Aircraft that were designed in the past century were expected to have a service life of 20 to 30 years; however, many aircraft are being flown longer or upgraded for life extension.

One issue that fleet owners face for their older aircraft is obsolescence of avionics systems. This article presents different strategies for addressing this growing problem, including replacement of obsolete line replaceable units (LRUs) using commercial off-the-shelf (COTS) products.

Using Custom COTS for Managing Obsolescence

All aircraft are sold with an estimated lifespan of service. Usually, it is anticipated that under normal usage, you would expect to get a number of years, e.g., 20, out of the aircraft before it needs replacing. An aircraft’s various onboard systems have supply and maintenance contracts designed with this lifespan in mind; however, many aircraft end up in service for longer than originally anticipated, which can create part obsolescence issues. It gets harder to find a way to replace or repair a part as time goes on and aircraft can be grounded if a necessary part fails and there is no replacement.

Extending Aircraft System Life

Some systems undergo significant technological upgrades, like flight data recorders moving from magnetic tape to solid-state recording.

During the design verification of a new aircraft type, a design life is estimated and assigned to all aircraft built to the type design. Traditionally, an aircraft is expected to be retired when the design life is reached. In recent decades, fleet operators have increasingly sought ways of extending the useful life of older aircraft to keep yielding economic benefit from them before needing to purchase a new fleet. Unfortunately, this means fleet operators globally are now faced with challenges associated with aging aircraft. In fact, the UK Ministry of Defence (MoD) classified 75% of their fleets as “aging.”1

Whether civilian or military, aging aircraft often require a range of structural and operational corrective actions to ensure continued airworthiness. A potential point of weakness lies within the avionics subsystems. These would have been designed with maintenance support contracts to meet the aircraft’s design life. Failure of one of these systems may ground the aircraft by removing a mission-critical function or by violating (or not being on) the aircraft’s minimum equipment list (MEL). Many of these systems can be difficult and expensive to replace or repair and since they are often very specific to the aircraft, modern COTS replacements may not be available.

Approaches to Managing Avionics Subsystem Obsolescence

A modular FDAU typically contains power, control, and acquisition modules to meet custom requirements with off-the-shelf capabilities.

Although it is understood that it is difficult to plan how long aircraft will remain in service and to estimate the failure rate of systems, the ideal solution to obsolescence is to have a sufficient supply of spares to cover the needs of the aircraft over its lifetime. This may include monitoring the reliability of various systems and taking advantage of any “last-time buy” options. Proactive obsolescence management programs are invaluable but even the best ones can, justifiably, not foresee extended aircraft use or unseen component failures.

One solution will be to look to the original equipment manufacturer (OEM) to see if they are able to repair the affected items under a new maintenance contract. An OEM may be happy in principle to do this but may be unable to if one or more critical components are themselves obsolete. Even if it is possible, the cost to redesign and requalify a unit using different parts, assuming fit and function variants exist, can be prohibitive.

Another common scenario is that an OEM may, over time, lack the expertise and resources to perform a repair. It is not uncommon for organizations to change focus over the years. An organization that specialized in hardware manufacturing could, for example, have moved away from hardware to purely providing data analysis services. This is also the case if the organization has been acquired and refocused or in extreme cases, has gone out of business altogether.

Similarly, one can look to other suppliers to see if anyone else can repair the item. It is possible the OEM has sold the rights of the item to a third party. This third party may provide a solution, although all the same issues that apply to the OEM may also apply to the third party. Assuming the item can be fixed, either by this or another third party, then the limiting issue is most likely to be the high cost, especially considering there is a risk of something else failing on a very old item.

Once repairs have been ruled out, it may also be possible to buy secondhand units. These may be sourced from resellers that make it their business to obtain stock from retired aircraft or participate in last-time buys themselves. This can be an ideal option, although there is a risk these secondhand items may encounter obsolescence problems in the near term. It is also possible to purchase secondhand aircraft directly rather than use a reseller. This is perhaps only economical if several different parts are being sourced.

The final option is to source a modern replacement. This has the advantage of removing additional concerns over obsolescence and may even offer desirable improvements like lower weight, smaller size, increased data quality, and additional functionality or features. One example is a move from magnetic tape to solid-state recording media that greatly simplifies data transfer.

The biggest problem is that custom avionics are expensive to design. For newer aircraft, this is a cost that is usually spread over a big production run. Unfortunately, this is rarely possible with aging fleets, as the numbers required are generally low. This can make this approach prohibitively expensive unless there is a COTS solution that caters to low production runs.

COTS Solutions

The flight test instrumentation (FTI) industry uses COTS extensively to rapidly build custom data acquisition, processing, and recording hardware. Hardware originally designed for this purpose is now commonly used in a variety of other aerospace instrumentation applications including space operational flight instrumentation, aircraft/system monitoring, flight data acquisition, remote interfacing, and data recording.

These solutions often offer a modular approach with interchangeable data acquisition modules that can interface with aircraft sensors and busses. These modules can be incorporated into a custom LRU that closely or exactly matches the functionality of the obsolete one. A good example of this is when a flight data acquisition unit (FDAU) is used with a flight data recorder (or black box). Rather than create a new custom unit from scratch, existing modules can be selected to quickly meet the I/O functionality needed.

Reverse Engineering Process

An existing COTS solution was quickly reformed to meet a specific customer form factor.

Replacing an obsolete LRU is not an easy task. A preliminary investigation stage will be needed to review all the available documentation including:

  • Interface control documents (ICDs)

  • Functional test reports

  • Environmental test reports

  • Aircraft installation diagrams

  • Aircraft wiring diagrams

A prototype can quickly be produced and tested by the customer to ensure that it functions correctly. A full set of verification tests will be conducted to show that all requirements are met. This approach ensures the customer is involved in the process from the start right up to final production deliveries.

Example Case Study

A customer approached Curtiss-Wright to provide an LRU to address obsolescence in its ARINC-717 FDAU where the OEM no longer supported the equipment. Standard COTS Curtiss-Wright data acquisition modules were integrated into an ARINC-600 chassis, as shown below (external panels have been removed).

The modular construction of the FDAU was configured to closely match the existing equipment with minimal non-recurring work and thus a short lead time. The result was a direct replacement to the existing unit with no rewiring of the aircraft, saving considerable cost and time.

This case study is a prime example of how a replacement unit can quickly be produced. A quick prototype can be built and used by the customer to confirm correct functionality with existing systems, saving years of development time.


Parts obsolescence is inevitable in the aerospace industry, especially in electronic equipment. The lifecycle of an aircraft is much longer than the availability of a significant number of electronics parts and often longer than contractually planned. This can eventually lead to challenges repairing or replacing aircraft parts, with serious consequences for the fleet.

There are several approaches to try to mitigate this obsolescence including last-time buys, repairs, and sourcing secondhand parts. While one of these approaches may work, eventually it is often necessary to use a modern replacement. This can be prohibitively expensive unless a COTS solution can be found that makes producing units in low numbers economical.


  1. Reed, S. (2010), “The role of R&D in support of the UK MOD’s fleet of ageing aircraft”, AA&S 2010, May 2010.

This article was written by Michael Doherty, Programs Manager, and Stephen Willis, Marketing Portfolio Manager, at Curtiss-Wright Defense Solutions, Ashburn, VA. For more information, visit here .