# Why don't airliners use in-air refueling systems?

Right now if an airliner wants to fly a really long distance (eg., a Boeing 787 flying from Seattle to Tokyo), it has to load itself down with lots and lots of fuel, which in turn weighs thousands and thousands of pounds. This, of course, makes the flight of the aircraft less efficient than it could be1. Thus, if the craft could theoretically carry half as much fuel, that should increase the fuel efficiency of the craft, right?2

Mid-way refueling seems like it would be a Good Idea™ at that point. Of course, landing would add a heck of a lot of time to the flight, so it seems the better option would be mid-air refueling. It would allow for the aircraft to be more efficient, without the need for stopping on a long journey.

Boeing and Airbus both make a few airplanes | that can do | mid-air refueling, in fact one of them is a highly modified 747-200 (properly called a VC-25) used as Air Force One:

(source: wordpress.com)

I assume that, because Airbus and Boeing's engineers and sales managers are really quite smart, they have a really good reason that they don't fit/sell this feature on any civilian transportation aircraft. But I'm not sure what that reason is.

Does anyone know why airlines do not use aircraft that are capable of mid-air refueling?

1 If I'm not mistaken, increased weight means an increased AoA to maintain level flight, which in turn increases induced drag from the wing. Less fuel would mean less induced drag or, if mid-air refueling were common practice, a wing that was designed to be more efficient because it was required to handle less weight.

2 I don't know by how much, if it's not that much, well, that might explain why nobody does this.

• Safety and logistics aside, I'm not sure the benefit of having another large aircraft burning fuel to refuel another aircraft burning fuel would offset the cost of just taking off with a larger fuel load to begin with. – Rhino Driver Aug 2 '15 at 19:48
• @RhinoDriver And that'd be especially true if the refueling plane had to fly a long way out of it's way to get to a single airliner. Where as the way the military uses them...is quite different I assume. Actually if you have a moment to describe the difference in mission (ie., answering this from the "why the military uses mid air refueling" perspective) in an answer, that would be really cool. Especially since you know directly :). – Jay Carr Aug 2 '15 at 20:07
• No problem Jay. Aerial refueling is a very expensive endeavor with very few practical uses. For the military, the increased price of fuel is well worth the increased tactical ability our aircraft receive. Whether that's playtime in a kill box, or blue water operations in the Navy (ie, no divert, where the carrier is the only option and being low on fuel requires a tanker), there are a myriad of tactical and administrative benefits the military pays for through aerial refueling. I'll post an actual answer later when I've got a bit more time. – Rhino Driver Aug 2 '15 at 21:00
• I'm not sure safety can be set aside... ATC exists expressly to keep airliners apart, putting two very large aircraft in close proximity, (10's of feet separation), one carrying an enormous fuel load and one carrying human cargo, is a recipe for disastrous inattention. – CGCampbell Aug 3 '15 at 17:34
• Given that the longest haul non-stop commercial flight is some 13,800km en.wikipedia.org/wiki/Non-stop_flight it would seem that airliners can pretty much fly as far as any commercial need would require. To put it another way, there is probably not enough economic benefit, and the solution to "not enough range" for commercial aircraft seems best addressed by increasing fuel capacity or making intermediate stops. Bear in mind that military and civilian aviation operate under different economic, safety, and logistical priorities, so what's good for one may not be for the other. – Anthony X Aug 13 '15 at 22:35

## 3 Answers

Fuel Quantity Unlike smaller fighter jets, you would need to offload a substantial quantity of fuel. For a B777, you're looking in the range of 60 tonnes of fuel for half a tank. The boom of a KC-135 (faster than a basket) can do around 3 tonnes a minute. The math comes to then 20 minutes of aerial refuelling. The KC-46 can do perhaps 180 tonnes, so you might squeeze out three refuelling operations from one flight.

Risk This is considerably more dangerous than the very conservative safety margins aircraft normally operate within. You would have one aircraft filled to the brim with fuel, and another aircraft with 300+ passengers. A quick search on YouTube is enough to propose this entire thing is very dangerous.

Furthermore, you need to consider that the aircraft needs safety margin to divert should refuelling not work, which cuts into the benefits you can expect.

Many intercontinental routes will be flown at night, making the manoeuvre more difficult.

Refuelling Area Looking at the route you proposed, the closest airfield would probably be in Alaska, at least 500km away. Guess another relevant area would be Iceland/Greenland. Even if you did reroute, you'd get away from the jetstreams that aircraft over the Pacific use lying considerably further south, reducing efficiency. The same scenario applies for eastbound Atlantic flights.

Expensive You would need to get another aircraft (~$200m for a KC-46) with special equipment and crew training. The receiving aircraft crew would also need special training. The airframe modifications would be complicated and require certification. Furthermore, you would need to get everybody to agree on some common standard. Logistics Planning is difficult. Each aircraft would have to be rerouted to intercept the tanker at a certain time and place. You want to refuel them perfectly one after an other, which is almost certainly not possible. There's just not the volume of aircraft movements feasible for this, especially for the ultra-long flights where it may have the greatest benefit. Other Landing Benefits Include changing crews and possibly offloading passengers. Applicability The number of flights where this can be practically implemented and used is very limited and for all intents and purposes, you can just land the plane itself and refuel it. Even for the route you propose, it's around 7,500 km which is not a lot for a B777. • Even the Air Force, with lots of experience in this, says in their tanker manuals: "Because of the magnitude of interrelated aerodynamic effects flying two aircraft at close vertical proximity is unsafe." – cpast Aug 2 '15 at 18:53 • – cpast Aug 2 '15 at 19:11 • Actually Iceland is a "pleasant place" for aircraft operations, being at low altitude, nice and cold all year round, and with no short-haul domestic routes! I can remember when all the records for "least unscheduled maintenance," "lowest IFSD rates," etc on the B757 were held by Icelandair. The only problem is the occasional volcanic ash cloud. Greenland, being mostly 9,000 ft mountains, is a different story, of course. – alephzero Aug 2 '15 at 22:42 • In terms of relative risk levels, regardless of the frequency of mishaps the military have with in-flight refueling, note that civilian aviation is MUCH more risk-averse, and the length of a refueling boom is somewhat less than the usual separation that the FAA tend to operate under, which are 1000ft vertically and 3 miles horizontally. – anaximander Aug 3 '15 at 16:01 • @MikeFoxtrot I think you can add one other benefit of landing over in-flight refueling: nice for the passengers to be able to exit the aircraft after a while. Eight hours is a long time (I would say too long) to be cooped up in an economy cabin, and there are airliners already in revenue service capable of significantly longer flights. IMHO a more passenger-friendly way to go further in a single hop would be to go faster rather than stay up longer. – Anthony X Aug 3 '15 at 17:46 Don't look at the fuel consumption of the airline flight in isolation. An airline would need to combine the fuel used by both the revenue-earning flight and the tanker, and then add the cost of operating it, too. Even if this could be shared by four or five revenue-earning flights, the total would still be worse. To find out how big the fuel saving by in-air refueling is, the Breguet equation is your friend. Let's assume an L/D of 18, a thrust-specific fuel consumption of$b_f$= 0.018 kg/kNs and a speed of Mach 0.82, which equates to$v$= 279 m/s in 11.000 m altitude. Now look at the mass fractions which go with ranges of 8000 km and 2$\cdot$4000 km: $$\frac{m_1}{m_2} = e^{\frac{R\cdot g\cdot b_f}{v\cdot L/D}}$$ Flying the distance in one go needs the plane to start with a fuel load equivalent to 32.5% of the landing mass, while an air-refueled flight needs only 2$\cdot$15.1%. The saving (fuel equivalent to 2.3% of airliner's landing mass) is real, but even if four others could benefit from the same tanker flight, it would need to cost less than the equivalent of the fuel price of 5$\cdot$2.3% = 11.5% of the airliner's landing mass. Since the tanker needs to carry 5$\cdot$15.1% = 75.5% of the airliner's landing mass in fuel, it needs to be an airliner-sized aircraft itself. Add more if the tanker needs to do even as much as to take off, let alone fly to a refueling point and wait there. And the fuel saving needs not only to pay for the tanker's fuel needs, but also for its crew, maintenance and depreciation. Aerial refueling is a great technology to make complex military scenarios possible, but is a very ressources-hungry beast. • It seems like the tanker just taking off and climbing up to the cruise altitude of an airliner would burn quite a lot of fuel... and that's if it would even be possible for a tanker loaded down with Jet-A to even climb up to the altitude of an airliner with light fuel load in the first place. If the airliner has to descend to meet the tanker, that's yet more wasted fuel. – reirab Aug 3 '15 at 2:57 • Might it become slightly more feasible if they used smaller planes for the flights? Like a B738, or A319? Maybe even a CR-145? Then you could have one tanker carrying fuel for several planes. Granted this doesn't address a whole list of other problems but...theoretically would it help? – Jay Carr Aug 3 '15 at 19:10 • @JayCarr: Using smaller passenger planes will make longer-range connections possible. Now we save not just some fuel, but make flights possible which could not be flown before. However, smaller planes are less efficient, and more cramped on the inside. The seat-mile fuel consumption would be larger, and I certainly will not look forward to flying transcontinental flights in a CR-145! – Peter Kämpf Aug 3 '15 at 19:59 • Looks like we can save, in perfect world, around 9.3% of total fuel. Assuming fuel is \$5 a gallon, and a 50,000 gallon capacity (777 long range seems to be just under that) there's a potential for a $23k saving in fuel. Bigger than I would have expected, but I doubt big enough to justify the cost of a second aircraft. – NPSF3000 Aug 4 '15 at 3:38 • @NPSF3000 MikeFoxtrot quoted \$200m for a KC-46, so let's go with that and your $23k savings per flight. In fact, let's make it \$25k/flight, and four refuelings per KC-46 flight, so each KC-46 takeoff saves \\$100k. That's 2,000 tanker flights until break-even. But that's before you have any people to fly or care for the tanker. Even assuming a perfect world, that's many thousands of trips for the tanker before break-even. The world is not perfect. One mishap and that carrier might very well be facing bankruptcy. People understand that crashes happen, but deliberately putting that fuel there? – a CVn Aug 4 '15 at 11:20

2019 update: site is dead (now a scam service). See wayback machine here:

The people from Cruiser-Feeder http://www.cruiser-feeder.eu/ try to make it happen.

Here is part of the abstract from a paper describing their approach:

In this paper it will be described how the safety of air-to-air refuelling has been assessed, and how proposed new or amended regulations and acceptable means of compliance have been defined

They even created a conceptual design of joint-wing tanker for civil operations: http://www.cruiser-feeder.eu/downloads/li-la-rocca---conceptual-design-of-a-joint-win.pdf

In their papers, you will find data for which flights mid air refuelling is worth it, how they want to make it happen, and some interesting facts.

• Interesting approach: Let the tanker fly behind and push the fuel up. This requires much less training of airliner crews. But when I see a joined wing proposal, I know this is from an academic ivory tower with little practical experience in aircraft design and construction. – Peter Kämpf Aug 3 '15 at 16:36
• @PeterKämpf, you mean sometimes theories don't work so well in the real world? I'm shocked! – FreeMan Aug 3 '15 at 19:08
• @FreeMan: I am even more shocked. The dishonesty (or is it cluelessness?) of the people behind the cruiser-feeder website is breathtaking. They are happy to gobble up EU research funds with unbelievable claims. A simple back-of-the-envelope calculation shows that the benefits are merely a tenth of what they claim - just for the isolated airliner. System fuel consumption will go up. – Peter Kämpf Aug 3 '15 at 20:13
• @PeterKämpf unfortunately I don't have much experience in this field of the wide aviation world, I posted the answer because I think it's definitely worth a read. Why are you so shocked about the cruiser-feeder concept? I think the idea behind the concept is very good. Let the plane start with a little amount of fuel to reduce its start weight and then refuell it in the air on its way to the destination. Why do you find a joined wing proposal a bad idea? As I said, I'm not experienced in this field of aviatics but I'm interested in your opinion about the Cruiser Feeder Concept. – jklingler Aug 4 '15 at 17:02
• @jklingler: This concept will never be certified - just think what might happen if the refueling fails. In the end, the airliner will need to be much like airliners today, and with the Breguet equation it is easy to see that the savings are modest. I encourage you to ask new questions, because the comments make it hard to explain all details. – Peter Kämpf Aug 4 '15 at 20:22