By Bjorn Fehrm

September 8, 2023, ©. Leeham News: We described the Preliminary design phase of an airliner development program over the last weeks. Now our project is transitioning into Detailed design.

It’s the most challenging part of the project as we now go from perhaps a thousand people involved at the OEM into tens of thousands and even more people at consultancies and suppliers.

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

Detail design

The Preliminary design phase sets the project up for its most demanding phase, Detailed design.

We wrote that an airliner typically has three to four million parts. While many of these are standard parts or minor variations thereof (rivets, fasteners, bolts, nuts, pipes, connectors, cables, chips and components in systems and avionics..) a million at least are custom parts. All these have to be designed to the precise requirements of the aircraft.

This is a HUGHE task, that engages not only most of the engineers of an OEM but the OEM will also contract consultancy companies to do part of the OEM design tasks. A large portion of the OEM’s organization will prepare the contracts, information flow, and rules on how to manage sub-suppliers and how to involve and govern these in the project.

The importance of a stable design

We wrote about the way a Preliminary design phase can be more precise in the definition of an aircraft and leave a minimum to interpretation. Mathematical models as specifications of functions are preferable over written specifications. Ideally, large parts of the aircraft exist in interacting mathematical models in a so-called Digital Twin.

Equally important is that the state of the overall design of the aircraft stays fixed. Let’s take an example;

• If the aerodynamic design team discovers that the authority of the ailerons needs to increase, and it affects the size of the ailerons in the wing, the volume and forces for the actuators in the aileron area and the sizing of the hydraulic system change.
• The changes ripple through the aileron desing team, the wing design team, the stress department, the hydraulic system supplier, the actuator supplier (if different), and also the flight control team with suppliers.
• All these deliver new information to the aircraft mass team, which now calculates an increased mass; the engine team, which sees increased power offtake to drive hydraulics; the flight dynamics team, which now has to recalculate many flight dynamics cases and the performance team that sees the aircraft mass increase, which means deteriorating performance.

The increase of the authority of the aileron could stem from the aircraft mass increasing and thus its inertia in the roll axis but also from a changed requirement for allowable crosswind during hot and high takeoff and landings.

Whatever the cause, it will send a shockwave of changes through the organization, which in turn increases the mass, which in turn increases the engine thrust demand, which in turn increases the engine size, which in turn beefs up the pylon, which in turn increases the powerplant stress on the wing, which in turn increases the wing mass, which in turn….

This chain reaction is called the aircraft project’s spiral factor. An increase of one kilo of mass somewhere leads to an increase of the aircraft’s empty weight by two kilos, which can result in an increase of the Maximum Takeoff Weight by three kilos to keep the range unchanged.

Each project and project part has its spiral factors; the above is made for illustration. I have been in projects with spiral factors above three.

Configuration management

The above illustrates a couple of things around Detailed design:

• It’s extremely dependent on a very good and stable definition of the aircraft from Preliminary design. The lower number of changes and the magnitude of changes will contribute to a more successful Detailed design phase.
• It’s very important for the project that project management has a tight grip on Configuration management. It means that there will only be an approved change to the aileron authority after all consequences are worked through and that a change is the best solution to the problem. It could also be that a pushback on the hot and high crosswind spec. is better. All trades must be made before something changes.
• When there is a change, it’s communicated to all involved in a uniform and consistent way, and every step in the change process is thoroughly documented, including all the new specifications needed for the new state of this part of the project. If project parts start to doubt what the present state and revision of specifications they interface with are, the project is in trouble!

There are more things that need discussion around Detailed design. We will continue the discussion next week and then also discuss what changes in the traditional way Detailed design has been made could lead to faster and more efficient projects.