U.S. Department
of Transportation
Federal Aviation
Administration
Advisory
Circular
Subject: HOW TO PREPARE
RELIABILITY ASSESSMENT PLANS
FOR AIRCRAFT SYSTEMS AND
EQUIPMENT
Date: 1/19/07
Initiated by: AIR-120
AC No: 20-157
1. WHAT IS THE PURPOSE OF THIS ADVISORY CIRCULAR?
a. This advisory circular (AC) shows you how to develop and use a reliability assessment
plan. An assessment plan documents the controlled, repeatable processes for assessing the
reliability of aircraft and engine electronic and electrical systems, including their
electromechanical elements and equipment.
b. We encourage manufacturers of aircraft systems, engines, and equipment to develop and
use an assessment plan. Each manufacturer who prepares a plan becomes the plan owner, and
the plan may be unique to the plan owner. Plan owners may apply for certification through one
of the following processes:
• Type certificate (Title 14 of the Code of Federal Regulation (14 CFR)) part 21,
Subparts B and F,
• Supplemental type certificate (14 CFR part 21, Subpart E),
• Amended supplemental type certificate (14 CFR part 21, Subpart D),
• Technical standard order authorization (14 CFR part 21, Subpart O), or
• Parts manufacturer approval (14 CFR part 21, Subpart N).
c. This AC is not mandatory and does not constitute a regulation. However, if you use the
means described herein, you should follow it in all important respects. This AC describes an
acceptable means, though it is not the only means, for manufacturers to use reliability assessment
plans to assess the reliability of aircraft and engine electronic and electrical systems, including
their electromechanical elements and equipment. This AC doesn’t apply to other systems and
equipment, such as mechanical or fluid systems. Those systems and equipment may involve
very different approaches to reliability.
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