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Recently, SAS announced that:

Withdrawing tax-free sales will reduce the overall weight of the aircraft, which in turn will reduce fuel consumption and emissions.

I could buy that this might be a polite way to say "we don't make any sales, consequently, the trolley with the items is just ballast; throw it away". It's not the first time that I hear it though: some time ago I remember an article in Aegean's Blue magazine about saving 15 tons of CO2 in one year just by replacing the pilots' flight bags with the electronic equivalent (EFBs). Unfortunately I don't remember in which issue was that.

Now assuming that a trolley weights approximately 25kg empty so let's say 30kg with the merchandise. The L/D ratio of a an Airbus A320 is 16,3 and thus a 30kg reduction in the weight will result in approximately 18,05N reduction in drag and and an equal reduction in the thrust required to maintain the same speed.

Compared to the ~150kN that 2 CFM56-5B4 produce in cruise, The 30 Kg reduction in weight will result in 0,00012% a 0,038% to 0,06% reduction in required thrust (depending on the aircraft gross weight), which seems like a drop in the ocean. Can this make a difference measurable by an airline company at the end of the month/semester/year/century?

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    $\begingroup$ It seems a perfectly reasonable question, and the discussion around it helps make clear what sort of things the author is asking about. If we can't have questions in which the author isn't perfectly certain about what to ask or how to go about asking it, what would be left would be pretty dull. $\endgroup$ – Daniele Procida Jun 6 '19 at 22:01
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    $\begingroup$ The opinion policy is zealously over-applied by a lot of the folks on here. Half of what is posted here is opinion. That is half the point; to gain knowledge from industry insiders, tribal knowledge and all that. If everything posted has to be backed up by links to factual data, what the point? Just look it up. $\endgroup$ – John K Jun 7 '19 at 3:34
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    $\begingroup$ @JohnK: FWIW, in the past 30 days, only 3 questions were closed as opinion based (see, double checking stats helps). :-) $\endgroup$ – ymb1 Jun 7 '19 at 4:06
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    $\begingroup$ @ymb1 Yes but there were a lot more attempts than that to close questions for being opinion based. See, relying solely on stats doesn't always help :) $\endgroup$ – Cloud Jun 7 '19 at 9:12
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    $\begingroup$ @zymhan a value which according to the highest voted answer at this moment, is wrong. So please stop writing disruptive comments on the thread. Also it's good to have in mind that paper calculations are usually different from reality. $\endgroup$ – Stelios Adamantidis Jun 7 '19 at 11:51
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Yes, the removal of payload will result in a measurable reduction in emissions.

First, I have to correct some math: the engines don't run at 150 kN in cruise - that would indicate a L/D ratio of only 4-5, since a typical A320 weighs around 60 tons mid-flight (give or take 10). Fuel consumption is roughly proportional to weight, so 30 kg out of roughly 60,000 kg is a 0.05% reduction, not a 0.00012% one.

As to whether one would be able to move the throttle down exactly 0.05% to benefit from the reduced weight and leave everything else equal, no, not even the autothrottle is so precise.

But that doesn't mean fuel consumption is unaffected: the balance of an aircraft's weight, lift, drag and thrust always affects its energy. At the exact same throttle setting, the aircraft will climb a fraction of a percent faster, and so finish its climb and go to lower fuel burn at cruise a few seconds sooner. In cruise, maintaining the same flight level, it can spend a minute or two at a bit lower throttle, or trimmed a bit down, which will make it arrive a few seconds sooner.

This is fundamental conservation of energy: if less of it is expended in drag, then either the engines have to work less, or the aircraft will climb or accelerate. This happens all the time - an airliner in level flight doesn't stay in perfect equilibrium throughout, but corrects for energy changes every once in a while. Even if it doesn't change how much fuel is pumped into the tanks that particular day, the effect is cumulative across all sources and persistent through random variations.

Overall the effect of weight on fuel burn is proportionate. Depending on the distance, everything aboard an aircraft can consume 10% to 50% of its weight in fuel each flight. A very rough ballpark is that, over the life of an A320, each pound of operating empty weight will cost an extra ton of fuel.

To be more specific, given the A320's fuel burn of roughly ~2,500 kg/hour, adding the trolley requires an extra 1.25 kg of fuel per hour it's in the air. For a 4-hour flight, that's an extra 5 kg of fuel. Over the 60,000 flight-hours an A320 will serve in total, that's 72,000 kg of fuel. So behind that trolley is 3 large tank trucks' worth of fuel to keep it in the air, and ~225,000 kg of CO2 emissions.

As an aside, in-flight sales of random junk really serve the airline, not the passengers. The profit margin on these items can be up to 100 times the tiny profit margin on economy tickets themselves. With airports packed full of duty-free shops, there's no shortage of shopping opportunities on an air trip. Cramped and burning fuel for every item on board, whether it sells or not, aircraft don't make for a very efficient or practical storefront.

Airlines only make up for this inefficiency by exploiting their cabin crews as free salesmen. Of course, airlines' environmental consciousness tends to correlate strongly with the price of jet fuel, so once sales profit no longer covers the extra burn, it's time to go green.

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    $\begingroup$ @trolley813 SAS owns mostly Boeing 737-800. Quick search shows it can produce 400 lbs of CO2 per flight. Leaving trolley out this gives 90g of CO2 reduction per flight. SAS flew 36 thousands times last year, and that gives 3.24 metric tons per year. Is it barely noticeable, you tell me. $\endgroup$ – Mołot Jun 7 '19 at 9:29
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    $\begingroup$ @Mołot It's still drop in the ocean. According to 2017 reports, the total amount of CO2 emissions in the world was about 37 gigatons per year. Even if we multiply these 3.24 tons by the total number of existing aircraft in the world (despite this figure is per airline and not per plane), which is about 39000, we get a total emission reduction by less than 1/250000. So, the game is not worth the candles: this surely won't significantly improve the environment. $\endgroup$ – trolley813 Jun 7 '19 at 10:25
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    $\begingroup$ @trolley813 but there are thousands if not millions of little ways to make little change that could add up to huge improvement. Not making any of them because making only one will not help seems unreasonable approach to me. $\endgroup$ – Mołot Jun 7 '19 at 10:29
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    $\begingroup$ @Mołot That figure seems off by two orders of magnitude - it should be 40,000 lbs, not 400. $\endgroup$ – Therac Jun 7 '19 at 10:34
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    $\begingroup$ @trolley813 "Worth the candles" is an opinion. How much difference does the ability of flyers to buy stuff they don't really need in-flight rather than in the duty-free shop make in the world average standard of living? SE isn't really about opinions, but personally I'd value it as even less than 1/250,000. $\endgroup$ – Therac Jun 7 '19 at 10:43
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Well, any reduction in Basic Operating Weight, which eliminating the trolley achieves, is an increase in efficiency, because anything not humans and their bags paying money, or kerosene, and not essential to getting from A to B, is ballast. So there is value in forgoing 65lb of ballast and whatever cash income it brings in (I suspect the real reason is it doesn't bring in enough cash flow to be worth the hassle of administration) in the accumulated savings over many years.

You can be sure that the financial and operations organization of SAS did a business case on whether to keep it on or get rid of it. Clearly the business case favoured the get-rid-of-it option.

And you always have to stick the word "emissions" in there because that gives you extra moral weight to prevent anyone from accusing you of a simple economic decision to cancel the service.

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    $\begingroup$ I think this is the correct answer. The bean counters determined it cost more to fly the merch around than they were making by selling it. They then got the marketing drones involved who came up with the "green" spin to make it sound good. $\endgroup$ – FreeMan Jun 7 '19 at 12:32
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    $\begingroup$ Also: Saving that 65 pounds of duty-free cart that is not generating revenue allows you to carry 65 pounds of extra cargo which is generating revenue. Of course, this wouldn't be reducing CO2 emissions, but it does increase profit. :) $\endgroup$ – reirab Jun 7 '19 at 16:22
  • $\begingroup$ That is kind of implicit in my post, but yes. Take away ballast = more payload, which is what the airline would really have in mind. In any case, I'm not a member of the CO2 bandwagon so that part is irrelevant to me and therefore, increased profit = good news. $\endgroup$ – John K Jun 7 '19 at 17:42
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When American Airlines switched from paper to iPad for the pilots, they saved 40 lbs (18 kg). This translated to "\$1.2 million of fuel annually". (forbes.com; 2013)

Of course this is across AA's fleet, which is huge. 963 planes as of writing this due to the c. 2013 merger with US Airways. Back in 2013 they had 605 planes, so it's an annual saving of \$2,000 per plane.

In 2013 the peak jet fuel price was \$125 per barrel, so it was a saving of 2,000 kg of fuel per plane per year. (1 barrel = 159 liters at 0.804 kg/l.)

To do it in another way:

To carry an additional 1,000 kg requires an additional 150 kg of fuel to be burned. (It's a general rule-of-thumb for flights that take 5–6 hours, see here for more.)

Using this ratio, we're looking at saving 4.5 kg of fuel per flight for not carrying a 30 kg trolley.

And medium and long-range planes (combined) make an average of 2.4 flights per day.

That's ~4,000 kg of fuel annually per plane.

In paper/iPad terms (18 vs. 30 kg), that's 2,400 kg (so it checks out).

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I paid for a flight Amsterdam to Accra this afternoon.

I selected the option to "Benefit from CO2-neutral travel".

fly CO2 neutral

Contribute directly to reforestation and conservation of tropical forest in Panama: this project promotes the restoration of ecosystems and biodiversity, and supports local development.

CO2 compensation for your flight costs:

GBP 5.80 [why in GPB, I don't know, I paid for the flight in Euros]

How did we get to this amount?

The calculation goes like this: by burning 1 kg (2 lbs) of fuel, 3.157 kg (6.959 lbs) of CO2 is emitted. So we start by determining the fuel consumption for your flight. This depends on the type of aircraft, distance flown, and number of passengers. Then, we calculate the average CO2 emission per passenger over a period of 3 months. This is how we calculate the amount you will have to pay to compensate for your share.

Now, flying my ~70 kg body plus ~10 kg of luggage to Accra and back consumes an additional X kg of fuel, and therefore emits about 3.157 * X kg of CO2.

The amount of extra fuel it costs may be relatively small, and it may well be a rounding error in each particular flight (a tailwind, or a couple of circuits of a holding pattern, may far outweigh any difference I could make), but it's a determinate and well-known amount (well-known by the airlines, that's for sure), and it corresponds to a determinate quantity of CO2 emissions.

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  • $\begingroup$ There is one very important difference to note here, though. The calculation that the airline is using here is dividing the total gross weight of the aircraft by the number of passengers to get a share of CO2 emissions per passenger. This is a very different number from the marginal CO2 emissions of a plane that is flying anyway carrying an extra x kg of mass (whether person or duty-free cart,) as the weight of the passengers is quite small compared to the gross weight of the entire aircraft. $\endgroup$ – reirab Jun 7 '19 at 16:44
  • $\begingroup$ Or, put more simply, they're charging you the CO2 emissions for ~700,000 lb gross aircraft weight / 400 pax = 1,750 lb, not just your extra ~175 lb of yourself + baggage. $\endgroup$ – reirab Jun 7 '19 at 16:47
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The other answers are correct, but too much on saving fuels.

Duty free trolleys are much more then just your 60 kg:

  • it should be handled and refill in various airports, it should be stored not only safely, but also securely (did you notice keys, and crew members have to sign some paper?

  • money handling. This needs a lot of papers, also accountants, and crew should be handled with cash. Logistically it is not simple, and delay the ever shorted airport times.

Then public relation will find some good words to justify cuts. It was so also for the slim seats, for not having anymore small bottles and cans for beverage (which were a lot quick to serve), or to reduce, to remove magazines. Reducing emissions make them to be seen as angels.

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Obviously it will reduce fuel consumption and emissions, if only by a tiny fraction. Which is good! But that is not the reason for their decision. They have simply decided that the service is losing them money.

I get a bit irked by such pronouncements, because it seems to me that they are taking me for an idiot. It's like hotels and towels. When I was a young man, hotels would replace all your towels every day. Now most of them ask you to leave your towels on the rack if you haven't used them, and they will only replace the towels on the floor. That's fine by me; but they justify it as being motivated by a desire to save water, for the sake of the environment. Yeah, right.

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  • $\begingroup$ water isn't free, so saving water IS saving money. Also, there's the cost of heating the water for laundry, the detergents, the cost of drying the towels after laundering, and the simple fact that in many areas, water conservation is a legal requirement due to limited supply. This is especially true in many popular resort locations, where salt encroachment is a serious problem. Considering the impact of your business practices on the long-term feasibility of your business model is being smart, not being trendy. $\endgroup$ – barbecue Jun 9 '19 at 15:24
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I think it’s a combination of factors: fuel saving not being the first reason, it certainly is a good motivation these days.

Then there are the logistics described above, costs due to security handling, thefts, percentage given to flight attendants, magazine printing costs, its weight and logistics handling.

And, obviously, the lessening interest of passengers in purchases. Airports have become shopping malls, and the 6-7 duty free retailers that cover 85% of the airports are also the same that provide the service on board airlines. So there’s really no competition, airlines produce the traffic and the Travel Retailers are making the money from this traffic avoiding to share it with the real ‘owners’ of the passenger traffic. Until 15 years ago, when airports duty free shops where not yet so huge and appealing, on board Sales conversion was 10% minimum. Now it’s down to 2% max. This says it all, in my opinion.

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Based on the other answers, I think the actual answer is No, the difference to emission is not measurable. None of the instruments concerned are sufficiently accurate.

I don't think the engines even have instruments to measure CO2 emissions. The fuel gauges may have enough digits to display a 0.05% change, but they probably aren't calibrated that accurately. Even if you did fit super-precise instruments, you couldn't control for all the other factors that effect fuel consumption.

However, the difference is still possible to estimate, as proved by the people who have done so in this question. When multiplied over a large fleet and a year of operations, it's probably quite accurate.

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    $\begingroup$ I am pretty sure that the accountants can measure annual fuel consumption with 0.05% accuracy, since they're getting the bills. And since carbon in=carbon out, that also means they know exactly how much CO2 is produced. $\endgroup$ – MSalters Jun 7 '19 at 10:01
  • $\begingroup$ @ MSalters - Yes, but how would they be sure that it was caused by removing the drinks trolley, and not passenger weight or headwinds? Or even that the fleet, routes and number of flights have changed from one year to the next? $\endgroup$ – Robin Bennett Jun 7 '19 at 10:07
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    $\begingroup$ @RobinBennett because they know that the trolley weights precisely x Kg for each flight. Even if I am on that flight and weigh precisely x Kg more than the average adult passenger (which is what they'll calculate W&B with), removing the trolley is still x Kg less being carried from A to B. I agree, that headwinds, holding patterns, etc. will burn more fuel, but removing that x Kg from the plane will burn less fuel given all the considerations for that flight. $\endgroup$ – FreeMan Jun 7 '19 at 12:38
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    $\begingroup$ But you can calculate, for any given flight, how much fuel each Kg cost. Multiply that fuel cost times the x Kg the trolley weighs and you'll know how much you spent by having it on board, or saved by not having it on board. $\endgroup$ – FreeMan Jun 7 '19 at 18:09
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    $\begingroup$ @RobinBennett I understand what you're saying, that in any given single flight, many variables will impact fuel consumption, and the impact of a single trolley may be subsumed in the background noise of other random effects. That's likely true, but it doesn't preclude measurement, just one specific type of measurement. ALL measurements are estimates, because no instrument is perfect. $\endgroup$ – barbecue Jun 9 '19 at 15:34

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