In-flight Refueling for Commercial Jet Airliners?

[brockrwood]

I was thinking about the rise of the large, twin-engine wide body airliners, such as the 777, 787, A350, and so forth.

In the olden days, the ETOPS requirements forbid a twin-engine airliner on long flights over oceans because, if an engine failed over the middle of the ocean, the plane could not fly far enough to reach an airport to land at. The new airplanes, with their HUGE twin engines, it is argued, are so reliable that they get to fly over long stretches of water with just two engines.

Then I thought about how the U. S. military regularly refuels military airplanes in mid-flight.

I wonder why commercial jet manufacturers never considered giving a commercial airliner an in-flight refueling capability. Even if the capability was never used, the mere fact that the plane could be refueled in the air would get around the ETOPS rules that prohibited twin engine jetliners flying long distances over water. No? There must be a reason (cost, probably), or the ingenious engineers and management at the world's plane manufacturers would have tried this already.

 

[OverTheTop]

I suspect risk and training requirements are a couple of the big-ticket items keeping this off the table. There would be a mass penalty too.

BTW, ETOPS=Engines turn or passengers swim

 

[brockrwood]

ETOPS=Engines turn or passengers swim

Ha! Ha!

 

[modeltrains]

I'll bet the answer to 'why not' is - money.
The cost of: the refueling aircraft's structure; the cost of the refueling aircraft's maintenance; the cost of the refueling aircraft's base; the cost of the refueling aircraft's crew; the cost of training Airliner aircrew to refuel; the cost of flight operations of the refueling aircraft; the cost of lobbying for FAA rules to be changed; and most importantly - the cost of all those aircraft in multiple dozens because you need refueling aircraft for operations, aircraft for training, and aircraft as reserves to cover maintenance issues.

In terms of traffic volume today, how many refuelers will be needed for the traffic volume, https://www.flightradar24.com/37.52,-97.04/2

 

[Steve Shannon]

I was thinking about the rise of the large, twin-engine wide body airliners, such as the 777, 787, A350, and so forth.

In the olden days, the ETOPS requirements forbid a twin-engine airliner on long flights over oceans because, if an engine failed over the middle of the ocean, the plane could not fly far enough to reach an airport to land at. The new airplanes, with their HUGE twin engines, it is argued, are so reliable that they get to fly over long stretches of water with just two engines.

Then I thought about how the U. S. military regularly refuels military airplanes in mid-flight.

I wonder why commercial jet manufacturers never considered giving a commercial airliner an in-flight refueling capability. Even if the capability was never used, the mere fact that the plane could be refueled in the air would get around the ETOPS rules that prohibited twin engine jetliners flying long distances over water. No? There must be a reason (cost, probably), or the ingenious engineers and management at the world's plane manufacturers would have tried this already.

In flight refueling only prevents engines from running out of fuel, which seldom happens suddenly or unexpectedly. It does nothing about the kinds of failure which are sudden and unexpected.

 

[boatgeek]

There's also limits on endurance of passengers, pilots, and crew. The longest flights now are 16 hours plus. I can't imagine trying to extend that. The only thing worse would be 16 hours in a 737 with two in-flight refuelings.

 

[brockrwood]

In flight refueling only prevents engines from running out of fuel, which seldom happens suddenly or unexpectedly. It does nothing about the kinds of failure which are sudden and unexpected.

I was thinking of it this way: An engine fails suddenly while flying over the Pacific Ocean. The engine has to be shut down. Now the plane flies more slowly on the remaining engine. The problem, it seems to me, is fuel, no? The remaining engine is still consuming fuel, probably more because it is throttled up to keep the plane's airspeed up. The plane runs out of fuel before it can slowly make its way to an airport to land at because on one engine, the plane simply can't fly as fast.

But, if the limping plane could be re-fueled in flight, it could make it to an airport. That was how I was analyzing it.

I could be completely misunderstanding the ETOPS restrictions. It wouldn't be the first time I misunderstood something!

My assumption is that twin engine jetliners can fly and land on just one engine, if the plane has enough fuel to get to an airport. I could be wrong.

 

[brockrwood]

There's also limits on endurance of passengers, pilots, and crew. The longest flights now are 16 hours plus. I can't imagine trying to extend that. The only thing worse would be 16 hours in a 737 with two in-flight refuelings.

Good point. I was thinking that the in-flight refueling would only happen in an emergency where, without the in-flight refueling, the plane would crash due to fuel starvation.

 

[modeltrains]

But, if the limping plane could be re-fueled in flight, it could make it to an airport. That was how I was analyzing it.

Ahh, a very important thing is being overlooked - aerial refueling sometimes fails due to mechanical issues or weather issues & if airliner has to refuel at a point where it still has enough fuel to divert to a landing field to refuel, you save a whole bunch of money and hassle by cutting aerial refueling out of the picture before it even begins.

 

[Steve Shannon]

I was thinking of it this way: An engine fails suddenly while flying over the Pacific Ocean. The engine has to be shut down. Now the plane flies more slowly on the remaining engine. The problem, it seems to me, is fuel, no? The remaining engine is still consuming fuel, probably more because it is throttled up to keep the plane's airspeed up. The plane runs out of fuel before it can slowly make its way to an airport to land at because on one engine, the plane simply can't fly as fast.

But, if the limping plane could be re-fueled in flight, it could make it to an airport. That was how I was analyzing it.

I could be completely misunderstanding the ETOPS restrictions. It wouldn't be the first time I misunderstood something!

But the failed engine isn’t using fuel so now you have that fuel available for the remaining engine, which probably isn’t using fuel at twice the rate. I would expect the plane would want to slow down to minimize drag (remember that drag is proportional to the square of velocity).

 

[brockrwood]

Ahh, a very important thing is being overlooked - aerial refueling sometimes fails due to mechanical issues or weather issues & if airliner has to refuel at a point where it still has enough fuel to divert to a landing field to refuel, you save a whole bunch of money and hassle by cutting aerial refueling out of the picture before it even begins.

We need to bring back the tri-jet! Those beautiful L-1011's and DC-10's should be roaring overhead!

 

[brockrwood]

But the failed engine isn’t using fuel so now you have that fuel available for the remaining engine, which probably isn’t using fuel at twice the rate. I would expect the plane would want to slow down to minimize drag (remember that drag is proportional to the square of velocity).

So what's the big deal with ETOPS? If a twin engine jetliner flying over an ocean could make it to an airport, on one engine, it seems like the ETOPS restrictions are not needed. I guess that is why the ETOPS rules do allow modern twin jets on long, over-water flights.

"ETOPS 180" means the plane must be able to reach an airport in 3 hours, if I understand it correctly. Even flying over the middle of the Pacific, is there a spot where NO airport is within 3 hours of one-engine flying time?

 

[boatgeek]

Based on the map here, it looks like the South Pacific is generally OK for ETOPS 180, but there may be some areas between California and the Galapagos that would be an issue. I can't verify the accuracy of the map...

 

[les]

But the failed engine isn’t using fuel so now you have that fuel available for the remaining engine, which probably isn’t using fuel at twice the rate.

Correct - the fuel for both engines would now be available for the one remaining engine, giving it "twice the fuel".
Also totally agree with the costs for having the tankers crew, and training would be prohibitive.

But how many flights/routes are limited by ETOPS? I just got back from a trip to Hawaii in a 2 engine plane (767). From California, it is over 2000 miles, so in theory if there is an issue half way, we have to fly over a thousand miles to find an airfield larger than a whale's back.
For business I've flown from NY to Tokyo non-stop in a 2 engine plane (14 hours in a tin can), but the routing is North over Alaska and then down to Japan. Not certain how much is over open water with no landing options. My understanding is that route is shorter and not caused by ETOPS limitations.

 

[modeltrains]

And with the flight ranges modern 2-engine jetliners already have from on-board fuel there is no practical need for aerial refueling:

Boeing 787 = 7,500 nautical miles, 14,000 km.
Airbus A350 = 8,000 nautical miles, 15,000 km.

https://www.boeing.com/commercial/787/
https://aircraft.airbus.com/en/aircraft/a350/a350-900

 

[cvanc]

Hmmm, I'm surprised no one has mentioned insurance yet. You think it could be obtained? What would it cost re: to now?

I kinda think this is a showstopper, unattainable at any workable price.

 

[boatgeek]

For business I've flown from NY to Tokyo non-stop in a 2 engine plane (14 hours in a tin can), but the routing is North over Alaska and then down to Japan. Not certain how much is over open water with no landing options. My understanding is that route is shorter and not caused by ETOPS limitations.

Alaska has a surprising number of really long runways from the strategic bomber days. I've had a flight I was on land in metropolitan King Salmon (population ~350) to refuel. They had a beautiful ~12000 foot runway in the middle of absolutely nowhere.

 

[rharshberger]

Safety is probably the #1 reason why not...aerial refuelling is one of the most dangerous things two aircraft can do, why put hundreds of lives at risk for something that can be avoided. Cost is just one ofcthe other reasons.

 

[Hobie1dog]

simply impractical

 

[Funkworks]

Reading this thread title caused me to picture an electrical cable instead of a fuel line. So I'm putting this idea out here: an electrical aircraft that can be recharged mid-air by a flying battery (cargo plane).

Don't ask any questions, I have no idea yet if it can be useful.

Carry on.

 

[boatgeek]

Reading this thread title caused me to picture an electrical cable instead of a fuel line. So I'm putting this idea out here: an electrical aircraft that can be recharged mid-air by a flying battery (cargo plane).

Don't ask any questions, I have no idea yet if it can be useful.

Carry on.

I'm seeing a wing walker on a biplane grabbing a trailing socket and plugging the charger in. Then they fly in formation for a 6-hour charging period...

 

[BEC]

Both the 787 and the A350 have ETOPS ratings over 300 minutes (5 hours). That pretty much covers the world without constraining route planning a great deal.

Just carrying the equipment for inflight refueling (not only the receptacle but also the plumbing, control valves, etc.) that you'd use only in an emergency — an emergency in which the tanker, which is likely no faster than your airplane, would be able to get to you in time — is just wasteful. That's fuel burn (or cargo/passengers you can't carry) on every flight against the one in millions per flight hour chance that you'd need it.

And Rich (@rharshberger) really nailed it. Flying two airplanes in close formation while transferring fuel from one to the other is really difficult with many chances for things to go wrong.

 

[user 13]

BEC is correct.

FWIW I have done air refueling (C-5) and fly ETOPS regularly (737).

ETOPS is much, much, much safer.

 

[Charles_McG]

Reading this thread title caused me to picture an electrical cable instead of a fuel line. So I'm putting this idea out here: an electrical aircraft that can be recharged mid-air by a flying battery (cargo plane).

Don't ask any questions, I have no idea yet if it can be useful.

Carry on.

Microwave beam it from orbit.

 

[cls]

Given how $ sensitive the airlines are, and how expensive the delivered in flight fuel is, seems like a no thanks. Also, no need as discussed above...

 

[Alan15578]

But the failed engine isn’t using fuel so now you have that fuel available for the remaining engine, which probably isn’t using fuel at twice the rate. I would expect the plane would want to slow down to minimize drag (remember that drag is proportional to the square of velocity).

An airliner gets great range by flying as high as practical. If you lose half your thrust, you will not be able to sustain the same speed or altitude and range will decrease. You will also have to crank in a lot of rudder and other controls just to trim the aircraft with an imbalanced thrust, increasing drag.

 

[watheyak]

Engine failure is one concern, but a scenario people don't often think about with Long flights over water is cabin depressurization.

A pressurization emergency would necessitate a descent to 10,000' where the air is breathable. The masks that drop from the ceiling only make enough oxygen to to last until you get down there.

Down at that altitude you burn around double the fuel, which obviously puts a serious dent in your range.

In the aircraft that I fly (9-seat business jet) I often find that in the event of a depressurization I have a "wet footprint" on the way to Hawaii, requiring us to bring supplemental oxygen for the passengers so we can fly at a higher altitude for a longer time. Sounds fun, right? Marginally practical in my airplane, not so much in an airliner.

 

[BEC]

An airliner gets great range by flying as high as practical. If you lose half your thrust, you will not be able to sustain the same speed or altitude and range will decrease. You will also have to crank in a lot of rudder and other controls just to trim the aircraft with an imbalanced thrust, increasing drag.

This is true, but remember that a twin-engine airliner has to be able to take off and climb out at a certain minimum climb rate (I don't know what it is - it's in the FARs) on one engine at full load as part of its certification testing. So yes, more drag than normal. But I'd be really surprised to see that even full climb out thrust of one engine would use more fuel than two cruising engines. This is all taken into account in granting ETOPS certifications.

Also, systems that might be driven primarily one engine, especially hydraulic systems where there are at least two and usually three or four different ones, have to have all control functions available regardless of which engine fails. That is why 757s that have ETOPs certification have a power transfer unit — a hydraulically driven hydraulic pump — to provide pressure to one of the systems driven from one engine to maintain capability in the event that that is the engine that failed. The airplane has three hydraulic systems. One is driven by an engine driven pump on one engine, one by the other engine and one by AC electric pumps (and electricity is generated by both engines).

This sort of thing also has to be taken into account for ETOPS certifications.

 

[watheyak]

This is true, but remember that a twin-engine airliner has to be able to take off and climb out at a certain minimum climb rate (I don't know what it is - it's in the FARs) on one engine at full load as part of its certification testing. So yes, more drag than normal. But I'd be really surprised to see that even full climb out thrust of one engine would use more fuel than two cruising engines. .

At sea level, one engine at takeoff power definitely burns more than two engines at altitude.

 

[BEC]

You know, as I typed that, I had to wonder if I was right or not. Propulsion is not my area of expertise. I wonder if, say, the fuel flows of takeoff thrust and cruise thrust for, say, GE90s, are available online. It would be interesting to see.

A quick look around the interwebs got me more confused.

Regardless….I am sure fuel flows are taken into account for ETOPS certifications.