FAA Requirements on EWIS EMI protection (25.1707)

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Continuing the examination of the FAA EWIS separation requirements detailed in 25.1707, this article examines the next part of the regulation dealing with EMI. The second paragraph from FAA regulation 25.1707 focuses on electrical interference and addressing this from the wire system level. The regulation states:

“(b) Each EWIS must be designed and installed so that any electrical interference likely to be present in the airplane will not result in hazardous effects upon the airplane or its systems.”

According to the FAA’s guidance document on this regulation (25.1701-1), the following sources for EMI should be considered with regard to how they affect the EWIS:

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EWIS Certification Compliance with 25.1707 – Part I

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Over the next couple of articles, I will discuss some of the finer points of the FAA regulation 25.1707 – System Separation: EWIS.

In each of these articles, we will go through an example of an unacceptable scenario and the means to address the scenario to be in compliance with the regulations. While these scenarios will not come from actual platforms, the hope is that they will provide sufficient insight into the critical items that should be considered.

Regulation 25.1707 starts with the following:

“(a) Each EWIS must be designed and installed with adequate physical separation from other EWIS and airplane systems so that an EWIS component failure will not create a hazardous condition. Unless otherwise stated, for the purposes of this section, adequate physical separation must be achieved by separation distance or by a barrier that provides protection equivalent to that separation distance.”

One of the first questions after reading the above statement is what is the criteria for determining an acceptable separation distance and how is this shown?

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MIL-HDBK-525 Overview – Task 6: Action Plan

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The eighth article in the series Electrical Wire Interconnection Systems (EWIS) Service Life Extension Programs (SLEPs) discusses the Enhanced Zonal Analysis Procedures (EZAP) process which aims to improve maintenance and inspection programs for all EWIS components. The EZAP process tailors the frequency of inspections based on the determined environmental conditions of a particular zone.

The EZAP procedure, originally developed by the FAA back in 2007, should be designed to maintain an aircraft’s EWIS airworthiness. This is achieved through defining general guidance checklists throughout the aircraft and inspection intervals. The EZAP logic below provides a step-by-step process for determining the proper inspection procedures.

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MIL-HDBK-525 Overview – Task 6: Action Plan

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The seventh article in the series on Electrical Wire Interconnection Systems (EWIS) Service Life Extension Programs (SLEPs) develops an action plan and mitigation techniques based on the EWIS risk assessment (Task 5).

There are multiple factors (project, platform, engineering and risk tolerances) to consider when determining what action and/or mitigation techniques need to be implemented. Mitigation techniques developed in this section consider all factors and the EWIS risk assessment (Task 5). Three important mitigation techniques discussed are design changes, replacement and maintenance changes.

Design changes: Identify and develop a mitigation strategy for components with high failure severity values. Some of the design changes may include harness rerouting, additional protective sleeving, or changing of circuit protection devices. Once a mitigation strategy is developed, a secondary risk assessment should be performed to determine the impact.

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F-22 Crash

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In a report that was released August 2013 by the USAF, the results of a Lockheed Martin F-22 aircraft crash point to a chaffed power feeder cable as the likely cause of failure. On November 15, 2012, the F-22, attempting to return to base, crashed just short of a runway in Florida. The crash resulted in total aircraft loss at a total cost of nearly $150 million.

The sequence of events from the moment of failure is shown in the figure below. The sequence of events shown start 23 minutes after takeoff when the first indication of problems appeared, “beginning with the MP’s left generator fail”.

This was immediately followed by oxygen generating system failure and making the radio’s inoperable, and the situational displays. Shortly thereafter, an attempt was made to reset the left generator which failed.

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MIL-HDBK-525 Overview – Task 5: Risk Assessment

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This is the sixth article in the series on Electrical Wire Interconnection System (EWIS) Service Life Extension Programs (SLEPs).  If you missed one, here are the links to be previous articles:

In task #5 of the MIL-HDBK-525, the focus is on the development of an EWIS risk assessment based on the data collected in Tasks 1-4. In practice, this stage applies common risk assessment processes (further details on general risk assessments process can be found in MIL-STD-882).

The data collected from maintenance and degradation results (Task 2 and 4) are categorized into a probability of failure (1-6), with a rank of ‘1’ given to failures that can be defined as ‘frequent’; guidance is provided in the handbook on how these should be combined. The failure severity, based on the assessments in task #1 and some of maintenance reports from task #2, is assigned one of four categories. 

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MIL-HDBK-525 Overview – Task 4: Component Assessment

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This article is the fifth in the series devoted to reviewing the guidance provided in MIL-HDBK-525 for Electrical Wire Interconnection System (EWIS) Service Life Extension Programs (SLEPs). If you missed one, here are the links to be previous articles:

Task four of the MIL-HDBK-525 focuses on component assessment. Two main goals within this task are to:

  1. Determine the current condition of EWIS components, and
  2. Assess the remaining component service life

These goals are accomplished through general assessment methods, techniques, and focus sampling from the fleet.

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MIL-HDBK-525 Overview – Task 3: Physical Aircraft Inspection

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This article is the fourth in the series devoted to reviewing the guidance provided in MIL-HDBK-525 for Electrical Wire Interconnection System (EWIS) Service Life Extension Programs (SLEPs). If you missed one, here are the links to be previous articles:

Covered in here is task #3 guidance, physical aircraft inspection. Applying the SLEP handbook guidance will improve the likelihood of properly identifying and correcting EWIS degradation issues. The basis for the guidance in task #3 comes from an amalgamation of military, industry, and regulatory publications on EWIS investigation procedures.

The principal causes of EWIS component degradation are identified as follows: vibration, moisture, maintenance, metal shavings and debris, repairs, indirect damage, contamination, and extreme temperature. Each factor has a different degradation effect on EWIS components, however all must be considered when performing physical aircraft inspections.

Entire article here

MIL-HDBK-525 Overview – Task 2: Data Analysis

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This article discusses the second task “Data Analysis” in the series on EWIS Service Life Extension Program (SLEP). This is based on the new guidance developed by the USAF for the sustainment of aging aircraft EWIS and has been integrated with MIL-HDBK-525. The previous articles can be found here:

The second task in MIL-HDBK-525 describes the data analysis processes that can be used to determine hot spots, scale of EWIS repairs, areas for focus during inspection (task #3 – covered in the next article), and the overall risk assessment process to support EWIS SLEPs.

The following analytical methods can be used to identify conditions and/or performance gaps: visual observation, comparative analysis, statistical analysis and and analysis process. Each method can provide beneficial information but only when used for a valid situation or environment with each described in more detail in the handbook.

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MIL-HDBK-525 Overview – Task 1: EWIS Documentation

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This series on EWIS Service Life Extension Program (SLEP) risk assessment techniques discusses the implications and ideas contained in the EWIS handbook MIL-HDBK-525; an overview of the handbook objectives was covered in this article. Each article in this series will review a task or a subtask within the EWIS handbook (a total of seven major tasks are identified in the handbook). In this article, the first task “EWIS Documentation” is discussed.

The first task outlined in the EWIS SLEP risk assessment process is to obtain data necessary to document an aircraft’s EWIS, identify the critical circuit paths, and gather equipment functionality. This information is to be used to perform a preliminary aircraft impact assessment of EWIS component failures.

The first step of the EWIS documentation process is to perform an aircraft functional hazard assessment. The preliminary assessment is performed utilizing two inputs: aircraft level effects (through device assessment or existing data) and aircraft zone information divided by environmental zone (i.e. temperature, vibration, humidity etc.). This task primarily consists of gathering wire data and electrical system data (i.e. connectors, splices, wires, circuit breakers etc.) needed to conduct the failure hazard assessment.

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