Design Assurance Level (DAL) and Software Level in DO-178C: A Deep Dive with Examples

DO-178C Design Assurance Level DAL

The aerospace industry is a realm of exceptional standards and unyielding precision. Among the vital elements that underpin this sector’s safety and reliability is DO-178C, the comprehensive framework for software development and certification in airborne systems and equipment. In this article, we’ll be delving into two critical aspects of DO-178C: Design Assurance Level (DAL) and Software Level. We will also illustrate these concepts with real-world examples to aid understanding.

The Foundation of DO-178C

Before delving into DAL and Software Level, it’s essential to understand the basis of DO-178C. This guideline is a product of the Radio Technical Commission for Aeronautics (RTCA) and the European Organization for Civil Aviation Equipment (EUROCAE). These bodies developed DO-178C to ensure the safety and reliability of software used in airborne systems and equipment.

DO-178C establishes a robust framework for developing and certifying software. It mandates stringent processes, methodologies, and software life cycle environments to enhance safety in the aerospace industry.

Understanding Design Assurance Level (DAL)

A cornerstone of DO-178C is the concept of Design Assurance Level (DAL). DAL is a classification system that categorizes the potentially catastrophic effects of software failure in airborne systems. The severity of these effects ranges from ‘no effect’ to ‘catastrophic.’ Based on this, DO-178C identifies five DALs, labeled A through E.

DAL A: Catastrophic – Software failure may cause multiple fatalities, usually with the loss of the airplane.

DAL B: Hazardous – The failure can cause a large negative impact, potentially leading to severe injuries or one fatality.

DAL C: Major – The failure can cause discomfort to occupants and might lead to minor injuries.

DAL D: Minor – The failure slightly reduces the safety margins or leads to passenger inconvenience.

DAL E: No Safety Effect – The failure has no impact on safety, airplane operation, or occupants’ comfort.

Each DAL requires different levels of rigor during software development and verification, with DAL A demanding the highest level of assurance.

The Intersection of DAL and Software Level

The DAL directly influences the ‘Software Level’ of a particular component in the DO-178C standard. The Software Level represents the rigor needed in software processes to ensure adequate safety. There are five Software Levels, corresponding directly to the DALs (A to E). Software Level A requires the most rigorous processes and assurance activities, while Software Level E requires the least.

To illustrate the relationship between DAL and Software Level, let’s consider an example. Suppose an aviation company is developing a new autopilot system, a critical system where failure could lead to catastrophic consequences. In this case, the autopilot software would be classified as DAL A, requiring Software Level A processes and assurance activities. This means the development team would need to follow stringent guidelines, including formal methods for software design and coding, rigorous verification processes, and comprehensive documentation to achieve certification.

Real-World Examples of DAL and Software Level in DO-178C

In-Flight Entertainment System (Software Level E/DAL E)

An In-Flight Entertainment (IFE) system is a prime example of a system classified as DAL E. This software’s primary role is to provide entertainment to passengers during flights via movies, music, and games. If the IFE software were to fail, it might cause inconvenience and dissatisfaction among passengers, but it would not compromise the safety of the aircraft or its occupants. Therefore, the IFE software corresponds to Software Level E in DO-178C, which means it requires the least rigorous development and verification processes. However, even at this level, basic quality assurance practices are still mandatory to ensure system functionality and reliability.

Engine Control System (Software Level A/DAL A)

At the opposite end of the spectrum, the software controlling an aircraft’s engine is considered critical, and any failure could lead to catastrophic consequences. Engine control software governs engine operation parameters like fuel mixture, ignition timing, and throttle position – all crucial for the safe operation of the aircraft. If this software were to fail, it could result in a total loss of engine power, endangering the aircraft and potentially leading to loss of life. Consequently, this software is classified as DAL A and requires Software Level A processes. This entails the most rigorous development and verification processes, including stringent design and coding standards, comprehensive testing methods, and extensive documentation to ensure that the software is reliable and safe.

Terrain Awareness and Warning System (TAWS) (Software Level B/DAL B)

Another example can be the Terrain Awareness and Warning System (TAWS), a system designed to prevent Controlled Flight Into Terrain (CFIT) accidents. A failure in the TAWS software could potentially lead to severe injury or even a single fatality, but it’s not typically classified as potentially causing multiple fatalities or loss of the aircraft, assuming other systems function correctly. As such, the TAWS would typically be categorized as DAL B, requiring Software Level B processes. While this still requires substantial rigor in the software development and verification process, it’s somewhat less stringent than for DAL A systems.

Fuel Monitoring System (Software Level C/DAL C)

The software used in an aircraft’s Fuel Monitoring System can be classified under DAL C. This system provides important data about fuel consumption and remaining fuel, helping pilots make informed decisions about flight range and when to refuel. If this software were to fail, it could cause major operational disruptions and might lead to minor injuries in severe cases, such as a forced landing. Therefore, the Fuel Monitoring System corresponds to Software Level C in DO-178C, requiring a moderate level of rigor in the development and verification process.

These examples illustrate the application of DO-178C’s DAL and Software Level in real-world aviation systems, highlighting the importance of this standard in ensuring varying levels of safety and reliability in airborne software systems.

These real-world examples underscore the importance of DAL and Software Level in DO-178C. By understanding and applying these classifications, we can ensure the safety and reliability of software in airborne systems, ultimately contributing to safer skies worldwide.

The Importance of DAL and Software Level in Airborne Systems

The Design Assurance Level (DAL) and Software Level are fundamental to the safety and reliability of airborne systems. They are the compass by which software developers and aviation engineers navigate the rigorous process of creating and certifying software for airborne applications.

Contributing to Safety and Reliability

The primary purpose of DAL and Software Level is to ensure the safety of airborne systems. By classifying the potentially catastrophic effects of software failure, DAL provides a clear risk assessment for every piece of software in an aircraft. This allows developers and engineers to prioritize their resources and attention to the systems that carry the highest risk.

The Software Level, determined by the DAL, further ensures safety by dictating the rigor needed in software processes. For example, Software Level A, corresponding to DAL A (catastrophic), demands the highest level of assurance. This includes stringent processes for software planning, development, verification, configuration management, and quality assurance. The rigorous processes at each Software Level ensure that every software component, irrespective of its potential risk, is reliable and performs its intended function.

Role in the Certification Process

DO-178C certification is a significant milestone for any software used in airborne systems. DAL and Software Level play a critical role in achieving this certification. They provide a roadmap for the certification process, outlining the activities, documentation, and evidence necessary for each level of assurance.

The certification authorities, such as the FAA in the United States and the EASA in Europe, rely on these classifications to assess the software’s compliance with safety requirements. They review the data produced at each Software Level to confirm that the necessary processes were followed and that the software meets the expected safety standards.

Without DAL and Software Level, it would be challenging to ensure the safety and reliability of software in airborne systems. These classifications provide a structured approach to managing risk, enhancing reliability, and achieving certification in the complex world of avionics software.


In the intricate domain of avionics software development, the Design Assurance Level (DAL) and Software Level in DO-178C stand as critical compass points, guiding the journey toward safer, more reliable airborne systems.

The exploration of these key concepts in this article has shed light on the rigorous processes that underpin the world of aviation software. The DAL, with its ability to classify the potential impact of software failure, allows for a structured approach to risk management. Meanwhile, the corresponding Software Level dictates the rigor required in software processes, ensuring every piece of software, regardless of its role, is developed and verified with an appropriate level of scrutiny.

These classifications not only contribute to enhancing the safety and reliability of airborne systems but also serve as crucial guidelines in achieving DO-178C certification. They enable the production of clear, comprehensive evidence of compliance with safety standards, thus facilitating the certification process with authorities like the FAA and EASA.

As we look toward the future, the significance of understanding and accurately applying DAL and Software levels in DO-178C will only become more pronounced. With the constant advancements in technology and the increasing reliance on software in aviation, these guidelines will continue to serve as invaluable tools for ensuring the safe and reliable operation of airborne systems.

In closing, whether you’re a software developer, aviation engineer, or simply a curious reader, developing a solid grasp of these concepts is a step forward in appreciating the complex, yet fascinating world of avionics software.

Design Assurance Level (DAL) and Software Level in DO-178C: A Deep Dive with Examples
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