White Papers

Why Dissimilar Redundant Architectures Are a Necessity for DAL A

July 09, 2018 | BY: Lisa Sarazin, Paul Hart, Rick Hearn

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For avionics systems requiring DAL A certification, adhering to the required <1 in 10-9 probability of failure is no easy feat. Take, for example, a flight control computer that relies on multiple air data computers and their collected inputs from sensors such as air speed sensors, altitude sensors, accelerometers, and gyroscopes in the roll, pitch and yaw axes. The flight control computer is responsible for reading data from these systems and calculating outputs to drive actuators for various aircraft components (for example, rudders, elevators and propulsion systems) in order to keep the aircraft in straight and level flight. Communication between these sensors and the flight control computer occurs at a high frequency, creating a controlled feedback loop.

Relying on a single computer to manage this loop would fall short of meeting the acceptable <1 in 10-9 probability of failure rate. The pitfall of a single channel flight control system is that any single point of failure in that chain can cause the entire system to malfunction. And, no matter how reliable your electronics are, unpredictable external factors can still cause a malfunction. For instance, if a UAV strikes a bird in flight and one of its probes becomes blocked, this can result in one of two major classes of errors: the probe can become completely inoperative or it can begin transmitting Hazardous Misleading Information (HMI) to the flight control computer. Either type of error can potentially prevent the flight control computer from properly calculating the desired output for any of the aircraft components under its control, and can ultimately lead to a disaster. For this reason, redundancy is critical in DAL A systems.

DAL A, Flight Standards

Figure 1: Achieving <1 in 10-9/Flight Hour Probability of Failure with a Dissimilar Redundant Architecture

Download the 'Why Dissimilar Redundant Architectures Are a Necessity for DAL A' white paper to learn more about:

  • Design Assurance Levels and Probability of Failure
  • Strengthening Redundancy with Dissimilarity and Complex Voting
  • Examples of Highly Redundant Systems


Lisa Sarazin

Author’s Biography

Lisa Sarazin

Marketing Portfolio Manager

Lisa Sarazin is the Marketing Portfolio Manager for the C4ISR division of Curtiss-Wright Defense Solutions. Lisa has previously held roles in Product Marketing and Content Strategy in the telecommunications industry.

Paul Hart

Author’s Biography

Paul Hart

Chief Technology Officer

Paul Hart joined Curtiss-Wright in 1982 as a graduate engineer and has worked for 18 years in the flight recorder business. Paul also worked for Thales for 2.5 years in helicopter flight management and was responsible for the mission systems group at Cobham Aviation Services for 7 years. In 2011 Paul re-joined Curtiss-Wright as the Director of Avionics Engineering and transitioned to the Avionics CTO.

Author’s Biography

Rick Hearn

Product Manager, Safety Certifiable Solutions.

Rick Hearn is the Product Manager for Safety Certifiable Solutions for Curtiss-Wright Defense Solutions. Rick has over 25 years of experience in design and design management positions in the Telecommunications and Defense industries including 11 years of experience in design management and program management at Curtiss-Wright Defense Solutions.

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