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I was wondering if this might make sense as commercial aircraft design has not changed significantly (from the outside) in the last 50 years?

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    $\begingroup$ What makes you think this hasn't already happened? $\endgroup$ – Dan Pichelman Jan 12 '16 at 15:01
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    $\begingroup$ Not changed much in 50 years? Aerodynamic efficiencies have improved massively, as has the materials used to build the fuselages and wings. These alone have produced massive fuel efficiency gains! $\endgroup$ – Gavin Coates Jan 12 '16 at 15:44
  • $\begingroup$ This question could use some clarification, as new models are obviously being built that combine older airframe designs with new engines (737, 747, A320 all spring to mind immediately). I assume you're asking if an airplane built in, say, the 1980s could be updated with 21st century engines? $\endgroup$ – egid Jan 12 '16 at 19:05
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You are absolutely right - refitting new engines to older airplanes makes a lot of economical sense. After all, two thirds of fuel economy gains in the last 50 years were due to better engines.

However, some conditions conspire to make this harder than it might look:

  1. The better efficiency was gained by increasing the bypass ratio of engines, which means that the new ones had more diameter for the same thrust. This makes them hard to fit to airframes designed for the smaller engines, and adding a longer landing gear would be too expensive - then it is indeed better to buy a new airframe.
  2. Designs with tail-mounted engines would have center-of-gravity problems with newer, heavier engines. Tail-mounting engines was very popular after the Sud Aviation Caravelle pioneered this, but rules out those designs (BAC One-Eleven, Vickers VC-10, Boeing 727, Illyushin 62, DC-9, Tupolev 134 and 154) for refitting with more modern engines.

Add to this that airframes have a limited life time, and fitting new engines makes sense for only a limited number of aircraft.

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    $\begingroup$ Regarding No. 1, the B737-300 transition to the CFM56 from the old JT8D engines used on the B737-200 overcame this drawback, but on a new model designation. $\endgroup$ – J Walters Jan 13 '16 at 2:14
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    $\begingroup$ @JonathanWalters: Yes, and that is why I linked to the article on the odd shape of the engine inlets. Developing those was a major effort, even for Boeing, so the big engine could be fitted into the restricted space between wing and ground. Note, however, that Boeing has since also added a newly-developed wing to capture the benefit of supercritical airfoils. $\endgroup$ – Peter Kämpf Jan 13 '16 at 5:00
  • $\begingroup$ Um, the DC-9 series is too capable of carrying modern high-bypass turbofans without having CG problems, as demonstrated by the DC-9-93/95 series. $\endgroup$ – Sean Aug 14 '18 at 3:48
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    $\begingroup$ @Sean: The V-2500 has 350 kg more dry mass, and those 700 kg at the tail need to be compensated somehow. That the engineers solved the cg problems does not mean that there were none. $\endgroup$ – Peter Kämpf Aug 15 '18 at 7:45
  • $\begingroup$ @PeterKämpf: No, but it does mean that they were solvable - which contradicts your assertion that having tail-mounted engines "rules out those designs... for refitting with more modern engines.". $\endgroup$ – Sean Aug 15 '18 at 20:43
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Yes, this can and does happen.

Two examples that I think are excellent illustrations of the concept are turbine conversions of the Beech 18 and the Douglas DC-3. In these cases the old radial piston engines were replaced with turbo-prop engines. Other examples also exist, including less drastic modifications.

An example of a Beech 18 modification is the Volpar Turbo Beech conversion which used Garrett TPE-331 engines. Information on that conversion here, and an example of the operational use of such modified aircraft here.

An example of a DC-3 conversion is the Basler BT-67, which uses PT6A-67 series engines. Information on that conversion here and here.

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It's certainly possible for aircraft to be retrofitted with newer engines. One example is the DC-8 Series 70, which had the older JT3D engines replaced with more efficient CFM56 engines. However, there are many reasons for this process not being more common.

There are many costs associated with changing engines. The engine may need a new nacelle design to fit the specific aircraft. This will probably require a new or at least revised strut design to mount to the wing. The wing may also need to be reinforced, as newer engines tend to be heavier. All of the systems attached to the engine, including electrical, hydraulics, and bleed air may also need changes. Avionics will also need to be updated to work with the new engine type.

Certification also should not be overlooked. Assuming the aircraft can use an existing engine that is already certified, the aircraft will still need to be certified to fly with that engine. This means a lot of paperwork with the regulators, and probably additional testing. However, engines tend to be designed for a specific aircraft type. For example, the CFM LEAP engine will power the A320, 737, and C919. All of these aircraft are similar in size and compete for the same market, yet CFM is doing testing and certification of a different version of the LEAP-1 for each of these aircraft. Each manufacturer chose to design their aircraft for a different engine version, which makes it seem unlikely that a new engine would just happen match the needs of an existing aircraft without extensive modification of the engine.

Aircraft fuselages are designed with a limited life span. As they age, maintenance costs increase as more work is required to repair damage from fatigue and regular wear and tear. Even with new engines, the airframe would still have a limited life span.

As other answers have pointed out, a new airplane design will be more efficient than just putting new engines on an old aircraft. Owners of the older aircraft would have to see a benefit in paying to upgrade their old aircraft, and having those aircraft out of service for an extended period, versus just buying a new plane that will be more efficient and will have many more flight hours left ahead of it. And some company will need to find enough of these customers to justify the large upfront costs listed above in making such an upgrade possible.

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    $\begingroup$ The CFM re-engines were the first thing I thought of. Basically the same engine upgrade was applied to KC-135s (the original Boeing 717) to produce the KC-135R & KC-135T. $\endgroup$ – egid Jan 12 '16 at 19:03
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While the design of aircraft may not look different to previous generations of aircraft, the technology behind them is vastly different, from lightweight carbon fibres, to far more aerodynamic wings. New design aircraft provide vastly better operating economics than aircraft based off an old design but with newer engines.

Take for example the A350, which Airbus claim delivers a 25% fuel reduction per seat when compared to existing aluminium long-haul aircraft. The A330neo on the other hand, which is a re-engined version of the A330 (but also containing revised wingtips and some other minor aerodynamic enhancements), is only expected to deliver a 14% fuel burn advantage per seat over the current A330.

And this is with a newly built A330neo rather than taking an old A330 and putting new engines on an existing frame.

There are many reasons why putting new engines on older airframes are not a good idea. First off, simply swapping out the engines is not feasible - the chances are the new engines will have different mounting points, and different fuel flow requirements, requiring a complete redesign of the fuel flow system. You would also need to revise the avionics, to take into account revised engine operating parameters. Operating procedures for pilots would also be different between models, leading to either the need for separate pools of pilots per engine type, or if pilots are crossing between models, an increased risk of pilot error leading to an accident.

But the biggest reason why this is not done, is that aircraft fuselages have a limited lifespan. With every flight, you are pressurising the aircraft, then de-pressurising it for landing. This causes metal fatigue to the frame. This alone is the single biggest cause of aircraft being retired. It is required that aircraft undergo a "D-Check" roughly every 6 years. This involves stripping the entire aircraft, and removing all paint from the outside, so the state of the fuselage can be checked for signs of metal fatigue. This is a hugely expensive and time consuming process, and as a result most aircraft only every go through a few of these before they are retired.

So re-fitting an aircraft with newer engines would not be feasible as the air frame would have exceeded its lifespan anyway and would need replacing, so far better to just buy a new build aircraft.

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    $\begingroup$ While you make some valid points, there are several examples of aircraft being retro-fitted with updated engines. You suggest that it would never be done, rather than saying that it can be done but isn't often. $\endgroup$ – Jon Story Jan 12 '16 at 16:04
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    $\begingroup$ The fact is that modifications are feasible, economical, and are done, despite the drawbacks you note. However, some of the notable examples of this type of modification include passenger aircraft which are not pressurized, negating that otherwise valid drawback. $\endgroup$ – J Walters Jan 12 '16 at 17:38
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    $\begingroup$ @JonStory yes, you're right. I was referring to modern aircraft in my answer. It may have happened in the past, but nowadays it's just not economically (and possibly technically) feasible. $\endgroup$ – Gavin Coates Jan 12 '16 at 21:13

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