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Selecting Aircraft for Inspection and Work Unit Codes

Maintenance & Sustainment

Key Takeaways
  • Air Force Materiel command instruction (AFMCI) 21-104 identifies the requirements for Controlled Interval Extension
  • This provides a clear set of aircraft for sample extraction for performance and degradation assesment.
  • Rev G of MIL-DTL-38769 now specifically calls out a Work Unit Code (WUC) for EWIS.
  • Listen to the podcast here.

A couple of weeks back, Lectromec published an article on condition-based maintenance and some of the thoughts that came out from the mechanical equipment and subsystems integrity program (MECSIP) managers meeting. To continue with that theme, this article focuses on two specific presentations and topics that were covered: sustainment authorization and EWIS working unit codes (WUCs).

Authorization to gather components

As a framework, MECSIP, incorporates the procedures for maintaining aircraft systems that include fuel, hydraulics, and EWIS (among others). The requirements of MECSIP are outlined in MIL-STD-1798.

One of the presentations during the MECSIP meeting was about the implications of the Air Force Materiel command instruction (AFMCI) 21-104 and the impact and support for the MECSIP process. The aircraft sustainment rests on the fact that all aircraft components have limited service lives. When the components are placed on an aircraft, the service life is estimated based on experience, lab data, and expert judgement. The extent of actual wear these components have endured is dependent on the initial component quality, operational life, and quality of maintenance.

AFMCI 21-104
Want to know which aircraft should be used for component testing? AFMCI 21-104 provides the guidance for it.

To grasp on how much wear has occurred requires thorough component inspection with the goal of assessing remaining component life and, if any, preventative maintenance actions that can/should be taken to extend service life. The support to such an effort is included in AFMCI 21-104.


The obvious goal of any maintenance program is to make the aircraft as reliable as possible. The goal for engineering support of aircraft is to use the best available data to increase reliability in the most efficient means possible. So too are the objectives of Controlled Interval Extension (CIE) programs; they are setup to control, “conditions for extending or reducing maintenance and inspection intervals without sacrificing safety of flight or reliability.” The policy memo outlines, “guidance and procedures for establishing and monitoring CIE programs for aerospace equipment.”

The air force policy memo seeks to make data-based preventative maintenance decisions, “By allowing a percentage of the force to exceed the established programmed depot maintenance (PDM) interval, then comparing the amount of wear and damage found on them with that found on baseline PDM aircraft, the possibility of extending the interval can be evaluated”.

What does this mean? This means that a percentage of the fleet is identified for sample removal and inspection. These aircraft are the guinea pigs for the rest of the fleet. From an implementation perspective, aircraft will have components removed as they reach an estimated end of service life. The sample size uses the following table:

Force Size

Sample Size

37 – 56


57 – 109


110 – 399


To go further, the policy seeks to perform Analytical Condition Inspections (ACIs) which is the, “…systematic disassembly and inspection of a representative sample of aircraft to find hidden defects, deteriorating conditions, corrosion, fatigue, overstress and other deficiencies in the aircraft structure or systems”

From an EWIS perspective, as the aircraft age, there are already a set of aircraft designated for sample extraction. As for the full process on how and what should be evaluated from the EWIS perspective, Lectromec has several podcasts, articles, and videos on the topic.

EWIS Work Unit Code

Work unit codes are the identifiers used by maintainers to identify which systems they are working on. The work unit code consists of a five-character set and is used to identify the system, subsystem, and component on which maintenance actions are performed.

From a data analysis perspective, work unit codes help for the analysis and review of where labor is being applied, and what work is being performed throughout the vehicle. At a high level, the work unit codes are assigned to systems and not against individual common hardware components such as nuts, bolts, etc.

Until the latest revision of MIL-DTL-38769 (now in revision G), there was no unique identification for the wiring system. The EWIS work unit code, system 35, is the latest addition to this specification.

From an application perspective, work unit codes are primarily associated with the functional system concept. What this means is that the components which comprise of a functional system will be those components which contribute to the actual function or activation of the system. The example cited in this specification suggests that a hydraulic or pneumatic power source will be included as part of flight controls if it is the final functional effect of the power source.

USAF Work Unit Code Breakdown
For years, the lack of work unit code for EWIS has been a stumbling block for accurate data. Hopefully the latest revision of MIL-DTL-38769 will help.

One exception to this functional system concept is the EWIS. Any wiring that is to be worked upon will be part of the system 35 work unit code and the remaining three characters of the code identify the specific functions/system.

Although this is now in a released specification, this still requires that each fleet implement it within their own processes. With hope, the work unit code for EWIS will make its way into each of the fleets and it will become easier for maintainers to gather the important data necessary to support their EWIS reliability targets.

Getting Ahead on EWIS

AFMCI 21-104 and the update to MIL-DTL-38769 provide tools to the maintainer to improve an aircraft EWIS. When these are combined with MIL-STD-1798 and MIL-HDBK-525, a framework is in place to justify and execute the proper level of assessment needed for the wiring systems within the fleet. Lectromec is available and ready to help you make the most of your EWIS assessment efforts.

Michael Traskos

Michael Traskos

President, Lectromec

Michael has been involved in wire degradation and failure assessments for more than a decade. He has worked on dozens of projects assessing the reliability and qualification of EWIS components. Michael is an FAA DER with a delegated authority covering EWIS certification and the chairman of the SAE AE-8A EWIS installation committee.