By Bjorn Fehrm and Henry Tam

November 14, 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. We are in the Detailed Design phase and working with the Certification Compliance plan.

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

Detailed Design – Compliance Planning

In our previous article, we looked at some of the technical aspects of ensuring we’re designing for compliance during the detailed design phase. We’ll take a closer look at why developing the detailed design for a certified product takes much longer than for a prototype, and at some of the procedural work we need to complete in this phase.

Why do new companies prototype so much faster than incumbents?

Something that has become very apparent in the industry is that many newcomers promise aircraft development timelines that are much faster than what’s seen by incumbents. At first (once they have funding), they seem to deliver on this promise – flights with new technologies or significant airframe modifications often happen in months instead of years.

Then things stall out. Too often, this is blamed on the regulators – they are a convenient wall to hide behind since they usually don’t publicly comment on specific applicant programs. Behind the scenes, these companies are realizing that a flyable product is a much lower bar than a certifiable product.

At the same time, incumbents often have design practices that prevent them from moving forward with design details that cannot be certified. This saves the cost of following a dead end in the design, but it often means going slower than a startup.

Flyable vs. Certifiable

Let’s look at a few differences in completing the detailed design of a prototype vs. a product.:

Safety Architecture

Most companies will not want to fly a piloted aircraft where it’s known that a single failure (such as loss of a flight control computer or engine control system) would result in loss of the aircraft. However, add a backup system and we’re now in the realm of acceptable risk.

In contrast, a certifiable design (especially in Part 25) will often require triplex redundancy – and dissimilarity. Why? Sometimes, to maintain availability at a once per billion flight hours rate, two systems aren’t enough. More often, because if you have two systems providing different data or different commands, which one do you trust? A third system can serve as a tie breaker to ensure the integrity of the system if one source goes haywire.

Simple in concept, but now we need logic for system health/monitoring. We need voting algorithms, validity ranges, persistence criteria to avoid erroneous system shutdowns, and many other integration design decisions to ensure three systems can work together in concert to provide the same function. This dramatically increases the detailed design effort.

Structural Margins & Optimization

Prototypes don’t have large payloads. Compliance requirements mean allowable loads knockdowns for temperature, damage, and life. Optimizing the structure to minimize weight while meeting the regulations on a Part 25 aircraft requires hundreds of thousands of hours of analysis and often materials-level testing to maximize credited strength characteristics.

A prototype with no passenger load need not have completed more than the most basic evaluations, and when we consider the “first flight” startup designs seen in recent years, they’re usually simply overbuilt in a way the incumbents would never accept. However, in a real product that sub-optimized weight can face a net present value reduction in the billions due to increased fuel costs and reduced payloads.

Verification Depth

We can fly a prototype safely so long as we’ve verified the functions we will use and the environments we will be exposed to. This may mean a very limited envelope (daytime, gentle temperatures, low altitudes, basic flight functions).

We can rely on having only the most highly trained pilots. If we have multiple flight modes or data sources, only one may have been demonstrated at the time of the first flight. The product life may be short.

For a certifiable product, everything on the aircraft must reliably perform its intended function throughout the most extreme environmental envelopes, and we need to prove it with data. Ensuring the design is ready for that challenge takes time. Even if the actual testing isn’t until the next phase, a certifiable design is expected to pass the test. This means each component of each piece of equipment going into the aircraft has been selected based on the full requirements it will have to meet. By the way, procuring such high-quality components can take a very long time.

Decision Making

A flyable design can be signed off by a chief engineer. On the scale of a Part 25 project, this is likely a safety review board including key engineers from multiple different functional areas, such as structures, systems, and propulsion. In contrast, certification is a consensus process.

For the detailed design to be completed and certification ready, it still needs the above approvals, as well as internal compliance team members, manufacturing personnel (manufacturability, production cost), quality and process assurance personnel, and the regulatory delegates (and/or the regulators themselves). Disagreement in any function can slow down or derail the design process.

Since we’re designing a certifiable product, we have to take the harder path on each of these (and many more) topics. If we want to reach the end, that means we’re going to need documented decisions (agreements) and very careful planning. A more flexible decision process that leverages incumbent know-how and startup fail-fast learning could provide the best of both worlds – but someone would have to take responsibility for the risk of failure.

Certification and Verification Planning

To ensure we have the best possible clarity on the certifiability of the detailed design, and in order to be prepared for the next phase of the program, by the time we exit the detailed design phase we need complete certification plans laying out exactly which regulations apply to which portions of the design and, equally importantly, what we will have to do to show the regulations are met. The criticality of this last part is easily overlooked.

Let’s use a simple example we’ve seen misunderstood before:

Assume we have an aircraft that will be certified to operate with outside air temperatures (OAT) up to 40 °C. This doesn’t mean our equipment will be compliant if we test it in a lab and show it operates at 40 °C. Why? Because the verification method requires that we conduct a temperature survey on the aircraft and show the actual temperatures in the installed location, which may rise above 40 °C if there’s a lack of airflow or the equipment in that location has significant heat rejection. If we know in advance that this testing will be required, we will perform thermal analysis and/or build in a margin when completing our detailed design.

So, in this phase, we will create a set of detailed compliance and verification plans. This allows us to ensure we’ve got the right design, and simultaneously coordinate (and negotiate) with the regulators to ensure we have consensus on what will be required to complete the program.

A conceptual description of this type of document can be found in the FAA’s ePSCP guide: https://www.faa.gov/sites/faa.gov/files/aircraft/air_cert/design_approvals/dah/ePSCP_guide.pdf

In practice, for a new aircraft like ours, the certification plan will likely be broken into 30-100 individual documents for different portions of the design. Each one will identify the boundary of the design covered in the document, the applicable regulations from our preliminary design phase, the specific methods by which we will show each regulation is met for that design, and the list of documents (analysis reports, test reports, inspection reports, etc.) that will be provided to the regulator.

This typically results in more than 1,000 documents that need to be produced for regulator approval. By Critical Design Review, CDR, we should have had initial conversations with the regulator on each of these plans in order to identify any risks of disagreement on the planned activities. Internally, we need to have a plan to conduct each of these activities.

Some compliance demonstrations can take over a year to perform, and cost hundreds of millions to build and to run. Each activity needs ownership, schedules, budgets, and stakeholder engagement to successfully complete the project. We cannot exit detailed design without a detailed understanding of what comes next.

Foreign Authorities

It is worth noting that there will be one additional layer of complexity to our consensus decision-making process. Foreign regulatory authorities for the countries we want to sell in.

In our conceptual design articles, we noted that each regulator may have Significant Standards Differences, Safety Emphasis Items, or other regulations that impose unique requirements or require special demonstrations. To prepare for a fast ramp-up and entry into international markets, these additional requirements must also be accounted for in the detailed design, and, for the most critical items, agreements should be reached with the foreign regulators at this time.

Tools for Speeding Up

There’s no magic bullet that will get us through compliance planning cheaply and quickly. This takes legwork. It takes an integrated team with foresight, attention to detail, and the ability to build consensus across many stakeholders. However, there are a few things we can do to help right-size the effort and avoid waste.

• Iterative (progressive) verification planning – To fully be prepared for the next phase, a detailed plan for each verification activity is desirable. However, the time and cost to prepare those plans can be high. Consider a progressive approach to this planning. For Critical Design Review, CDR, we need to know the specifications of key test facilities, the number of pre-production builds we will need to complete, and the approximate amount of time the verification activities will take. Consider a multi-phase planning approach that specifies key resources and stakeholders first, specific test criteria second, and defers detailed-level planning until closer to execution. This path reduces uncertainty while balancing resource cost and rework risk.
• Templating & workflows – Sometimes it’s the old answers, not the new ones, that let us move quickly. Each design release and compliance plan has a cycle of development, review, and signature. To prepare for certification, each drawing needs dimensioning and tolerancing, configuration management reviews, and stress analysis. Tens of thousands of drawings, thousands of compliance documents. Up-front investment in workflows, templates, and a mindset that doesn’t pass defects or accumulate technical debt will prove invaluable. During prototype design, most startups see these as things that slow us down. For certification design, these are the tools that make us go fast.