By Bjorn Fehrm

October 20, 2023, ©. Leeham News: We are discussing the different design phases of an airliner development program. The typical phases and their time use and manning are described in the Gant chart in Figure 1.

After covering Conceptual, Preliminary, and Detailed design, we now discuss Prototype manufacturing and Testing.

Figure 1. The development plan for a new airliner. Source: Leeham Co.

Prototype manufacturing and testing

When Detailed design has progressed about halfway, we see the first parts and system components coming out of prototype manufacturing and going into testing.

It’s not as simple that “X” number of prototype aircraft are built and tested. Every major system component is built and tested individually first to see that its functional characteristics are as predicted by the modeling software in its digital twin.

First versions are often not final mass or functionality but are representative for key functions, and these can be tested and verified. Gradually, the system’s components converge to full specification and are built into system “iron-bird” test beds, Figure 2.

Figure 2. The hydraulics, electrical, and flight control system “Ïron Bird” of the A350, with the wing parts folded back to save space. Source: Airbus.

Historically, each major system, like Hydraulics, Electrical power generation, Flight control system, Landing gear, Fuel system, Bleed air system, Environmental Control System, APU system, and Avionics/Cockpit, had their iron birds and test environments.

More and more, these are then combined either physically or network-wise to work in an ever more complete and interconnected ground copy of the aircraft’s systems.

It allows the control software to test the normal behavior of the systems, but also the exhaustive testing of the failure cases that have been mapped and cataloged in the Failure Modes and Effects Analysis (FMEA). The failure mode testing can easily be several times more exhaustive than the normal mode testing.

Every possible reaction by the systems of a control software hang or fault, the non-response of a system component like an electrically controlled valve, etc., must be tested and verified so the aircraft cannot end up in a dangerous situation because of the malfunction.

The mass race

The building of prototypes of structural or system parts is the first chance to check the manufactured mass of the part. Weight creep, or more correctly, mass creep, is the ever-present enemy of an aircraft project.

When prototype components are built, be they structural or system pieces, they are carefully weighed to the last gram, and the mass and mass center of gravity are entered in the project’s mass control database.

Aircraft projects have a special mass control organization that shall at all times have 100% control of the present mass status of the aircraft per hardware revision of each part. Mass excess compared to predicted mass from Preliminary and Detailed design is legio in every phase of an aircraft project.

Several times during a project, weight reduction campaigns are started to get the aircraft back to what was defined in Preliminary design. Ultimately, a well-managed aircraft project ends up with about 3-4% total weight creep. Anything more, and Preliminary or Detailed designs have not done a good job.

Flight test safety

Before a flight test aircraft can take to the skies, it must have a flight test permit from the regulator of the project. Such a permit will only be issued when a number of agreed compliance reports are present, some with data from simulations and others from testing.

There are system functionality with failure mode tests and structural tests. We will go deeper into the project’s structural tests next week.