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I was researching about the Boeing 737 MAX, and came across some information on a Business Insider page:

But in the 2010s, Boeing tried to replicate the success of the 737NG with the 737 Max. But this time, it wasn't simply competing with Airbus; it was playing catch-up. Boeing began to discuss a successor for the 737 as early as 2006, looking at both putting new, more efficient engines on an existing 737 airframe, or starting from scratch with a brand new airframe. Boeing knew that Airbus was similarly exploring an A320 replacement, but both companies were still in early stages.It was still trying to decide in 2010, when Airbus launched the A320neo family (neo = new engine option). The jets used the original A319, A320, and A321 airframes, but used new engines that offered a 15-20% increase in fuel efficiency, consequently lowering operating costs and giving the planes longer ranges. Airbus has since released two longer-range variants of the neo family — the A321LR and A321XLR.

Then the article began talking about how it could take up to 10 years for Boeing to create the new aircraft, so instead of creating a completely new airplane, Boeing decided to just add bigger engines.

Now my question is, why does it take so long to develop a new passenger aircraft?

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    $\begingroup$ I'm trying to develop a passenger aircraft prototype right now in fact, and trust me, it takes years. You have to consider every single tiny detail. The ECAM and cockpit, where the fuel tanks should be located, how high the landing gear should be, how big the plane should be, what the MTOW, MLW, maximum cruise alt. and stall and max. speeds should be. And many times, you hit calculations wrong or forgot something. So, you have to do them all over again $\endgroup$ – Air Canada 001 Apr 26 at 14:36
  • $\begingroup$ Closely related. $\endgroup$ – mustaccio Apr 27 at 2:28
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    $\begingroup$ One word - gravity $\endgroup$ – MaxW Apr 28 at 20:51
  • $\begingroup$ And if you think the engineering takes a lot of time, have a look at how the contract lawyers are moving... ;) $\endgroup$ – bogl Apr 29 at 13:51
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It's similar with military aircraft. In a mature field it becomes ever harder to create something that is clearly better than what we have already.

Since every new development costs so much, the consequences of failure are immense. Therefore, management will try to lower the risk of failure which again drives up development cost and time. It is a self-reinforcing spiral.

Add to that the fact that we now have only two major civil aircraft producers which both are heavily backed by their respective governments and are too big to fail. This creates political interference which makes major decisions even more complex and precludes bold, risky decisions. Only the most safe* option will be chosen after years and years of deliberations.

And I refuse to believe that we don't know enough yet in aerodynamics or materials science. When a wing test lets the wing break at a load factor of 1.54 (where 1.5 is the goal) on the first try, we are pretty good already. Same for performance: New models hit their predicted fuel consumption within 2%! And regarding the many subsystems: COTS. Of course, if you insist on re-inventing the wheel all over again for a new airliner, then those 10 years are acceptable.

* "Safe" meaning the one promising the highest bonus to management for the time they hold their position, and with their current state of incomplete knowledge. Sadly, the ethos of companies like Newport News Shipbuilding, whose motto was: 'We will build good ships here — at a profit if we can, at a loss if we must, but always good ships.' has been lost to the Harvard MBA culture with its focus on short-term results.

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  • $\begingroup$ Incase if the company isnt backed by government,will time required to make a new plane be reduced by 30% (roughly) of the total time? $\endgroup$ – Auberron Apr 26 at 5:45
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    $\begingroup$ @Auberron It is very hard to quantify this. Without government backing, another competitor would not even stand a chance to get his designs certified in time to bring the advantage of a faster development to market. Or to get credit once something goes wrong and more capital than initially planned is needed. My personal opinion: With a hand-picked team and no interference (both ways!) the plane would be done within four years. But the risk of commercial failure would be quite real. $\endgroup$ – Peter Kämpf Apr 26 at 6:41
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    $\begingroup$ The maturity of the field is really important. In the 1920's and 1930's (and even into the early jet era) there were some hilariously wrong designs, but if a badly designed aircraft killed a bunch of people, then they just hoped the next one will work better. If today a new design had a significant fault killing a lot of people, they won't be forgiven as easily. $\endgroup$ – vsz Apr 26 at 15:22
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    $\begingroup$ @DohnJoe: Right, that one is a good example. I should qualify "best" better. In that case, what is best is determined by management and their desire to maximize profits. If you had asked the engineers, they would had decided on a new airframe or at least a bigger stabilizer. With their limited knowledge at that time, management decided on what they considered best. Now we know the engineers should had made that decision. Sadly, the ethos of companies like Newport News Shipbuilding ('We will build good ships here — at a profit if we can, at a loss if we must, but always good ships') is no more. $\endgroup$ – Peter Kämpf Apr 27 at 7:48
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    $\begingroup$ upvoted, especially for the last paragraph :) $\endgroup$ – Nubarke Apr 27 at 11:04
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There are many unavoidable factors that contribute to lengthy aircraft development:

  • Aircraft development involves a great variety of subsystems. Examples include: the aerodynamic surfaces, the mechanical structure (the former two are physically the same), propulsion, electric systems, hydraulic systems, avionics, landing gear, passenger cabin, cabin air management, cargo hold, emergency systems, de-icing, operations, personnel, pilot training, building a dedicated flight simulator, manufacturing, supply-chain, design for maintenance and recycling, writing maintenance and operation manuals, etc.

  • All aircraft systems are highly coupled. If you change one component, it has effects on the entire aircraft. This is especially true for aerodynamics, total mass, center of gravity, and fuel consumption.

  • Everything is designed close to the limit. While most components of factory machines, tractors, and the like can be designed with huge safety factors, all aircraft components are typically just as strong, as stiff, as large, ... as they need to be. Typical safety factors are just 1.5x or 2x the worst expected operating conditions. With higher safety factors the aircraft would probably not fly, and you definitely couldn't compete with other companies.

  • The snowball effect. If during later design stages any little component needs more power or mass or has more drag than accounted for in the preliminary design, that effect multiplies. An example: The forces on a tail control surface are higher than expected. So you need a bigger cylinder to move it. That adds weight and moves the center of gravity backwards. To keep flying with that little added weight, you need more fuel. But that means you need an extra tank. You wanna put that one in the front of the plane to move the center of gravity back where it needs to be but then you need an extra pipe and fuel pump that goes to that tank. All that adds even more weight. And soon you're too close to the sound barrier that flying faster won't solve it, so you need bigger wings. Et cetera.

  • Iterations. Because of the three previous points, designers will often be forced to re-consider previous decisions and re-do parts of the design effort. If that happens too often, the aircraft never gets done. Therefore during preliminary design aircraft designers take great care to properly estimate what's possible in terms of global design parameters (weight budget, fuel budget, wing shape, wing location, engine number, engine location, engine thrust, etc.) and fix those. And the same goes for the major parameters of any subsystem during preliminary subsystem design. This reduces the need for iterations but they still play an important role in aircraft development. Many individual design sequences have to be repeated until the final design of every subsystem.

  • Aerodynamics are poorly understood. This is especially true the closer you get to the sound barrier (airliners get close) and in the boundary layer (and that's all the air molecules in the direct vicinity of the aircraft; all the neat aerodynamic equations we work with basically operate on the aircraft plus the boundary layer, the shape of which we don't know). Hence designing for aerodynamics is guesswork. And the only time you actually get to check whether your aerodynamic design has the required properties is when you do the flight tests. It takes lots of time during the design enhance your chances that the flight tests go well. See the paragraph on modelling.

  • We know even less about material science. For aerodynamics we have partial differential equations derived from fundamental physics that model the airflow pretty accurately. We just don't know how to solve them. But there are no remotely accurate ways to model the failure modes of aluminium (or any other material) based on fundamental physical or chemical laws alone. On top of that, modern aircraft designs are incorporating more and more composite materials which are even harder to model. So that's even more trial and error.

  • Aeroelasticity. That's the dynamic interactions between aerodynamics and mechanics of materials. These interactions can cause the wing to vibrate so badly that it breaks. This field is particularly hard to understand because we already have such a hard time with aerodynamics and materials. Yet we need to design for it which involves lots of modelling and testing.

  • Most aspects of aircraft design require multiple stages of modelling. Because of our limited understanding we can't just go ahead, make some guesses, build the bird, and hope it flies. That might kill our test pilot. And we'd have to start from scratch. So we make lots of models of reality. For instance, for wing design we start with back-of-the-envelope calculations, then build computer aerodynamic models to narrow down our options, then put a few simple model wings in the wind tunnel and see how they perform. Then we repeat these processes with winglets and flaps added to the wing. Later we do the fight test which is also just a model of the infinite amount of possible operating conditions. To check many of the mechanical components (small and large) we'll design test setups to figure out how they deform and when they break. This video shows what this looks like for a full wing. We do modelling for all the subsystems mentioned in the first bullet. All this modelling is very time and resource intensive. For instance, several full planes have to be built for the last stage of modelling, the actual flight tests. It takes lots of effort to design those models in such a way that they model reality as closely as possible. Especially computer models are often way off. So on top of all the efforts to build, validate, and run models, iterations are required when a model turns out to be wrong.

  • Manufacturers need to prove that aircraft won't fall out of the sky. This requires lots of paperwork, analysis, and tests. And dealing with the responsible aviation authority (the FAA in the US) is not always fast.

  • Managing all the people involved inevitably results in delays. Due to all the complexities mentioned above, thousands of people are involved in developing a new airliner. And they don't just work at the plane manufacturer. Many subsystems are designed by other companies. And they have suppliers too. Managing all the people and companies involved can't always be perfectly efficient.

  • non-technical constraints All the technical issues mentioned above already mean that it takes at least a decade to get a new airliner off the ground. Additional delays often result from bad luck, politics, lack of finance, lack of experienced engineers, lack of urgency, management mistakes, and incompetence at any level.

If this description sounds circular, that's because it is. In aircraft development it's easy to end up going in circles if the systems engineering isn't done very well.

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    $\begingroup$ Don't forget the fact that they've got to write an Aircraft Maintenance Manual detailing every possible maintenance task for every single system and subsystem that could be dozens of "books" long when printed out on paper. $\endgroup$ – nick012000 Apr 27 at 17:04
  • $\begingroup$ The key point here is the necessity for low safety factors, everything else is just a consequence of that. An average petrochemical plant is just as complex, but when the required safety factor is 4, you can easily design for 8 for good measure and no matter what turns out not to be exactly right, you can still be pretty sure that you'll end up above the design requirement. So you hardly ever need to start from scratch and redo a bunch of things. This makes the design process way cheaper but the product somewhat more expensive. You can't afford that with aircraft. $\endgroup$ – TooTea Apr 28 at 8:23
  • $\begingroup$ "Manufacturers need to prove that aircraft won't fall out of the sky." That is my main issue why everything takes ages. Developing a sensor for functional safety levels required for aviation will easily take twice the time then it normally would. And this carries on through all the stages... $\endgroup$ – Arsenal Apr 28 at 15:54
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By the time an aircraft program is announced to the world, it would already have had a few years of conceptual development with a team of a dozen people. The business/market objectives, basic dimensions and layout, performance and weight objectives, and critical design decisions that define the aircraft would have been made. At least one or two wind tunnel tests would also have been performed.

The next 2 years would be preliminary design, concentrating on finalizing the major aircraft systems requirements, Outer-Model-Line design, control surface sizing, and selecting the Tier 1 suppliers. Many wind tunnel tests would be performed at this phase, and the design proceeds in iterative fashion until the loops are closed around all major disciplines. At this point, the original team of a dozen engineers will have ballooned to hundreds of people.

The next 2-3 years would be detailed design, iterating amongst aircraft manufacturer and suppliers to ensure the requirements can be met/are met and the kinks are worked out. The production on multiple prototype flight test vehicles (FTVs) would also start at some point. A lot of pilot-in-the-loop simulations are proceeding behind the scene to fine-tune the flight control systems, avionics, and working toward safety-of-flight for the first flight.

This is followed by 1-2 years of development and certification flight testing. Development flight testing ensures that the aircraft and systems behave satisfactorily and per certification, and things are tuned if they do not. To condense flight test schedules, each flight test vehicle will be relegated to particular areas: for example, one for aerodynamics/performance/controls, another for pneumatics/hydraulics/electricals, etc. Once the tuning is completed, then the certification flight testing begins with the regulation agency flight crew onboard.

If all goes well, then the airplane gets the primary agency's type certification and the entry into service follows soon after.

The above is a fairly rosy development schedule. Any major design issues discovered along the way (the later in the program the worse the impact) could easily stretch the schedule by years, adding billions to the development cost. Examples: electrical system certifiability in A380, manufacturing and integration issues with B787, control law and engine explosion in A220, fabrication issues and weight blow-up in Lear 85 (that ended up killing the program).

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  • $\begingroup$ That "safety of the first flight" bit is significant. In the 1910s, an airplane design was something you could bang out in your garage in a few months, and "first crash" was at least as common an outcome as "first flight". Safety expectations have changed a bit since then. $\endgroup$ – Mark Apr 28 at 2:30
  • $\begingroup$ @Mark Safety for all the subsequent flights is a huge issue as well. Development for life critical systems is heavily regulated and formalized, leading to a rather glacial pace of progress. Consider how incredibly safe airplanes are, even though they operate at high speeds in the air. There is no simple "safe mode" they can fall back to in the event of an unrecoverable error, the way almost every other device can (machines, cars and trains simply stop!). Failure rates of trains, cars or smartphones would be totally unacceptable for airplanes. $\endgroup$ – Peter - Reinstate Monica Apr 28 at 12:37
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Aircraft are complex machines. A Boeing 747 consists of ~6 million parts, all of which have to be designed, tested and certified.

Compare this to cars, which have ~10,000 parts. Developing a new car takes 2-3 years.

Aircraft also operate close to the state of the art (i.e. using the most advanced materials, design techniques and construction methods available). To create a design that is noticeably better than its predecessor, you may have to advance the state of the art, which means your design process starts with fundamental research. Then that research has to be made production-ready, the new products have to be tested to make sure they're safe to use etc.

Testing and certification is a massive undertaking. For each of those 6 million components, you have to have a paper trail that proves it's been designed, made and assembled correctly. You have to run large-scale tests (e.g. accelerated fatigue tests where an entire airframe is put inside a hydraulic rig to simulate flight loads) which can take months to complete.

Then there's the tooling. For many of those 6 million parts, you need custom tools and jigs so you can produce the parts repeatably and reliably. You need to develop the manufacturing process.

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I found this Wikipedia Article very helpful in answering your question

The aircraft design process is a loosely defined method used to balance many competing and demanding requirements to produce an aircraft that is strong, lightweight, economical and can carry an adequate payload while being sufficiently reliable to safely fly for the design life of the aircraft. Similar to, but more exacting than, the usual engineering design process, the technique is highly iterative, involving high level configuration tradeoffs, a mixture of analysis and testing and the detailed examination of the adequacy of every part of the structure. For some types of aircraft, the design process is regulated by national airworthiness authorities.

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It doesn't take a decade to develop a new passenger aircraft from scratch. Two years is plenty long enough.

The issue with Boeing is that they wasted the best part of 10 or 20 years fooling around with ideas (for example the Sonic Cruiser) that no customers were interested in, and that Boeing's high level management couldn't choose between making a derivative of one of their existing products or designing something new.

And of course during those decades, many of the experienced Boeing engineers who actually knew how to develop an aircraft in two years found some other employment that was more interesting than playing about with "paper aeroplanes" and watching their managers fail to make any decisions and commit money to implementing them.

There are some wonderful stories from airline customers during that ten years. For example, on one occasion a Boeing sales team was giving a presentation of their "latest strategy" when the airline team leader stopped them and said "This is exactly the same presentation as you gave us five years ago, and you haven't actually delivered any of it in those five years. So why are you wasting our time telling us the same fairy tales all over again?" The Boeing team denied this was the same message - until the customer did a quick search through their filing system and came back with the exact same presentation material dated five years earlier. End of sales pitch.

On the other side of the Atlantic, Airbus devised a strategy (not necessarily the "best" strategy, but good enough), got on with implementing it, and were catching up with Boeing at the rate of more than one year per year...

Working for an engine manufacturer and interfacing with both companies, the difference was chalk and cheese. Airbus knew where they were going and weren't afraid to kick @** to get there. Boeing were generally regarded as a bunch of clueless time-wasters, with the few guys who actually knew what they were talking about being 10 years or more behind the state of the art.

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    $\begingroup$ Can you give any example of a postwar airliner that took 2 years to design? I've looked up a few. BAC 1-11: 9 years. Airbus A320: 7 years (longer if you consider the preliminary studies as well). A330: 10 years. $\endgroup$ – Hobbes Apr 27 at 8:54
  • $\begingroup$ I suspect that sales-info is just generally silliness whenever stuff gets technical. Nutritional supplements, especially stuff like coconut oil, seem to be extreme examples, but it's just hard to believe even the sales reps in a tech store. The more complex stuff gets, the more lax the standards seem to be. Highly regulated fields can be an exception, but even then it seems like softer/fuzzier claims can fly under the radar, especially in informal contexts and when they're hard to verify. $\endgroup$ – Nat Apr 27 at 15:30
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Joe Sutter started the design of the 747 in 1965 and first flight was in February 1969. Design practices and manufacturing processes have improved massively since then, so the critical issue as to why it takes so much longer to design commercial aircraft 50 years after the 747 is probably safety, but as demonstrated by the MAX fiasco, not flight, aircraft or passenger safety, rather management and shareholder safety. The 747 was a gamble that nearly bankrupted Boeing. It paid off because of individuals who had the courage, foresight and authority to make risky decisions that would not pay off in the current or even the next quarter.

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I'm making a comment above an answer because I think it is an important factor: Safety.

One difference between airplanes and other complex devices is the requirement of an unusual level of safety. The reason is that there is no simple "safe mode" airplanes can fall back to in the event of an unrecoverable error, the way almost every other device can (machines, cars and trains can usually simply stop!).

Development for life critical systems with such tight safety requirements is heavily regulated and formalized, leading to a rather glacial pace of progress. Every line of code, every part specification and design, every revision is checked and re-checked, documented and archived, audited and approved, at multiple levels from the detail to the overall construction. The result is that airplanes are incredibly safe, even though they operate at high speeds in the air. Failure rates of trains, cars or smartphones would be totally unacceptable for airplanes.

The only technical construction I can think of that has similar safety requirements, and which may not have an easy path to a safe state either, are nuclear power plants, and they take a long time to develop as well, even though there are almost no weight restrictions: They usually do not fly.

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