January 23, 2026, ©. 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 have looked at the preparation work around entry into service; now we talk about post-certification work and the support of the new airliner generating revenue flights for the airline customer.

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

            ** Special thanks to Andrew Telesca for helping with this article **

Post Certification / Sustaining Phase

After getting the type certification, the program transitions into the Post Certification or Sustaining Phase.  This phase could last for decades.  We now go into some of the activities for this phase.

Adding New Interior and New Features

During this phase, the OEM needs to develop various Layouts of Passenger Accommodations (LOPA) for its customers.  Different airlines have different needs for the type of seats, the number of seats, galley configurations, etc.  These may sound trivial but, for example, changing the number of seats would require adding/removing passenger service units, in-flight entertainment equipment, emergency equipment, the aircraft center of gravity, etc.  This may deviate from the original type certification configuration and require the OEM to add new data to the type certificate so that the OEM can install these equipment on the final assembly line.

The OEM can also introduce new features to the aircraft.  For example, it can add features to the avionics suite.  It can also introduce new SATCOM to offer higher bandwidth.  Similarly, the OEM needs to add new data to the type certification data to enable the installation of these equipment at the final line.  There may also be opportunities for incremental performance improvements (such as engine efficiency upgrades) or cost reductions (perhaps changing to a new supplier). However, unless the change has “no appreciable effect on the weight, balance, structural strength, reliability, operational characteristics, or other characteristics affecting the airworthiness of the product,” additional certification effort will be required.

Maintainability, Reliability, and Safety Improvements

Customers always want a reliable and easily maintainable aircraft.  If an aircraft is not flying, it is not generating revenue.  A reliable and maintainable aircraft minimizes downtime, which helps maximize the airline’s return from the asset.

An aircraft OEM regularly receives data on component failure.  By aggregating this information, the company has an idea of which components are failing and their impact.  The OEM can then prioritize the required improvements to get the most out of a finite budget.  An OEM also periodically runs conferences with airlines to receive feedback and obtain buy-in for the proposed improvement list. This failure data also supports decisions on spares production rates and profit margins.

We introduced the Maintenance Review Board in Part 14 of this series.  Back then, the OEM executed activities related to initial type certification.  The OEM continues the Maintenance Review Board, using the Maintenance Steering Group-3 process, to continuously improve maintenance intervals of components covered by this process based on actual fleet data.

As well, an FAA Part 25 Type Certificate holder has certain obligations to follow the FAA Part 26,  Continued Airworthiness and Safety Improvements for Transport Category Airplanes, to address topics such as aging aircraft safety.

Obsolesce

Just like some of the electronics at home, components on an aircraft can also be obsolete.  Think about computer chips.  Over the life of an aircraft program, many computer chips would have come and gone.  Unless the supplier of the system bulk buys enough chips to last the life of the program (not just for new aircraft but for repair/replacement as well), some changes due to obsolescence will be required.  Under normal circumstances, a system supplier would notify the OEM about an upcoming obsolescence and work together to make necessary design changes to not impact aircraft production.

Airworthiness Directives & Continued Airworthiness

It is inevitable for an aircraft to find issues in service over time, either due to deficiencies in design, manufacturing, or supplied parts. This is why, under FAA 14 CFR 21.3, operators are required to report in-service issues. Prior to type certification, the OEM will have gained agreement with the regulator on a continued airworthiness plan stating how they will receive and manage these reports, typically through implementation of a failure reporting and corrective action system (FRACAS).

Not all issues are of equal severity, so this system allows a range of responses. Sometimes, in-service issues are anticipated wear or failures with no need for further action once the root cause is determined. Other times, minor issues are found that result in a service letter or bulletin from the OEM to operators letting them know about recommended part swaps or additional inspections. However, if the issue is serious enough, it can result in the regulator issuing an Airworthiness Directive (AD) to all operators of the aircraft type, requiring a mandatory safety action.

ADs can have a major impact on both the OEM and the airlines. For example, after the lithium-ion battery fire on the 787 in 2013, the FAA issued an AD that grounded the entire fleet for more than three months. When a report identifies a potentially unsafe condition, the OEM and regulator work together to look at the severity of the hazard, likelihood of additional failures, number of aircraft in service, and aircraft utilization rates to calculate a response time. Sometimes the result is an immediate grounding, while other times the airlines may be able to continue operating for months or years, incorporating the required corrective action during another scheduled maintenance activity. However, if the airline fails to incorporate the AD the aircraft will eventually no longer be considered airworthy, and if the airline continues to operate beyond that point can face significant fines or other consequences from the regulator.

Product Change Rules

We’ve mentioned a couple of times the role that certification plays in the cost of post-TC changes to the aircraft. The great majority of the time, these changes will be “non-significant” changes. This means that the regulations that were applicable at the time of TC continue to be applicable to the revised design, preventing a ripple effect that could cause large-scale changes if a high-impact new regulation has been released. For example, when the Electrical Wiring Interconnect System (EWIS) regulations were released, it did not mean that every equipment change that impacted wiring suddenly required revised harnessing and separation standards for the entire aircraft.

However, sometimes, either through accumulated small changes or one larger change, a project is deemed “significant.” This means that the change, and everything impacted by the change, must meet the latest regulations unless an exception can be negotiated (typically based on an argument that meeting the latest requirements would not materially improve safety). FAA AC 21.101-1B gives many examples of significant changes, but they include considerations like a >10% weight or thrust increase. We often associate significant changes with the release of a new minor model, such as a 787-8 turning into a 787-9.

Challenges for Startups

By the time a startup reaches this stage, it is no longer a startup.  However, there are still challenges for new entrants.  From past experiences, some companies believe that their engineering work is completed when a type certificate is issued.  This can’t be further from the truth.  As mentioned above, engineering effort is required to certify new LOPAs, new features, or improvements.  Technical teams still need to conduct root cause analyses and identify fixes for certain incidents.  The OEM needs to have sufficient engineering talents and the type design knowledge to ensure that the fleet stays airworthy.