Safety Design Philosophies - Lecture 14

Fail-Safe & Safe-Life Designs in Avionic Systems

Course Overview

This section presents an overview of the design techniques that can be employed for the various design philosophies used in avionic systems design. Upon completion of this course, students will be able to:

  • Understand the concept of factor of safety and its importance in avionic systems.

  • Comprehend the philosophies of safe-life and fail-safe designs.

  • Grasp the fundamental process for conducting damage tolerance assessments.

  • Appreciate the various assessment techniques applicable in avionic systems design.

Factor of Safety

Definition

The factor of safety (FS) is a numerical ratio that reflects the load carrying capability of a structure in comparison to the expected loading. This includes various types of loading such as static, impact, fatigue, and wear. The purpose of applying a safety factor is to ensure that the design can withstand unexpectedly high loads or material/design defects without failing. Consequently, factors of safety help decrease the probability of failure or, conversely, increase the probability of success.

Importance

Factors of safety are integral to design due to the inherent ignorance in designs stemming from variability in materials, manufacturing processes, maintenance, and real-world experienced loads. Engineers must consider:

  • High quality and consistency in materials, manufacturing, maintenance, and inspection.

  • Accurate control or understanding of actual loading conditions and environmental factors.

  • Availability of reliable analytical or experimental data.

In industries such as commercial aviation, rigorous controls throughout fabrication and inspection allow for lower factors of safety (typically around 1.3) despite the high stakes involved.

Determining Safety Factors

The degree of ignorance is one factor in determining the needed safety factor; however, the potential harm from failure is also crucial. If failure poses little inconvenience, a small factor may suffice, while a significant potential for life-threatening consequences justifies a larger factor. Minimum safety factors are mandated in specific cases like pressure vessels; however, often, the best guide is experience gained from similar designs. Generally, factors may range from approximately 1.3 to 5.

Fail-Safe and Safe-Life Designs

Definitions

Aerospace engineers have developed two primary philosophies—fail-safe and safe-life—to manage fatigue loading in designs:

  • Fail-Safe Design: This philosophy implies that failures will occur, but the system or component is designed to fail in a way that mitigates potential loss.

  • Safe-Life Design: Here, the component is engineered to function adequately without failure for a predetermined service life, after which it is replaced based on testing and analysis that estimate its lifetime.

When to Use Each Philosophy

The decision to implement either the fail-safe or safe-life design depends on judgments made on a case-by-case basis. Factors influencing this decision include:

  • Costs of failure: Physical harm, equipment loss, and service disruptions.

  • Costs of implementation: Involves increased design, testing, and production costs against potential decreases in product performance. Both philosophies aim to enhance air travel safety, but total safety is not achievable due to inconceivable risk factors.

Techniques for Safe-Life Designs

To ensure that components don’t fail during their operational lifespan, safe-life designs require extensive testing and analyses, usually focusing on fatigue. The design must include:

  • Generous safety factors to cover for unforeseen failures, ensuring inspection capabilities during service.

Techniques for Fail-Safe Designs

Several techniques are employed to implement fail-safe designs:

  1. Redundancies: Prevent single-point failures (e.g., aircraft typically have multiple engines).

  2. Back-Up Systems: Provide alternative systems to avoid catastrophic losses.

  3. Multiple Load Paths: Structural design ensuring that if one component fails, the load transfers to others without systemic failure.

  4. Intentional “Weak Link”: Use of components designed to fail easily (e.g., fuses, shear pins) to protect more expensive parts.

  5. Physical Law Compliance: Designing systems for fail-safe characteristics, e.g., ductile failure in natural gas pipelines.

  6. Early Detection: Ensure cracks or defects can be detected early before they reach critical sizes through non-destructive testing (NDT).

  7. Leak-before-Break: Pressure vessels can crack while creating detectable leaks, preventing catastrophic failures.

  8. Crack Arresters: Mechanisms like riveted straps in aircraft to contain crack propagation.

Damage Tolerance Analysis for Active Inceptor Systems

Damage tolerance analysis assesses component performance under cyclic loads with flawed conditions:

  • The analysis outputs stress and fatigue life, establishing necessary design margins against both pilot and ground handling loads.

  • Critical components undergo rigorous evaluations to ensure they meet strength and safety requirements through a variety of potential wear mechanisms, including fatigue, corrosion, and maintenance-induced defects.

Analysis Procedure

  1. Define Damage: Initial defect sizes are established for screening flaws.

  2. Conduct Fracture Screening: Assess potential weaknesses.

  3. Perform Residual Strength Analysis: Look at initial and final flaw scenarios.

  4. Apply Deterministic Crack Growth Analysis (DCGA): To track how crack size changes over time under loading conditions.

Crack Growth Analysis

Each stage outlines the relationship between crack growth rates and cyclic loads, factoring in material properties and environmental effects on performance.

Design Loads and Structural Requirements

Overview of Design Loads

Different loads affecting aircraft include:

  • Ultimate Loads: Maximum expected loads that an aircraft can endure.

  • Limit Loads: Safe operational limits before yielding.

  • Operating Loads: Common in-flight forces experienced during regular operations.

Design Requirements

As per regulations, structures must:

  • Remain elastic under limit loads and withhold ultimate loads, verified through stringent analytical approaches ensuring margins of safety based on established load allowables.

Corrosion Prevention

The design of aircraft must mitigate corrosion throughout its service life by:

  • Utilizing corrosion-resistant materials and protective measures.

  • Implementing maintenance schedules for preventative measures.

Regulatory Implications

The regulatory landscape has evolved, encapsulating key amendments related to structural integrity:

  • Amendments stipulate detailed evaluations of fatigue and damage-tolerance, mandating residual strength assessments under service-induced damages.

Historical Perspective

Changes within regulations reflect accumulated data from service experience, indicating a shift towards more analytical methods in designs and inspections to ensure safety and airworthiness.