Chapter 28: Aerospace Applications |
A Brief Summary of Code 39 in the Aerospace Industry |
The aerospace industry represents perhaps the most demanding environment for any identification technology. Components must endure extreme temperature variations, high vibration, exposure to harsh chemicals, and the relentless passage of time. In this context, the reliability of a barcode is not merely a matter of convenience---it is a matter of safety and regulatory compliance. Code 39, with its self-checking structure and forgiving print tolerances, has found a natural home in aerospace manufacturing and maintenance. Boeing and Airbus, the two giants of commercial aviation, have long used Code 39 for part numbering on rivets, panels, and wiring harnesses, where durability and simplicity outweigh data density. This chapter explores why this decades-old symbology remains a trusted workhorse in the skies and on the assembly line, and how its technical characteristics have shaped its adoption across numerous other industries. |
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Introduction: The Unseen Language of Flight |
Every commercial airliner is a marvel of modern engineering, composed of millions of individual parts working in perfect harmony. From the smallest rivet holding a fuselage panel to the complex wiring harnesses that carry signals throughout the aircraft, each component must be traceable, accountable, and replaceable. This traceability is achieved through a universal language---the barcode. Among the various barcode symbologies available, Code 39, also known as Code 3 of 9, holds a special place in aerospace history and practice. |
The opening scene for this chapter is set on the factory floor of Boeing or Airbus. As a massive wing section moves down the assembly line, workers scan the barcodes on rivets, brackets, and hydraulic lines. These scans update inventory systems, verify that the correct parts have been installed, and create a permanent digital record of the aircraft's construction. In this environment, a barcode that fails to scan is more than an annoyance; it can halt production, compromise safety, or lead to costly rework. Code 39 rose to this challenge not because it is the newest or the most data-dense symbology, but because it is extraordinarily robust and forgiving. |
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The Technical Foundation of Code 39 |
To understand why Code 39 prevails in aerospace, we must first appreciate its technical design. Developed by Dr. David Allais and Ray Stevens of Intermec in 1974, Code 39 was the first barcode symbology capable of encoding both numbers and letters. This alphanumeric capability was revolutionary at the time, allowing part numbers, serial numbers, and other identifiers to be encoded directly without translation tables. |
The '3 of 9' Encoding Principle |
The name 'Code 39' derives from its encoding structure. Each character in the Code 39 symbology consists of nine elements: five bars and four spaces. Of these nine elements, exactly three are wide and six are narrow. The wide-to-narrow ratio is typically between 2.5:1 and 3:1. This design is what gives Code 39 its distinctive appearance and its robustness. Because the pattern of wide and narrow elements is so distinct for each character, even a poorly printed barcode can often be read successfully. The barcode reader looks for the presence of wide elements, and because there are always exactly three wide elements per character, the scanner can verify that it has correctly identified a character before moving on to the next. |
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Self-Checking and Error Resilience |
One of the most important features of Code 39 is that it is 'self-checking'. This means that a single printing defect---such as a bar that is slightly too wide or too narrow---cannot accidentally transform one valid character into another. If a scanner encounters a character that does not have exactly three wide elements, it knows an error has occurred and can reject the scan or request a re-read. This self-checking property is achieved without a mandatory check digit, though many implementations add an optional Modulo 43 check digit for an additional layer of security. In the aerospace industry, where mistakes can be catastrophic, this inherent error detection is invaluable. |
The Start and Stop Characters |
Every Code 39 barcode is bookended by an asterisk (*) character, which serves as the start and stop delimiter. This tells the scanner where the barcode begins and ends and, crucially, in which direction it should be read. The asymmetry of the Code 39 start and stop patterns allows the scanner to determine the orientation of the symbol, enabling omni-directional scanning. When you see a Code 39 barcode, the asterisks are often included in the human-readable text beneath the bars, though some implementations suppress them for aesthetic reasons. |
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Aerospace Applications: Where Durability Meets Simplicity |
The aerospace industry is the perfect crucible for testing the durability of Code 39. The environment in which an aircraft operates---from the freezing temperatures of high altitude to the heat of an engine nacelle, from the vibration of takeoff to the humidity of a tropical airport---is incredibly punishing. Barcodes applied to aircraft parts must survive all of this while remaining scannable for decades. |
Part Numbering on Rivets and Fasteners |
Consider the humble rivet. A single aircraft may contain hundreds of thousands of them, each with a specific part number indicating its size, material, and heat treatment. These rivets are often tiny, leaving little room for a large barcode. Yet, Code 39, with its relatively low data density, must be printed with sufficient size to be readable. In practice, aerospace manufacturers use specialized printing techniques, such as dot peen marking or laser etching, to apply Code 39 barcodes directly onto the surface of the rivet or fastener. The self-checking nature of Code 39 allows these marks to be somewhat imprecise---which they often are, given the curved surface of a rivet head---and still be decoded reliably. |
The use of Code 39 on rivets is a testament to its versatility. The symbology can be printed with a wide range of bar-to-space width ratios, making it adaptable to different marking methods. This flexibility is crucial when moving from a flat panel to a curved rivet head to a flexible wiring harness. |
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Panel Identification and Traceability |
Aircraft fuselage and wing panels are large, complex assemblies. Each panel carries a barcode that encodes the part number, serial number, assembly date, and other relevant data. These barcodes are typically printed on durable labels made from polyester or polyimide, which can withstand the harsh environment. Code 39 is often chosen for these labels because it can be printed with standard industrial printers and read with almost any type of barcode scanner, from handheld laser scanners to fixed-mount imagers. The simplicity of the symbology---no checksum calculation, no complex encoding rules---means that it is easy to integrate into legacy manufacturing systems and supply chain management software. |
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Wiring Harnesses and Electrical Systems |
The wiring harness of an aircraft is its nervous system, carrying power and data to every component. These harnesses can be miles long and contain hundreds of individual wires. Each wire must be identified with a part number, wire gauge, and destination. Code 39 is frequently used on wire labels, which are wrapped around the wire itself. The low density of Code 39 is actually an advantage here, as the bars are relatively wide and easy to read with a contact scanner, even in cramped and poorly lit conditions. The 'quiet zone' around the barcode---the blank margin that must be left clear to enable scanning---is also easier to maintain on a wire label than with a smaller, denser barcode. |
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The Choice for Aerospace: Why Not Code 128 |
Given that Code 128 is more data-dense and supports full ASCII natively, why do aerospace giants like Boeing and Airbus often stick with Code 39The answer lies in the concept of legacy infrastructure and the trade-off between density and ruggedness. In many cases, Code 39 is 'good enough' and its simplicity reduces the risk of scanning errors. |
Historical Adoption |
Code 39 was standardized by the U.S. Department of Defense under MIL-STD-1189 and later MIL-STD-130, which required the use of Code 39 for all government property marking. This led to widespread adoption throughout the defense and aerospace supply chain. Boeing and Airbus, as major defense contractors, inherited these standards and their supply chains became built around Code 39. Changing to a more modern symbology would require a massive, costly, and time-consuming update to every scanner, printer, and software system in the supply chain. |
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The Ruggedness Factor |
Code 39's wide bars and self-checking nature make it more forgiving of print defects than Code 128. In a factory environment where labels are exposed to grease, solvents, and physical abrasion, this ruggedness is paramount. A Code 39 barcode that has been partially scratched or scuffed may still be readable, whereas a Code 128 barcode, with its narrower bars and more complex encoding, might fail. |
International Standards |
The International Organization for Standardization (ISO) has standardized Code 39 under ISO/IEC 16388, ensuring that scanners and printers from different manufacturers can interoperate. This standardization gives aerospace companies confidence that they can source labels and equipment from a global supply chain without compatibility issues. |
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Code 39 Beyond Aerospace: A Survey of Industry Use Cases |
While aerospace represents a particularly demanding application, Code 39's influence extends far beyond the tarmac. Its combination of robustness, simplicity, and alphanumeric support has made it a standard in numerous industries. |
Military and Defense (LOGMARS) |
The U.S. Department of Defense's LOGMARS (Logistics Applications of Automated Marking and Reading Symbols) program was one of the earliest large-scale adopters of Code 39. LOGMARS required Code 39 barcodes on all supplies, equipment, and parts to streamline logistics and inventory management. This mandate cascaded down to thousands of defense contractors, creating a massive installed base of Code 39-compatible equipment. Even today, despite the availability of more advanced symbologies, Code 39 remains a requirement for many government contracts. |
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Automotive Industry (AIAG) |
The automotive industry, through the Automotive Industry Action Group (AIAG), adopted Code 39 for part identification and tracking. Similar to aerospace, automotive manufacturing involves a complex supply chain of parts from different suppliers. Code 39 barcodes on parts ensure that the correct components reach the assembly line and that quality control data can be traced back to the original manufacturer. |
Healthcare and the Health Industry Bar Code (HIBC) |
The Health Industry Bar Code (HIBC) standard, used for medical devices, pharmaceuticals, and hospital supplies, builds upon Code 39. In healthcare, the accuracy of barcode scanning can be a matter of life and death---administering the wrong medication or using the wrong medical device can have fatal consequences. Code 39's self-checking property provides an extra layer of safety, reducing the risk of misidentification. |
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Inventory Management and Asset Tracking |
For internal inventory management, asset tracking, and library management, Code 39 is often the default choice due to its universal scanner compatibility and lack of licensing fees. Because virtually any barcode scanner manufactured in the last thirty years can read Code 39, it is an ideal symbology for tracking assets across multiple departments or facilities. A Code 39 asset tag, for instance, might be used to label an IT laptop, a piece of medical equipment, or a library book. The barcode can encode a simple asset ID that combines letters and numbers (e.g., 'IT-LAP-00421') and can be printed on durable label stock for long-term use. |
Warehousing and Logistics |
In warehousing, Code 39 is used for inventory labels, shelf labels, and bin labels. Its wide bars make it easy to scan from a distance or with a handheld scanner, even in the dim lighting of a large warehouse. While Code 128 and GS1-128 are increasingly used for logistics, Code 39 remains a strong presence in many older or smaller-scale operations. |
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The Extended Code 39: Encodable Characters |
The base Code 39 symbology can only encode 43 characters: uppercase letters A-Z, digits 0-9, and a handful of special characters (-, ., $, /, +, %, space). This limitation is a significant drawback when dealing with lowercase letters or the full ASCII character set. To address this, an extended version of Code 39, known as Code 39 Full ASCII or Code 39 Extended, was developed. |
How Code 39 Extended Works |
Code 39 Extended encodes all 128 ASCII characters by using pairs of base Code 39 characters. For example, a lowercase 'a' is encoded as '+A', a lowercase 'b' as '+B', and so on. This encoding method effectively doubles the length of the barcode for any extended character, significantly reducing data density. However, it allows Code 39 to encode any ASCII character without changing the underlying decoding hardware or software. |
Advantages and Disadvantages |
The primary advantage of Code 39 Extended is that it offers full ASCII compatibility while maintaining backward compatibility with existing Code 39 scanners (with appropriate software updates). The primary disadvantage is the reduced data density; a Code 39 Extended barcode is often twice as long as a standard Code 39 barcode for the same information. In aerospace, where space is often limited, this can be a significant constraint. |
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Applications |
Code 39 Extended is less common than standard Code 39, but it finds use in applications where lowercase letters or special characters are essential. For instance, some healthcare applications require the encoding of patient names or medical procedure codes that include lowercase letters. Some electronics manufacturers use Code 39 Extended to encode data that contains a mix of uppercase and lowercase characters, such as a serial number or MAC address. However, in many cases, the availability of Code 128, which provides full ASCII support with higher density, has led to Code 39 Extended being less widely adopted than the standard Code 39. |
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Scanning and Printing Considerations |
For Code 39 to perform reliably, especially in demanding environments like aerospace, careful attention must be paid to printing and scanning. |
Print Quality |
The quality of the printed barcode is crucial. If the bars are not sharp, the spaces are not clear, or the contrast is insufficient, the barcode may not scan. Factors that affect print quality include printer resolution, label material, and the ink or toner used. In aerospace, labels often need to be printed on durable materials that resist abrasion, chemicals, and heat. Polyester and polyimide labels are common choices, often with a protective over-laminate. The X-dimension, or the width of the narrowest bar, must be sufficient for the scanner to resolve. The minimum recommended X-dimension is 0.19mm (7.5 mils), though 0.33mm is often preferred for reliability. |
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Quiet Zone |
A Code 39 barcode must be surrounded by a 'quiet zone'---a blank margin at least 10 times the width of the narrow bar on both sides of the code. This quiet zone allows the scanner to distinguish the start of the barcode from the surrounding text or graphics. In the tight spaces of an aircraft wiring harness, maintaining an adequate quiet zone can be a challenge, but it is essential for reliable scanning. |
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Scanner Technology |
Modern barcode scanners, including both laser and imaging-based scanners, can decode Code 39 easily. Laser scanners are simple and reliable for standard Code 39, while imaging scanners can also read 2D barcodes and are more versatile. In aerospace applications, scanners must be ruggedized to withstand drops, dust, and moisture. Many are also designed for hands-free operation on the assembly line, mounted on a stand or integrated into a workstation. The universal compatibility of Code 39 is one of its greatest strengths; it can be read by almost any scanner, from the cheapest consumer-grade model to the most advanced industrial imagers. |
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Software and Integration |
Code 39 is easy to integrate into existing systems. Because it requires no checksum calculation, it can be generated with a simple barcode font. To create a Code 39 barcode, you place an asterisk at the beginning and end of your data string and then display that string using a Code 39 font. This simplicity makes it a popular choice for generating barcodes in Microsoft Word, Excel, or other common applications. |
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The Technical Trade-Offs in Depth |
Code 39's technical characteristics are a classic example of engineering trade-offs. It prioritizes reliability and simplicity over data density and modern features. Understanding these trade-offs is key to understanding where and why Code 39 is used. |
Data Density and Symbol Length |
The most significant drawback of Code 39 is its low data density. Because each character requires nine elements, including three wide ones, Code 39 is much longer than Code 128 for the same data. In fact, a 10-character Code 39 barcode can be roughly 40% wider than the same data encoded in Code 128. This means that Code 39 is not suitable for very small items or for applications where the barcode must be printed in a very small area. For example, a microprocessor or a tiny medical device might not have enough surface area to accommodate a Code 39 barcode. |
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Character Set Limitations |
The standard Code 39 character set is limited to uppercase letters, numbers, and a handful of punctuation marks. This is insufficient for many modern applications that require lowercase letters, the full ASCII set, or special characters from other character encodings. While Code 39 Extended solves this, it comes at the cost of even lower data density. For these reasons, Code 128 has become the preferred symbology in many new implementations where data density and character support are critical. |
The Enduring Appeal of the Older Standard |
Despite these limitations, Code 39 continues to be used in legacy industries and applications where changing to a newer symbology would be costly and disruptive. It is also often chosen for new applications when reliability and scanner compatibility are paramount. In aerospace, for example, the supply chain is so large and so established that switching to Code 128 or another symbology would require an enormous effort. Moreover, the data encoded on aircraft parts is often relatively short: a part number and a serial number, for instance. In such cases, the additional density of Code 128 is not needed, and the ruggedness of Code 39 is more valuable. |
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Case Study: Boeing and Airbus Implementation |
Returning to our central example, the use of Code 39 by Boeing and Airbus is a case study in supply chain management and standardization. |
Rivet and Fastener Marking |
On the assembly line, rivets and fasteners are delivered to the workstation in bags or bins, each with a label containing a Code 39 barcode. The worker scans the label to confirm the part number before installation. This simple act updates the inventory system, triggers reordering, and creates a record that the specific rivet is now part of that aircraft. Because the rivets are small, the Code 39 barcode must be printed on a label that is large enough to be scanned but small enough to fit on the part's packaging. The label may also include human-readable text, such as the part number, as a backup. |
Panel and Major Assembly Marking |
Larger components, such as wing panels or fuselage sections, are marked with Code 39 barcodes that can be read from a distance. These barcodes may be printed on metal plates or ruggedized labels that are riveted or glued to the component. The data encoded may include the part number, the serial number, the assembly date, and the manufacturing plant. These barcodes are scanned at various points in the production process to track the component's progress and ensure that it is installed correctly. In addition, the barcode may be used to link the component to its digital manufacturing records, which include test results, quality control data, and maintenance history. |
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Wiring Harness Identification |
Wiring harnesses in an aircraft are long and complex. Each wire in the harness is identified with a Code 39 barcode label that is wrapped around the wire near its connector. These labels are typically heat-shrink sleeves that are printed with a Code 39 barcode. The barcode encodes the wire part number and the destination connector or terminal. This allows technicians to identify the wire quickly and accurately during installation or maintenance, reducing the risk of wiring errors. |
Quality Control and Regulatory Compliance |
In the aerospace industry, quality control is paramount. Every part that goes into an aircraft must be traceable, and every installation must be documented. Code 39 barcodes are a key tool in this traceability chain. |
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Traceability and Recall |
If a defective part is discovered, the manufacturer must be able to identify every aircraft that contains that part. The barcode on the part, along with the records of when and where it was installed, enables this rapid traceability. In the event of a recall, this capability is essential for quickly identifying and fixing affected aircraft. The use of a common, standardized barcode like Code 39 ensures that this traceability can be implemented across a global supply chain. |
Documentation and Record Keeping |
The data from barcode scans is fed into digital systems that create a comprehensive record of each aircraft's assembly. This record, known as an aircraft logbook or digital twin, includes every part number, every test result, and every installation procedure. This record is used for maintenance, repair, and overhaul (MRO) throughout the aircraft's life, as well as for legal and regulatory compliance. The quality of the data input---the barcode scan---is therefore critical to the quality of the record. |
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Audit and Compliance |
Regulatory agencies, such as the Federal Aviation Administration (FAA) in the United States and the European Aviation Safety Agency (EASA) in Europe, audit aircraft manufacturers to ensure they comply with strict safety and quality standards. A robust barcode system, based on an industry-standard symbology like Code 39, is a key element of demonstrating compliance. The use of Code 39 provides an auditable trail that can be inspected and verified at any time. |
The Future: Will Code 39 Survive |
Given the emergence of more advanced barcode symbologies, such as Code 128, GS1-128, and Data Matrix (2D), one might wonder if Code 39 will eventually be phased out. The answer, based on its continued use in large-scale, long-lifecycle industries, is likely 'no' for the foreseeable future. |
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The Persistence of Legacy Systems |
The aerospace and defense supply chains are notoriously slow to change. The cost and effort required to update the thousands of printers, scanners, software applications, and supply chain partners that rely on Code 39 are simply too high. As long as the existing systems continue to work, there is little incentive to upgrade. |
The Role in New Implementations |
Even in new implementations, Code 39 is often chosen because of its robustness and universal support. For applications where density is not a primary concern, the simplicity of Code 39 is a significant advantage. The need for a check digit is optional; the barcode is self-checking, and printers can produce high-quality Code 39 labels easily. There is no licensing fee associated with using Code 39, making it an attractive choice for smaller companies or organizations on a budget. |
The Rise of 2D Barcodes |
While 2D barcodes like Data Matrix and QR codes are becoming more popular, they have not displaced Code 39 in many industrial and aerospace applications. 2D barcodes can hold far more data and can be read even when partially damaged, but they require imaging scanners, which are more expensive than the laser scanners commonly used for Code 39. Moreover, the data encoded on a typical aircraft part---a part number and serial number---is often short enough to be encoded in a small Code 39 barcode. In this situation, the cost and infrastructure advantages of Code 39 keep it relevant. |
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Case Study: The Price of Obsolescence |
There are examples in the industry where companies have attempted to phase out older symbologies only to encounter resistance from their own supply chain. A tier-3 supplier that makes hundreds of different parts for a tier-1 supplier may not want to upgrade its barcode printing system if it only supplies to that one customer. The cost of a printer upgrade and software retraining might be a significant burden, while the benefit---to the customer, not to the supplier---is minimal. This inertia is a powerful force in industrial supply chains and helps ensure the longevity of Code 39. |
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Detailed Summary: The Aerospace Advantage and the Wider World |
As we have seen, Code 39 is far more than an old barcode. It is a testament to the power of good engineering, where a simple, rugged design can become a global standard for decades. |
Code 39 in Aerospace |
In aerospace, Code 39's role is defined by the environment. The self-checking feature means that even when a barcode is printed on a small rivet or a curved wire label, the reader can be confident in the data it decodes. The low data density is not a drawback when the information to be encoded is short and must be readable with wide bars and large quiet zones. The simple encoding and no-mandatory-checksum structure mean that Code 39 can be integrated into older systems without massive software upgrades. And the widespread adoption, driven by the U.S. Department of Defense, created a global ecosystem of compatible scanners and printers that Boeing and Airbus continue to leverage. |
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Code 39 in Other Industries |
The same properties that make Code 39 effective in aerospace---ruggedness, simplicity, and universality---make it valuable across a broad range of other industries. |
Military and Defense: The LOGMARS legacy ensures that Code 39 remains a standard for identification and logistics across all branches of the U.S. military and its contractors. |
Automotive: The automotive industry uses Code 39 to track parts and assemblies, ensuring that the right components reach the right production line. |
Healthcare: In the form of HIBC, Code 39 provides a safety-critical layer of identification that helps prevent medical errors. |
General Industry: For internal asset tracking, inventory management, and warehouse operations, Code 39 is often the default choice because it works with almost any scanner and is free to use. |
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The Technical Strengths Revisited |
The technical characteristics of Code 39 are the foundation of its success: |
Self-Checking: This is its single most important feature for industrial use, where print quality is variable. The code's structure ensures that a misread character is almost impossible. |
Simple Encoding: The nine-element, three-wide structure is easy to understand, easy to print, and easy to decode. |
Wide Bar Tolerances: Code 39 can tolerate a wide range of bar-to-space width ratios, making it adaptable to different printing methods and substrates. |
No Licensing Fees: Code 39 is an open standard with no royalties, which has encouraged its widespread adoption. |
The Drawbacks and the Competitive Landscape |
Low Data Density: Code 39 is not the best choice for small items or when a large amount of data must be encoded. Code 128 and 2D barcodes offer better density. |
Limited Character Set: Standard Code 39 cannot encode lowercase letters or the full ASCII set without a significant density penalty. |
Competition from Code 128 and Data Matrix: In new applications, Code 128 and Data Matrix are often preferred for their higher density and better character support. |
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Conclusion: An Enduring Legacy |
Code 39 is not the newest, the fastest, or the most efficient barcode. It is, however, one of the most resilient. Its longevity in the aerospace industry is proof that, in the real world of greasy hands, cramped compartments, and legacy supply chains, simplicity and ruggedness often win over technical sophistication. The barcode on an aircraft wing panel is not just a label; it is a link in a chain of safety, quality, and accountability that stretches from the factory floor to the skies. As long as aircraft are built and maintained, Code 39 will likely remain part of that chain. Its continued use, alongside the more modern symbologies, is a reminder that in technology, as in aviation, sometimes the old ways are the best ways. |
cs@easiersoft.com