By Bjorn Fehrm and Henry Tam

October 24, 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. Today’s topic, the Preliminary Design Reviews, PDRs, are marked in the chart.

Figure 1. A generic new Part 25 airliner development plan with PDR marked in time. Source: Leeham Co. Click to see better.

Preliminary Design Review, PDR

Preliminary Design Review (PDR) is a key review to support the Preliminary Design phase exit.  PDR for an aircraft development program is actually not a single event but a series of reviews to ensure that all work packages have reached a sufficient level of maturity before progressing onto the Detailed Design phase.

As mentioned previously (borrowed from NASA’s definition), the objective of the PDR is to demonstrate that the preliminary design meets all system requirements with acceptable risk and within the cost and schedule constraints, and establishes the basis for proceeding with detailed design.

It shows that the correct design options have been selected, interfaces have been identified, and verification methods have been described. The PDR should address and resolve critical, system-wide issues and show that work can begin on detailed design.

PDR Checklist

NASA’s PDR definition that we quoted in the last article is a good general definition.  Each company or organization has a slightly different PDR checklist tailored to its specific situation.  Some items depend on the business model, supply chain strategy, or program type.  Others depend on the company’s capabilities or lessons learned.

Here is a sample product/maturity list from the NASA Systems Engineering Handbook, Figure 2. The PDR is in Phase B, the fourth column. You have the PDR subjects in the rows and the required status in the PDR column cells:

Figure 2. NASA checklist for PDR. Source: NASA. Click to see better.

Separate PDRs roll up to Aircraft level PDR

Each structure, system, and interior work package needs to conduct its own PDR.  These PDR results then roll up to the aircraft-level PDR.  The aircraft-level PDR result, along with maturity review results from manufacturing, customer support, etc., rolls up to a program-level Preliminary Design phase exit review to evaluate the program’s readiness for the next phase.

Products (documents, datasets, etc.) are usually jointly generated by the aircraft OEM and suppliers.  These products are peer-reviewed to ensure that they have a sufficient level of maturity. To ensure independence, reviewers are often experts within the company but not a part of the program.  This can be a challenge for start-ups.

Let’s use the baselining of the interface definition on the NASA list in Figure 2 as a case study.  What does it mean in practice? 

If we use the fuselage as an example, the team would have worked with many structures, systems, and interior teams during the Preliminary Design phase to identify interfaces.  Examples of interfaces with fuselage include wing, empennage, windows, doors, hydraulics system (pumps, accumulators, controllers, penetrations, etc.), avionics (electronic boxes, antennas, etc.), landing gear system (nose wheel, controllers, etc.), EWIS (brackets, penetrations, etc.), insulation, side walls, cockpit door, overhead bins, and so on.

These teams would have agreed on relevant interfaces and released Interface Control Documents or Drawings.  Interfaces are then frozen so that all teams can start the detailed design work using the same set of assumptions.

Having a baseline does not mean things cannot be updated.  For instance, a six-inch hole in the fuselage through a pressure bulkhead needs to be fully analyzed and agreed upon during this phase.  Yet, a local quarter-inch hole entirely within the pressure vessel may be okay to finalize during the next phase.

Keep in mind, some flexibility is still needed to execute the design.  Going forward, the team needs to ensure that changes are carefully evaluated and executed.  Ideally, only minor interface updates are required during the next phase.

Sequencing of PDRs

Proper sequencing of PDRs is an important part of planning.  For example, the fuselage PDR needs to take place before the interior monument PDRs so that there is a defined volume and a set of attachment coordinates for interior components.

The program should have structures, systems, and interiors PDRs completed before having the aircraft-level PDR because the aircraft-level PDR requires data such as weight, drag, thermal, reliability, etc., from work packages to properly assess the maturity of the aircraft design.

The integration team also needs to ensure requirements are traceable from market level to systems (or even items) level and have a good understanding of the current requirements compliance status.

Clearity before the PDRs

Hopefully, all non-compliance items are discussed and negotiated prior to the PDR, not during it.  In addition, are there sufficient design margins left?  The team needs to ensure it will not require a miracle to meet contractual obligations.  Having negative margins at this stage is likely an issue, not a risk.  On the other hand, the team needs to make sure that the aircraft is not too over-designed, penalizing the aircraft with unnecessary weight and cost.

Risk management is also an important topic on the PDR agenda.  These risks can be technical or program risks.  For example, a new cabin window design may not comply with regulations due to a new, desirable marketing feature.  The team needs to identify the risk and come up with mitigation plans, such as having a cabin window configuration that does not have this feature.  Team members also need to know when they can gain more knowledge about the risk, retire the risk, and launch the mitigation plan.

Cost and Schedule reviews are critical

Cost and schedule are crucial topics as well.  Although the team has put together a plan for the program in previous phases, a phase exit is a good time to revisit the plan with respect to deliverables, resources, schedule, and cost.  Are we still within cost and schedule constraints?  If not, this is a good time to have some difficult conversations.  Cost will only increase after this point because of staffing, parts manufacturing, testing, and infrastructure commissioning.

At the end of each PDR, the work package and its supplier usually have a list of actions based on the discussions during the review.  These actions could include finishing the release of datasets within a timeframe or updating the presentation/document/analysis based on discussions during the meeting.  If the team has properly prepared for the review, there should not be any major surprises.