By Bjorn Fehrm and Henry Tam

October 10, 2025, ©. Leeham News: We do a series about ideas on how the long development times for large airliners can be shortened. New projects talk about cutting development time and reaching certification and production faster than previous projects.

The series will discuss the typical development cycles for an FAA Part 25 aircraft, called a transport category aircraft, and what different ideas there are to reduce the development times.

We will use the Gantt plan in Figure 1 as a base for our discussions.

Figure 1. A generic new Part 25 airliner development plan. Source: Leeham Co. Click to see better.

Interior Preliminary Design

As mentioned in Part 7 of this series, the interior is where the OEM and its customers (i.e., airlines) generate brand loyalty.  Let’s delve deeper into the activities of the interior team during the Preliminary Design phase.

Prior to Preliminary Design

Over the past two phases, the interior team has worked with industrial design to come up with a concept of the cabin.  They might have built engineering mockups (virtual or physical) to assess headroom, overwing emergency exit location, galley workflow, lighting, etc.  Suppliers could also be involved in translating concepts into potential offerings.

Similar to system suppliers, interior suppliers could offer new, modified, or existing products to an OEM.  Unless the OEM is looking at commodities such as galley inserts (e.g., coffee maker, oven, trolleys, etc.) and oxygen masks, some customization is required.

Figure 2. The preliminary design of the Spacejet interior with pivot bins. Source: Mitsubishi.

Detailing Requirements

One of the key activities during this phase is to define the certification basis. The team has to know not only applicable rules but also acceptable ways to demonstrate compliance with these rules.  For example, the FAA Part 25 Airworthiness Standards states:

• 25.795(d) Each chemical oxygen generator or its installation must be designed to be secure from deliberate manipulation by one of the following:

(1) By providing effective resistance to tampering,

(2) By providing an effective combination of resistance to tampering and active tamper-evident features,

(3) By installation in a location or manner whereby any attempt to access the generator would be immediately obvious, or

(4) By a combination of approaches specified in paragraphs (d)(1), (d)(2) and (d)(3) of this section that the Administrator finds provides a secure installation.

This text is somewhat vague.  Relevant guidance materials, such as Advisory Circular (AC) 25.795-9, provide additional explanations on how to comply with these rules.  For example:

A tamper-evidence system installed for compliance with § 25.795(d) is intended to notify crew members that someone is trying to gain access to a COG (Chemical Oxygen Generator, our comment). The system should provide aural and visual warnings to immediately notify crew members so they can provide direct response in a timely fashion. For example, visual indication should be provided so the crew can identify which COG location is being tampered with while performing their normal duties. Aural alerts should be distinct from other alerts and clearly audible to the crew members expected to respond to the alert If an alert is provided to the flight crew, the alert should be presented in accordance with § 25.1322.

And if the guidance is still insufficient, the team needs to make proposals to and negotiate with authorities to come up with an acceptable approach.  The team will then synthesize information from various sources to refine requirements.  Finalizing requirements is a crucial activity during this phase because having a concrete set of requirements will help minimize rework, making the development go smoother and faster.  We will go into more details on requirements definition from a certification standpoint in the next article.

Conducting Trade Studies

During this phase, the team needs to conduct many trade studies.  The chemical oxygen generator system is an excellent example.  Should the team select a mechanical tamper-proof design?  Should the team add sensors to detect tampering and then display the information on the cabin control display?

Or should the team include an alarm system on each chemical oxygen generator unit?  Each solution has its pros and cons in terms of weight, cost, complexity, and so on.  A tamper-proof system may need a stronger enclosure and a more sophisticated latch for the oxygen generator, which usually means extra weight and cost.  Adding a sensor to the enclosure for detection may seem lighter at first glance.

Yet, the team needs to consider the weight, cost, and complexity related to extra wires, brackets, electronics, and software.  In addition, other suppliers have to be involved, making the design more difficult to coordinate.  The team could also install some electronics and a speaker in each oxygen generator unit.  It is a self-contained solution, but still has weight, cost, and complexity impact.

Keep in mind, this is only one out of many trade studies that would be conducted during the Preliminary Design phase.  This is why a multi-disciplinary team is necessary to properly evaluate trade-offs, identifying the optimal design solution for the aircraft.

Integrating the Design

In parallel, engineers start to “install” interior monuments (such as galleys, overhead bins, dividers, etc.) into the digital 3D model.  These monuments are typically represented by design envelopes (boundaries without a lot of details inside) with external interfaces identified at this stage.  The model allows engineers to assess connections between different systems, such as potable water and a lavatory.  The model also enables engineers to resolve clashes, such as a door hinge interfering with a corner of a galley.

Preparing for Engineering Tests

The team also needs to start preparing for some engineering tests to be conducted in the next phase.  Although not intended to formally demonstrate compliance to regulatory authorities, these tests will add value to the program by derisking the design.

For instance, a seat supplier may design the main structure of the proposed seat during this phase.  A prototype seat based on this design will then be manufactured and tested against certification requirements, such as crashworthiness, early in the next phase.

The derisking exercise would give an indication of the suitability of the design.  If the prototype fails the test, the supplier still has time to update the design to ensure that the final product will comply with relevant rules.

Evaluating Manufacturability

Design engineers need to work closely with the manufacturing team as well.  They must consider the installation of interior monuments.  One of the considerations is to get these monuments through the aircraft doors.  For example, a completed lavatory may be too big to go through the door.  The design team needs to split up the lavatory to make installation possible.  However, splitting a monument typically has a weight impact.  The design team and the manufacturing team must work together to find the optimal solution.

Similarly, engineers need to ensure that there is enough space for mechanics to install monuments.  If a design requires an installer to use a wrench to turn a bolt, there needs to be sufficient clearance and space for this activity, Figure 3.

Figure 3. The installation of seats at the Boeing 737 Renton FAL. Source: Boeing.

This may sound obvious, but sometimes these details get overlooked.  This is why having manufacturing engineers involved during design has a profound impact on the end product.

Assessing Serviceability

Maintainability and accessibility are important considerations.  When the aircraft is on the production line, installers generally have reasonable access to many locations on the aircraft.

When maintenance technicians need to work on an aircraft in the field, they don’t have such a luxury.  If they need to take apart the interior to reach a problematic part, airline profitability could be affected because of departure delays, missed flight, or extra work hours.  As a result, maintainability and accessibility must be considered at this stage.

Managing Risk

Identifying risks and coming up with mitigation plans are important activities during this phase.  We already discussed how the team could derisk challenges associated with seat crashworthiness.  There are also other technical and programmatic risks the team needs to think through to ensure that a plan is in place when a risk materializes.

Speeding Up the Interior Preliminary Design

Managing the scope of the interior is critical to the success of the interior team.  Stakeholders often add scope to the project when they receive feedback.  Sometimes, feedback could help with the implementation.  Other times, it could require unnecessary change at this stage, adding more work to an already busy team.  The unplanned work could lead to delays for the interior team as well as other teams relying on the interior team’s output.

Separating research & technology projects from a development program can also reduce risk and help maintain the schedule.  Innovation, without a doubt, is critical for an OEM to successfully compete in the airliner market.  Yet, innovation needs to stay ahead of a development program so that it doesn’t hold up the program.  As an example, if the team needs to use an exotic material for seats, the material development project should be completed by now so that material properties and manufacturing processes are well documented.

The alternative is to minimize the dependency of new technologies.  The team could incorporate next-generation wireless communications technology into the aircraft.  However, if the technology development does not finish on time, the aircraft can still be delivered without installing the system, and it can be retrofitted to the in-service fleet at a later time.