- MIL-HDBK-522 provides guidance on wiring system inspection. Rev B was recently released.
- In addition to three new areas covered, dozens of additions have been made.
- Guidelines on solder contacts, conduits, and terminal junctions have been added.
Back in November 2018, the US Navy released the second revision to the military handbook on “Guidelines for inspection of aircraft electrical wiring interconnect systems” (MIL-HDBK-522). This handbook, last reviewed here, is a guide for the aircraft EWIS inspection and provides a lot of detailed examples of wiring evaluation from beginning to end of the EWIS.
The handbook does a great job up front detailing the importance of EWIS. The handbook’s forward identifies key reasons why EWIS inspections should be done. Pulling directly from Rev B
“All aircraft are filled with miles of wiring and hundreds of wiring devices that connect and transfer power and signals to and from electrical components. Virtually all aircraft systems rely heavily on some type of wiring for safe operation. Much like the structural components of an aircraft, the health and integrity of the EWIS can be significantly compromised due to premature aging, damage, and failure of wiring insulation. It is integral to the overall maintenance and sustainment of all aircraft that the EWIS be treated as a system and as a system be afforded the same level of importance as the aircraft structure and other critical flight control systems.”
In Rev B, three new sections have been added: solder contact inspection, conduit inspection, and terminal junction inspection.
There are dozens, if not hundreds, of guidance on how to solder, best practices for soldering, and how to identify poor soldering, and there is a reason for this: it still is a common issue. It is easy to argue why one might consider crimping versus soldering, but for those in need of low resistance connections, it is difficult to beat solder (NASA even showed some performance benefits of doing both crimping and soldering).
Within the MIL-HDBK-522 Rev B, specifically focuses on the criteria for inspection solder contact terminations such as the SAE AS39029/73 and /74 contacts. To go with this was a set of figures to help identify acceptable and unacceptable configurations.
Example of the AS39029/73 contact specifically designed for coaxial cables. Here, the cable is stripped back and the center conductor is inserted into the barrel; the position of it is verified through the inspection hole in the center. The shield is secured to the contact and its position is verified through the left inspection hole (obscured by the solder band under the clear plastic).
This is an example of minimal acceptable solder flow. The inspection window is not full of solder, but the ground wire (in this case for a twisted pair configuration) is not visible.
This is an example of maximum solder flow and is acceptable. The inspection hole is filled, and the wire is not visible. While there is some discoloration of the heat shrink sleeve, it is minimal.
This is an example of an unacceptable configuration. The solder band around the shield inspection window has not been fully melted.
This is an example of correct soldering job, but there was too much heat and caused damage to the heat shrink sleeve.
Wiring conduits are used throughout aircraft for several reasons including harness management, physical protection, and environmental protection. The MIL-HDBK-522 Rev B added this section to help maintainers with inspection of these conduits.
The handbook identifies three performance criteria for review of conduits:
Capacity: The conduit should be filled. Ideally, the conduit should not exceed 80% capacity. In addition to leaving room to handle joints and bends, the reason for this 80% is explained in the deformation inspection criteria.
Deformation: The deformation should not exceed 20% of the conduit diameter; beyond this, wire/cables within the conduit may my pinched and damaged. This photo shows an unacceptable configuration.
Cuts: Cuts in conduit are unacceptable. This will likely cause damage to the wires/cables within the conduit.
Terminal Junction Inspection
The last new section to be added to the MIL-HDBK-522 Rev B is on terminal junctions. In particular, the inspection of the terminals focuses on ensuring good connection and seals.
This is an example of an acceptable terminal junction. The unused positions are filled, there is no damage to the grommet surface, seals around the wires are undamaged.
While there is a gouge in the grommet, the gouge does not impact any of the wire seals and it does not extend to the base of the chamfer. This is an acceptable condition.
This is an unacceptable condition. The grommet has been damaged near one of the wire seals and the impact to the wire-grommet seal is uncertain.
While the gouge does not appear to impact the grommet-wire seal, the depth of the gouge extends to the bottom of the chamfer. This is an unacceptable condition and the grommet would need to be replaced.
While this article focused on the three new sections of the MIL-HDBK-522 handbook, there are dozens of additions to the other sections within the handbook. It would be advantageous for anyone working in aircraft maintenance, not just those working with the wiring system, to review the entire document. At the very least, a quick skim of the document to see what is considered acceptable would be helpful in helping to identify EWIS issues early.