Hard-Time的正式定义是什么时候出现的？
FAA AC 120-17A中提到的方法就是被称为MSG-2的方法。另一个材料同样来自1978年（但实际正文成文时间应该在1972到1978年之间，因为有提到相关年份的资料），即：United Airlines联合US DOC发布的Reliability-Centered Maintenance[2]，其中完整介绍了RCM的诞生、MSG-1的应用和MSG-2的诞生。另外，其引用的材料提到1970年3月25日（下文Marcy怀疑为错误，应为“March”）ATA已经发布了MSG-2方法，可能其中已经包含了Hard Time的定义，只是暂时原文未找到，无法验证。
我们详细看下1978年Reliability-Centered Maintenance中提到Hard-Time的内容：
——正文·87页
——附录·480页
——正文·87页
而现行有效的FAA AC中，能找到的Life-limited part定义均指向在指定期限内“更换”，且不包含“翻修”（FAA Order 8110.42D CHG 1例外）。LLP的定义中也抛弃了1978年文件中提到的对“经济性”的考虑，而是强调了“安全”和“强制”。
Life-limited part means any part for which a mandatory replacement limit is specified in the type design, the Instructions for Continued Airworthiness, or the maintenance manual.
By a procedure approved by the FAA, operating limitations must be established which specify the maximum allowable number of flight cycles for each engine life-limited part. Engine life-limited parts are rotor and major static structural parts whose primary failure is likely to result in a hazardous engine effect. Typically, engine life-limited parts include, but are not limited to disks, spacers, hubs, shafts, high-pressure casings, and non-redundant mount components.
——IATA Guidance Material and Best Practices for Life-Limited Parts (LLPs) Traceability
无论是1978年的RCM，还是某发动机厂家现行有效的维修指南，均可以看出Hard-Time和LLP之间是有联系的。
Hard Time: Hard time limits,such as Life Limited Parts (LLPs), which are controlled through Chapter 05-11/12-XX in the LEAP 1AEngine Manual. Chapter 05-11/12/21-XX the "Airworthiness Limitation" section for life limited parts requires the adoption of an Airworthiness Directive (AD) to mandate the change.
出处源头不同。Hard-Time的要求出现在MRBR和MPD中；LLP的要求出现在ALS（AWL）、特殊运行要求和Engine Manual中。
Life-Limited Parts are including in the Airworthiness Limitations Section of the Instruction for Continued Airworthiness.
确定的时机不同。Hard-Time项目确定发生在MRBR准备阶段，LLP项目则出现在型号设计阶段。
……necessitate the development and execution of an engineering plan, a manufacturing plan, and a service management plan for each life-limited part. These three plans form a closed-loop system that links the assumptions made in the engineering plan, to how the part is manufactured, and to how the part is maintained in service. Engineering, manufacturing, and service management must function as an integrated system and recognize the effects of actions in one area on the entire system.
记录要求不同。在规章层面，Hard-Time中涉及定期Overhaul的项目，需要记录自上次翻修的时间；而LLP则需要记录当前的状态（至少包括自新时间和剩余寿命）。但在商业规则层面，LLP的要求非常繁复，比如经常被提及的Back To Birth要求，虽然BTB本身也没有明确的定义。感兴趣可以查看IATA文件Guidance Material and Best Practices for Life-Limited Parts (LLPs)[3]。
The current status of life-limited parts of each airframe, engine, propeller, rotor, and appliance.
The time since last overhaul of all items installed on the aircraft which are required to be overhauled on a specified time basis.
总结：越写越无力，终于从一团浆糊写成了一团乱麻。还有很多问题没解决，先放一放，后面有更多研究时再补充、纠正。
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