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In aviation, the '''safe-life''' aircraft design principle, also known as "safety by retirement," assumes that a structure can be designed to withstand a specific number of flight cycles or hours without significant degradation, and is then retired from service before any potential fatigue cracks can cause failure.

== History and Evolution ==

=== Early Aviation ===
Initially, aircraft structures were designed based on engineering judgment, focusing on strength to withstand known loads.

=== Emergence of Safe-Life ===
As aviation moved towards metallic structures, the concept of "safe-life" emerged to address the potential for fatigue cracks.

=== Assumptions ===
The safe-life approach assumes that a structure, when properly designed and maintained, will remain free from defects and will not experience significant degradation before reaching its designated retirement point.

=== Fail-Safe and Damage Tolerance ===
Over time, the limitations of the safe-life approach became apparent, leading to the development of [[fail-safe]] and [[damage-tolerant]] design principles, which focus on redundancy and the ability to withstand damage.

=== Current Use ===
While safe-life principles are still used in some applications, especially for helicopter structures, fail-safe and damage-tolerant designs are now more common, particularly in larger aircraft.

== Key Characteristics of Safe-Life Design ==

=== Defined Service Life ===
Structures are designed to have a specific, predetermined service life, expressed in flight cycles or hours.

=== No Redundancy ===
Safe-life structures typically do not rely on redundancy or alternative load paths to prevent failure.

=== Retirement ===
At the end of its designated service life, the structure is retired from service, regardless of its apparent condition.

=== Focus on Fatigue ===
Safe-life design emphasizes preventing fatigue cracks from initiating and propagating to critical sizes.

=== Examples ===
Safe-life design is often used in helicopter structures, where the entire structure is designed to be replaced after a certain number of flight hours.

[[Category:Aircraft design]]
[[Category:Aviation safety]]