System Design

Understanding System Design in Airframe, Systems, and Power Plant

In ATPL theory and practical operations, “system design” describes how aircraft structures and systems are engineered to meet safety, reliability, and regulatory requirements. At the airframe level, certification rules such as EASA CS-25/CS-23 and FAA 14 CFR Part 25/23 mandate design philosophies that manage failure risk: safe-life, fail-safe, and damage tolerance. At the systems level (hydraulics, electrics, flight controls, power plant), regulations such as CS/FAR 25.1309 require that no single failure leads to a catastrophic outcome, driving redundancy, segregation, and robust failure detection. Understanding how these philosophies translate into maintenance procedures, inspection intervals, and operational limitations is essential for pilot decision-making and exam success.

Safe-life design is based on replacing a component after a specified number of cycles, flight hours, or calendar time—before fatigue cracks are expected to initiate. Such life-limited parts (LLPs) include examples like certain landing gear elements, helicopter rotor hubs, propeller hubs, and engine rotating parts. They are tracked meticulously in aircraft records and must not be used beyond their declared life. This is a “no detectable damage” philosophy: continued airworthiness is assured by timely removal rather than by scheduled crack detection. For pilots and maintenance planners, this means MEL deferral does not apply to expired LLPs; compliance is mandatory via the Aircraft Maintenance Program (AMP) and the Airworthiness Limitations Section (ALS).

Fail-safe and damage-tolerant designs address the possibility of damage or element failure during service. In a fail-safe structure, redundancy and multiple load paths allow the aircraft to safely carry required loads for a defined period after a primary element fails, giving time for detection and repair. Damage tolerance goes further: the structure is analyzed and tested to tolerate a prescribed initial flaw, with crack growth predicted so that scheduled inspections (e.g., eddy current, ultrasonic NDT) will detect it before it becomes critical. Modern transport-category aircraft rely heavily on damage tolerance per CS/FAR 25.571, including considerations for widespread fatigue damage in pressurized fuselages. The same principles appear in systems: dual or triple hydraulic circuits, segregated wiring routes, independent power sources (IDGs, APU, RAT), and monitored flight-control actuators ensure fail-operational or fail-passive capability consistent with the system’s Development Assurance Level and Failure Hazard Assessment.

What this System Design question bank covers

• Key design philosophies: definitions and differences between safe-life, fail-safe, and damage tolerance, with practical examples.
• Life-limited parts: cycles/hours/calendar limits, tracking, regulatory basis (ALS/AMP), and operational implications for dispatch and maintenance.
• Structural integrity and inspections: multiple load paths, crack growth assumptions, NDT methods, and interval setting under aviation regulations.
• Systems redundancy and independence: compliance with CS/FAR 25.1309, segregation against common-cause failures, and failure condition classifications.
• Exam-style tasks: identifying correct versus incorrect statements about replacement policies, redundancy, allowable damage, and continued airworthiness procedures.

For ATPL candidates, link each philosophy to its operational consequence: safe-life means mandatory replacement at the limit; fail-safe relies on redundancy to maintain safety after a single failure; damage tolerance mandates inspection programs that assume and manage imperfections. Together, these principles underpin aircraft systems and structures, ensuring regulatory compliance and safe, reliable operations throughout the aircraft’s service life.