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Why is it that commercial airlines are so concerned with checked baggage weight, such that they charge extra just for the privilege of checking in a suitcase with a limited weight (e.g. 15kg), and then if you should go even a couple of kg over the limit, they charge quite a bit per kg?

My first guess is they see it as a way of making extra money, but is this the only reason?

The reason I'm so puzzled about it is that they don't seem the least bit bothered about the weight of carry-on luggage, not to mention the passenger's weight (e.g. I weigh about 70kg more than my daughter, yet our ticket prices are the same). And then the weight of the aircraft itself - e.g. an Airbus A320 weighs 42.6t, with a max take-off weight of 78t according to Wikipedia. So why are they so concerned about an extra 3-4 kg in the checked-in luggage?

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  • $\begingroup$ Relevant: travel.stackexchange.com/q/52669/12011 (The answers to that question answer this one.) $\endgroup$ – reirab Aug 16 '18 at 1:47
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    $\begingroup$ It is primarily a safety issue. Profit is secondary. Extra 3-4 kg if abused by all passengers on board is around 1/2 a ton. $\endgroup$ – slebetman Aug 16 '18 at 3:38
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Airlines do care about your carry-on weight and even your weight. Many have carry-on weight limits, which low-cost carriers usually enforce, weighing the carry-on and tagging it as cabin luggage. Full-service airlines rarely bother.

One reason they don't measure your weight is that most people wouldn't feel comfortable getting weighed in front of strangers, then charged for that. Only Air Samoa currently weighs each passenger and tickets them by weight. In the West, weight is a contentious issue, so an assumed average is used for passengers. Such assumptions have crashed small planes, and in special circumstances like sports charters, actual weight is used. Finnair weighed volunteering passengers to get a better sample.

Weight costs money to carry, and not just in fuel burn. Luggage shares its volume with cargo, which airliners from the A320 up are designed to carry in the hold. Lately, cargo tends to get become more profitable than passenger service. Electronics and other high-value items cost more in depreciation and time value to ship by sea and land than by air, creating massive demand for air freight.

The A320 is 42.6 tons empty, but that's not the number that matters. The number that matters is its 78 tons MTOW, and how short of that it comes after adding pax, luggage and fuel to the 42-ton OEW. That spare weight is money in standby cargo.

Giving potential 175 passengers an extra 20 kg each (long-haul FSA vs average LCC allowance) could, if everyone used it all, add up to 3.5 tons total. Freight rates vary widely, but on a 3,000 mile flight that's worth \$7k-\$10k in freight revenue, or \$40-\$60/pax. Admittedly that's on the upper end of the range, but you can see there's money to be made. For an A320 that only costs ~$7k/hr to fly, it's a lot.

Less ethically, moving weight and volume from luggage to carry-on is profitable for a mixed passenger/cargo airline. Checked-in luggage has to be weighed in all cases, to ensure each piece is within safe limits for manual handling. This known weight then eats into the takeoff weight calculation, leaving less for cargo.

As airliners are designed to take off with one failed engine and carry reserve fuel for failures and diversions, this extra power works as weight safety margin when nothing fails. In practice weight margins rarely get pushed - most flights are volume-limited, with enough weight margin to take off at reduced power. Still, less luggage means more cargo means money.

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    $\begingroup$ For pax and carry on, airlines are allowed to assume a standard adult passenger weight of 190lbs in summer and 195 in winter. Carry on bags are assumed to be 16 lbs. (this is in North America). The numbers are based on assessments of typical weight distributions within a random group of people and carry on luggage. The 190lb weight used to be 180lbs, but this was increased due to the observed chubbyness increase in the population in recent years. $\endgroup$ – John K Aug 15 '18 at 20:08
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    $\begingroup$ The average pax weight being too low and lots of heavy baggage contributed to the crash of Air Midwest flight 5481. $\endgroup$ – Michael Hampton Aug 15 '18 at 21:23
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    $\begingroup$ "That's why there are carry-on weight limits." Many (most?) airlines don't have carry-on weight limits. Instead, they have a size limit to make sure it will fit in the provided space. It's mostly just LCCs that have carry-on weight limits so they can find an excuse to charge you more. Some mainline carriers do have weight limits for carry-on, but they're typically generous and often not enforced. $\endgroup$ – reirab Aug 16 '18 at 1:49
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    $\begingroup$ @reirab many airlines have both but the weight limit is hardly ever enforced. E.g. KLM has a max weight of 12kg for carry on (klm.com/travel/nl_en/prepare_for_travel/baggage/…) and if at the checkin desk you've clear trouble lifting your carry on bag they will want to weigh it (I've had it happen, with severe back problems...). $\endgroup$ – jwenting Aug 16 '18 at 4:39
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    $\begingroup$ Also note that while passenger won't simply weigh more if allowed to, many will take bigger luggage, so some limit has to be placed on it. $\endgroup$ – Jan Hudec Aug 16 '18 at 5:46
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Never mind the weight of the passenger and its flying consequences. For checked luggage, the main constraint is the maximum weight that has been negotiated with the baggage handlers' union. The passenger is assumed to be handling their carry-on luggage, and if they are stupid enough to injure themselves doing it nobody else particularly cares. But checked luggage is handled by employees, who often lug bags all day long. So keeping to a weight limit is important to preserve their health and safety.

You can draw your own conclusions about working conditions from the fact that domestic US limits are so much higher than international ones. And of course none of this prevents airlines from separately gouging the passengers with extra fees as much as possible.

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    $\begingroup$ I just came home and my suitcase has a big orange sticker on it saying "DANGER! HEAVY" with a hand-written "22 kg" on it. And, having schlepped it to and from the airport, I can definitely attest to the fact that handled wrongly, you can badly hurt yourself. $\endgroup$ – Jörg W Mittag Aug 15 '18 at 22:06
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    $\begingroup$ Note that overweight carry-on is entirely not without consequences. The bins are now designed for crash deceleration, but only with carry-on within the usual limits. On the other hand, less luggage in the hold means more room for cargo, so the profit motive is for more carry-on and less-check in. $\endgroup$ – Therac Aug 16 '18 at 4:52
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    $\begingroup$ While airlines do have a maxlimit per piece, the ones that are more concerned about high piece weights is actually the airport and not the airlines. Most baggage handling systems (BHS) are designed around a max 32kgs per piece. If there are heavier bags being consistently run thru the system this will result in high wear and tear on the system which may result in delay/damage to bags which the airlines usually blame the airport! $\endgroup$ – Anilv Aug 16 '18 at 6:04
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    $\begingroup$ But I'm still allowed to check in overweight luggage, I'm just charged for it - does that fee get passed onto the baggage handlers (as compensation for having to handle overweight luggage) or something? $\endgroup$ – colmde Aug 17 '18 at 8:09
  • $\begingroup$ @Therac I've often seen three pieces of hand luggage in my overhead bin and thought: well, if all three are as heavy as my backpack, this thing will be significantly overloaded :D I'm pretty sure I commonly overflow the 12kg hand luggage limit in most EU carriers. $\endgroup$ – yo' Aug 17 '18 at 8:21
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If an airplane is too heavy when it tries to take off, it might not be able to take off safely (it might still lift off, but be unable to maintain flight in the event of an engine failure).

Additionally, the weights of the things that can render an airplane overweight (mainly passengers and their baggage) are not necessarily centered within the airplane's safe center-of-gravity (CG) range; thus, overloading a plane with passengers and baggage could potentially move the CG beyond its forward or aft limits, resulting in loss of control upon liftoff or shortly afterwards.[1]

The absolute upper limit on how heavy an airplane can be and still take off safely is known, conveniently enough, as its maximum takeoff weight (MTOW); this is the maximum weight at which an airplane is capable of lifting off and remaining airborne, even if it loses one engine at the worst possible moment during the takeoff. Note that the calculation of a particular airplane's MTOW assumes that it will be taking off from a long, dry, sea-level runway with a high local atmospheric pressure, engines at full power, a stiff headwind, and flaps and slats fully deployed; when taking off from a shorter runway, or from one covered in slush, or from an airport at high altitude, or with a low-pressure system over the airport, or with a tailwind, or with noise restrictions that require the use of less-than-maximum power when taking off, what matters is instead the maximum permissible takeoff weight, which is less than the MTOW (how much less depends on all of the above factors, along with any others that would make it harder to take off).

As a result, airlines try very hard to keep their planes from taking off overweight. There are eight major contributors to an airplane's weight:

  1. The dry weight of the airplane itself and of those of its resources that aren't consumed during normal flight (such as hydraulic fluid).
  2. The weight of the fuel and oil carried by the airplane.
  3. The weight of the airplane's crew.
  4. The weight of the airplane's passengers.
  5. The weight of the passengers' checked baggage.
  6. The weight of the passengers' carryon baggage.
  7. The weight of any freight being carried in the cargo hold.
  8. The weight of other miscellaneous consumables (such as the peanuts and soda, henceforth known as "passenger chow").

Here's a diagram showing most of these contributors:

enter image description here

(Image by mins from this post.)

Of these, the weight of the airplane itself is fixed and unchangeable from flight to flight (unless maintenance and/or repair work has been done on the plane in the interval between flights, which could slightly alter this weight). And the amount of fuel and oil carried is determined by the amount of same needed in order to get the airplane from its source to its destination, fly around in a holding pattern for an hour or so, and then fly to the diversion airport listed in the flight plan, plus a generous margin of safety to account for such things as headwinds, fuel leaks, engine failures, or all of the above simultaneously. This leaves six weights that can be minimised:

  1. There are only a few crewmembers onboard the airplane, and there is far less variation in physical parameters among crew than there is among passengers (the geometry of the flightdeck and the controls therein imposes hard upper and lower limits on height, and height is fairly strongly correlated with weight); therefore, although the weight of the crew is known exactly and can be minimised, there is little weight that can be saved by doing so.
  2. The weight of the passengers can be estimated, but, as Therac said, can't generally be measured exactly, for fear of lawsuits by the large of frame. The total passenger weight can be minimised, if need be, by removing passengers from the airplane, but the airlines avoid doing this if at all possible, because taking passengers off a flight costs them money - and they can't try to offset this by charging heavy passengers more than light ones, again for fear of lawsuits.
  3. Checked baggage can be weighed exactly, and restricting its weight doesn't lose the airline any money (they generally charge passengers based on their number of checked bags, not - up to a certain point, at least - the weight of their checked baggage).
  4. Carryon baggage - see 5. However, as the weight of the average passenger's checked baggage is much more than the weight of their carryon baggage (16.7 kg/passenger versus 6.1 kg/passenger, respectively, according to this study), there is more weight to be saved by minimising the weight of the passengers' checked baggage than by minimising the weight of their carryon baggage (and, as Therac pointed out, the airlines do care about the weight of your carryon baggage - this is the main reason why they limit the number and size of your carryons).
  5. Freight (if any) can be removed to lighten the airplane, if necessary (and its weight can be measured exactly), but, as with removing passengers, the airlines don't like to do this except as a last resort, as it costs them money (since now they don't get paid for transporting the freight that had to be offloaded).
  6. The amount of passenger chow carried is either constant from flight to flight, or else depends on the number of passengers onboard; either way, it can't be independently decreased (at least, not without pissing off the passengers), and the possible weight savings would be minuscule anyway, due to their very small weight.

As you can see, most of the contributors to an airplane's weight either can't be lightened at all (airplane, petroleum byproducts, passenger chow), would incur significant cost penalties in lightening (passengers, freight), and/or wouldn't free up enough weight from being lightened (crew, passenger chow); this leaves baggage (both checked and carryon), and, since the average passenger has more checked baggage than carryon baggage, checked baggage represents a greater opportunity for weight reduction than carryon baggage does.

[1] As happened to, as previously stated in this comment by Michael Hampton, Air Midwest Flight 5481.

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    $\begingroup$ "since the average passenger has more checked baggage than carryon baggage" - at least in Europe, I suspect the median passenger doesn't have any checked baggage - it's all carry-on. Certainly I have made many more flights without than with. $\endgroup$ – Martin Bonner Aug 16 '18 at 6:02
  • $\begingroup$ @MartinBonner: In Europe, that's probably true, but passengers stateside tend to carry a LOT of checked luggage - and I suspect that US travellers are considerably more likely to fly than are European travellers, what with the distances being much greater and the rail alternative being much shittier. $\endgroup$ – Sean Oct 15 '18 at 3:17

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