Bjorn’s Corner: New aircraft technologies. Part 26. Preliminary design

By Bjorn Fehrm

August 25, 2023, ©. Leeham News: We described the different phases of an airliner development program last week.

We will look at the Preliminary design phase this week, what work is done, the tools used, and how it can be made more efficient.

Figure 1. A typical new airliner family development plan. Source: Leeham Co..

Preliminary design

We said last week that a new airliner project starts with the Conceptual design phase. It would be more correct to say that the formal aircraft project starts when a project transitions from a Conceptual design phase into a Preliminary design phase.

As discussed, the Conceptual phase is continuous at airliner OEMs, exploring new technologies and configurations against market opportunities defined by the Marketing and Sales departments.

The actual aircraft program starts when Conceptual design morphs into Preliminary design. The Company management has decided there is a market need that has a size that motivates the high workhour content and thus the cost of developing a clean sheet new airliner.

The long life of a new airliner

As before, let’s assume the project is about developing the next-generation airliner for the heart of the market segment. We want the development program to cut the typical seven to eight years to EIS (Entry Into Service). But we also understand that the work result shall develop a new aircraft family that will have an operational life of at least 50 years.

The design work has to take the 50 years of operational life into account; thus, designing with flexibility for changes and upgrades to the airplane for capacity, engines, and aerodynamics is important. Shall we count on one wing size for all variants and all years, or shall there be, e.g., two wing areas and/or spans? Should the supplier teaming and production follow previous practices, or should there be changes? These are studies in Preliminary design that have to “get it right.”

In Preliminary design, you take the most promising concepts and ideas from Conceptual design and detail them further. To do it, a number of tools are used.  The initial tool is similar to our Aircraft Performance and Cost model, APCM. It has a geometry section that feeds the Aerodynamic drag section, the Weight estimation section, the Cabin overall configuration part, and the Production cost part. The geometry section can use the modern generation of 3D design suites, like Dassault 3DEXPERIENCE, to capture the geometry in digital 3D solids format from stage one.

The output feeds overall dimensions, wetted areas, etc., into a first principles calculus model. It will generate preliminary performance, weight, and cost data. The results will later be further refined with detailed tools for aerodynamics like CFD (Computer Fluid Dynamics) and for the structure, FEM (Finite Element Model) structural analysis to get a better estimate of drag values, skin geometry, and weight. These are either part of the overall 3D design suite or can work seamlessly with the suite. All data from the Preliminary design tools are stored in a central data repository, often the data management part of the 3D design suite.

For engines, the OEM will ask for data from engine OEMs for engine candidates but most likely also use engine modeling tools like GasTurb to get thrust, fuel consumption, weight, and size estimates for engine iterations they do without going the engine OEM loop.

The process is iterative, comparing how, for instance, two candidate designs downselected from the Conceptual phase will pan out. At some stage, one design will be chosen and refined further.

Now, the different parts of the aircraft are analyzed deeper. The overall shape and aerodynamics are studied in more detailed CFD runs. Models of the aircraft are made to run in wind tunnels as low-speed aerodynamics are difficult to predict accurately with CFD.

The weight estimation and production cost analysis are important as any weight creep will affect promised performance to launch customers, and the prices of the aircraft are set after production cost analysis has projected the cost learning curve for the aircraft.


When program and company management are satisfied that the aircraft’s aero and weight data is solid, the engine OEM’s data is reliable, and production cost analysis has matured, the aircraft can be formally launched.

It means the Sales teams get the authority to give binding offers to customers. Typically, one large customer is presented as the launch customer when the formal launch is publically announced, together with preliminary data for the aircraft.

All data for the aircraft are now detailed to the level where it can give solid input for the detailed design of the aircraft. When a sufficiently stable detailed level is reached, the part of the aircraft design is frozen so that detailed design doesn’t have to chase overall design changes, which means the parts go through constant redesign.

Efficiency improvement potential

The Preliminary design phase is probably the part of the program that is hardest to make big gains regarding work hours and calendar time used for the phase. There is a limited amount of people involved, and the tools define the aircraft on an overall level, like:

  • The overall shape of the aircraft and its aerodynamic surfaces.
  • The forces that are created from the aerodynamic shapes, engines, and landing gear during type missions.
  • The airframe structural integrity needed to handle these forces.
  • Weight estimates based on the shapes, volumes, and forces.
  • The performance specifications for the aircraft system based on the above and other data. For instance, the actuation capacity for the flight controls is given from the aero data, which decides the hydraulic system’s overall sizing. The environmental and mission specifications set the load together with the different cabin configurations for the ECS (Environmental Control System).
  • The aero, weight, and system data define the engines and the APU performance-wise. Power and bleed offtakes together with thrust needed at different points in the mission size the engines.

One area that will have an important efficiency effect downstream in the project is how design specifications are created from all the above activities. Traditionally, it has been written text reports and specifications.

Such documents have several problems:

  • What is often a mathematically defined output from an analysis activity is translated into words by a person. This translation is not 1:1 accurate. Then, this text document is read by, for instance, the subsupplier that shall supply the ECS system. Here, we have a second interpretation hazard, where language understanding can play a role.
  • The above is the reason that specifications, as far as possible, are replaced with mathematical models, as these diminish the scope for misinterpretation. These are harder to create as they require the creator to be exact and 100% logical, but then the models can serve as input to a system model the supplier creates for the ECS. It can also serve as the control input for a design review to check that the offered system meets the specification model.

Another change is the elimination of distributed paper or electronic documents (PDFs), as these have an eternal revision problem. Instead, the models and any data or written information needed in addition are stored on collaboration systems as living items, with strict version and revision control and real-time access for all in the project. Ideally, this shall be part of or integrated with the company’s 3D design suite. It’s an important part of a paperless project, where all information is stored in an online information system with strong workflow and revision control.

A third action is to compose cross-functional teams when downstream specification and instruction work is done to make sure detailed design, system, production, and support aspects are covered in the information that gets created.


The major gains from improvements in the Preliminary design phase will not be a faster phase but a more accurate, unambiguous, and up-to-date information set to the thousands of users of the data from Preliminary design. The cross-functional teaming also makes sure the information is complete, covering all aspects of how to create an aircraft project for its lifecycle.

The early capability to capture design information and specifications in an integrated digital design suite like the Daussuakt 3DEXPERIENCE will give gains for the rest of the project.


15 Comments on “Bjorn’s Corner: New aircraft technologies. Part 26. Preliminary design

  1. You also need provisions for form and size tolerances, added materials for reparability like provisions for oversize rivets and blends. There is a large amount of brackets that often is not modelled in preliminary design that add cost and mass that I think traditionally has been added as a % on component mass and cost. There is a danger getting stuck too long in preliminary design like on the A380 as technology evolves that you might miss (a whole engine generation) or make the wrong selection GLARE instead of carbon…

  2. This is a really nice, informative look behind the scenes for amateurs like me.
    Thank you, Bjorn Fehrm.

  3. Thank you so much, for this comprehensive overview, for what is possibly, one of the most difficult things Humans can build.

    I imagine that, for an engineer, it must be thrilling to be part of a team, that will design, a brand new aircraft, using new technology, and focusing in the future.

    This is a very complex task, with a lot of unknowns. You are designing today, for a future that no one can predict, or understand.

  4. Program Schedule

    I understand the economics, and pressures, behind trying to design and build fast, but for the last five decades, some the most successful Aircrafts, were designed with passion, creativity, ingenuity, and safety in mind. I do not believe time was paramount.

    Some of the programs took longer, other programs took less time, but what they all had in common was professionalism, responsibility, and love for what they were creating.

  5. Its when you get details like Bjorn presents you realize how complex an aircraft build is and why few can do it.

    Mitsubishi is one that comes to mind that failed at it despite extensive corporate resources (clearly the drain and any return was no sustainable with US Scope clauses requiring a re-design. )

    BBD really failed as well though they got a complete aircraft over the line, they could not sustain it into production.

    Its amazing any new aircraft get built.

    • Normally you design something that replaces an existing aircraft, like the 757 replacing the 727. The A330 replacing the A300/767 and the A380 intended to replace the 747-400.

  6. When Boeing designed the B-747 it was a revolutionary new aircraft
    There was no other previous aircraft to look for inspiration
    The B-747 was the first wide body aircraft ever build

    The B-767 was the first, two crew cockpit, wide body aircraft, to cross the Atlantic with two engines

    When Great Britain and France decided to build the Concorde, it was a new clean sheet. From the airframe, to the wings, engines, and avionics, all was new.

    When Airbus designed the A-320 family, they created a lot of new engineering from zero, like the incorporation of new materials never used before in aviation, or the revolutionary new way of flying, using digital fly-by-wire, and side stick control

    It must have been very exiting, to be part of the engineers that made the new possible

    • Wasn’t the A300 the first, two crew cockpit, wide body aircraft, to cross the Atlantic with two engines?

      • Tintin_in_France
        (What a nice Nickname!)

        I am sorry, I was wrong…
        You are completely right!

        Airbus has the merit of designing, and building, the first wide-body, twin engine, that entered service in (1974)
        And then, ETOPS certified (in 1977)

        Passengers loved that plane!

      • Original A300 was a 3 crew wide body short/medium range carrier.
        It eveloved into a 2 crew (A300-600) but (as far as I remember) never crossed the Atlantic as a passenger version (whereas A310 did it)

        • Transatlantic A300 passenger flights:
          LH: DUS-JFK, FRA-JFK
          Iberia: MAD-JFK

  7. Long haul, wide body Aircraft

    From an Airline perspective, the distance between exit doors 1L/R, and 2L/R is very important. This issue may seem trivial, but for a legacy carrier this is paramount.

    First, and Business Class passengers, board and disembark the aircraft, through exit door 1L, and Economy Class passengers through exit door 2L.

    Airlines like to have all Business Class seats between exit doors 1L/R and 2L/R. In some type of aircrafts, this cabin is not big enough to seat some 48 passengers, and airlines are forced to spread, the Business Class seats, in two cabins (passed exit door 2L/R).

    This is confusing for some passengers, and logistically more complex for the crew, to organise a great service. Passengers like to fly in a differentiated cabin, separated from the Economy Class cabin, by toilets, and galleys, and not by a curtain, because of noise, and privacy.

  8. A reason for which nowadays developpement are more costly and take more time is the exponantial requirements for specification requirements validation activities (i.e. proving that each requirement is correctly libelled, traced, achievable…).
    It may seem simple at a first look but this key activity is terribly time and people consuming since the one how wrtite the specification should not be the one that validate it, who should not be the one who cross check… Not to mention when you have several project layers from customer to sub-sub-supplier…

  9. I agree with Bjorn conclusion: from my experience, all the project where management imposed shorter / lower cost preliminary design phase took a severe back-fire afterwards for missing key activities.
    It is a project phase where it is critical to take into account every aspect of the design (safety, functions, performances, costs…) but also industrial and operationnal impatcs.
    APQP standard even tends to increase preliminary design phases compared to the following to allow a better anticipation of the needs.

  10. Airline Shrinking Toilets:

    Some Airlines, trying to squeeze more seats on the planes, have opted to install small toilets in some aircrafts. This is a main concern for all passengers, but specially for elders, passengers with reduced mobility, and of course pregnant women

Leave a Reply

Your email address will not be published. Required fields are marked *