Coming up with failure rates of electrical components is always a difficult task. Historically, when failure modes and effects analyses were created for aircraft systems, the level of detail of EWIS component failures were severely lacking. Some organizations would create a single failure number for an entire circuit, regardless of the wires or connections made in that circuit.
Using modern tools and design technologies makes it possible to have a more thorough evaluation and accurate representation of the EWIS failure rates within fault trees. This still presents an issue with gathering the necessary information to determine what is the best failure rate to use. One often cited standard is the military handbook MIL-HDBK-217. Although this document has not been updated in years, it still is a useful data source. Here we review the failure rate presented for circular connectors and what this suggests when used on aircraft.
Background
The purpose of the MIL-HDBK-217 is to “establish and maintain consistent and uniform methods for estimating the inherent reliability of military electronic equipment and systems”. Further, the handbook provides a common basis for predicting future maintenance needs. Those who have maintained any platform will acknowledge that proactive maintenance is the best way to keep cost down. The handbook seeks to provide the data to make that predictive maintenance possible.
Just like any other component, connector reliability cannot be assumed. Getting the right number into fault trees is necessary to get the best failure projection.
As identified in the introduction, the handbook has not been updated in more than two decades; the latest revision of this handbook is from 1995. Tools and technologies have significantly improved in many areas since that time and at the very least, the reliability has remained consistent if not improved. While the failure rates presented in the handbook may be a little out of date, this does provide a conservative basis for the estimation of part reliability.
Of course, failure rates do not exist in a vacuum. As additional data becomes available and service history information is thoroughly reviewed, organizations can improve or modify these models to best reflect what they see and practice. The handbook provides the general case and the general direction; it is the responsibility of the user to make it specific to the organization and application.
Connectors
MIL-HDBK-217 Section 15 addresses connectors. At its core, the handbook provides a formula and a set of parameters that can be used to estimate the reliability.
The first item to note here is that the failure rates presented are for mated connectors. The MIL-HDBK-217 suggests that, if a connector is attached to a Line Replaceable Unit (LRU), then the failure rate should be divided in half with the failure rate being assigned to both the LRU connector and the EWIS connector. The reason for this is to provide better reliability numbers on the LRU. EWIS engineers should be grateful to hear that they are not responsible for the full failure rate of the connector.
Failure Rate Factors
The connector failure rate presented in MIL-HDBK- 217 is comprised of four factors: the base failure rate, the active number of pins, the environmental factor, and the mating/unmating factor.
Base Failure Rate
The base failure rate factor uses the connector insert material to separate connectors into four possible groups. These groups are:
|
Insert Material Type |
Common Insert Materials |
Temperature Range (C) |
|
A |
Vitreous Glass, Alumina, Ceramic, Polyimide |
-55 to 250 |
|
B |
Diallylphtalate, Melamine, Fluorosilicione, Silicone, Rubber, Polysulfone |
-55 to 200 |
|
C |
Polyteyrafluorethylene (Teflon) |
-55 to 125 |
|
D |
Polyamide (Nylon), Polychloroprene (Neoprene), Polyethylene |
-55 to 125 |
Since each of these insert materials have different temperature ranges for operation and different performance histories, there are different base failure rate formulas for each of them. Unfortunately, this creates a more difficult approximation for each configuration. The main difference between the formula is the maximum temperature rating and a couple of scalar factors.
Where this becomes more complicated is the connector temperature rise estimation. While the temperature rise within the connector can be quickly determined through laboratory testing, a generalized formulation for connector rise is difficult. Different insert materials, contact arrangements, airflow within a compartment, etc., can dramatically change the connector internal temperature. MIL-HDBK-217 guidance on estimating the temperature increase is a formula considering the combination of contact gauge and amps per contact.
Next Article
Without service data, estimations of component failure rates must use previously established best performance estimates. MIL-HDBK-217 provides a good starting basis for basic component failure rates for use in fault trees. In Lectromec’s next article, we will review some of the consequences of using this failure model and what it predicts for EWIS reliability.
Michael Traskos
President, Lectromec
michael.traskos@lectromec.comMichael 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.