioning this but found people mentioning this here and here.

Since flight attendants often help with those bags, it's possible that this limit is also a way of protecting the cabin crew.

Yet another reason to keep a limit is safety. To avoid the baggage-retention-related incidents as explained here.

Your question is if the plane is almost empty. Of course, the fact that the plane is almost empty is not known to anyone when packing. To allow this the airline company would have to inform customers about the weight limit increase in a clear way beyond any doubt. Even with the existing fixed limits, there are already tons of questions (as you can easily find in SE) about luggage weight. This uncertainty would only bring extra costs in customer support and time loss when boarding customers.

Even if this was feasible in a practical manner, companies may be unable to predict how many passengers there will be for a given flight with enough anticipation. If there is a delayed flight they might have to re-route customers filling what was previously an empty plane.

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    Very unlikely. Weight restrictions for carry on vary wildly from airline to airline and they all using more or less the same aircraft types. If there were any technical reasons, there wouldn't be that much variability – Hilmar Mar 11 at 13:08
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    While there are official weight limits for the overhead bins, many airlines do not weigh carry-on bags at all and manage just fine. – Zach Lipton Mar 11 at 20:42
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    @Hilmar actually a very common maximum weight that you find is 10Kg. Having a lower limit only means the company wants to restrict either the weight in the plane or the weight that flight attendants have to lift since many times they have to help with bags. – nsn Mar 12 at 10:27
  • United doesn't appear to have a carry on weight limit at all. How can this answer be correct if that is true? – phoog Mar 12 at 19:36
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    I fully acknowledge that aircraft manufacturers do put limits on the bins, but major global airlines, including United as phoog notes, have no weight limits besides "whatever you can lift into the bin" and don't usually weigh carry-on bags at all, so airlines appear to be imposing limits for commercial reasons and not out of some clear necessity. Nobody seems to think United is operating unsafely because they don't weigh carry-ons. – Zach Lipton Mar 13 at 1:15
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There is no reason for the airline to not enforce the rule, regardless of how full the plane is. Customers will either pay to put their bag on the plane, or they will pack less. Even if the plane is nearly empty, the airline has an opportunity to collect baggage fees, and they have no compelling reason to waive that opportunity. Why would they refuse to take $25+ from anyone who wants to check their bag?

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    particularly if the plane is near empty. – ZeroTheHero Mar 12 at 2:26
  • This is the real answer. The other answers focus on logical arguments relating to the weight of the plane, fuel, etc. but the real answer is "because they can". Airlines, as with any other business, are not going to unnecessarily reduce their profits. – JBentley Mar 13 at 11:55
  • Indeed, they are not Santa Claus, if they would allow people to take more bags just because the plane is half empty, they could just as well allow a few additional people to fly free of charge. – Count Iblis Mar 13 at 17:34
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For one: to be safe.

There is an average passenger weight (which includes also carry on baggage). This affect the configuration of the airplane, the required fuel, and the emergency fuel.

Because these calculations are done by maybe by dispatcher, but the pilot need to verify and input in the plane computer, one do not want that there was different opinion on the inserted numbers, and so maybe going on low fuel (and this means paperwork and possibly penalties, but for sure an expedite landing, so good for passengers).

Additionally carry on is checked on very last moment. How do you know that airplane is half full? If your estimates were wrong, it will take time to correct, and it could cause delay to the airline (or just denied boarding). Maybe at last minute an other airline has to cancel a flight, so they will rebook many passengers on your flights (and this will take some time, do load new baggage and doing the new paperwork), but so now you have extra weight which was not accounted.

Let's also say: it easy money for airline, and so they enforce such rules, OTOH the average weight seems to be lower then modern passengers.

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As many have said, it affects the weight of the plane, which other have said affects how much fuel the plane consumes, which of course costs the airline money. However, one thing few, if any, have mentioned is the safety implications; if the plane is heavier and it needs to make an emergency landing, there are implications that are going to be needed, such as time and fuel dumped. Additionally, there have been accidents on the smaller (turboprop) planes that have been attributed to excess weight on the plane (here is an example). I'm not particularly sure how much this relates to the decision of airlines themselves to charge extra for extra baggage, but do keep this in mind.

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