Understanding wire failure risk levels for aircraft EWIS

At all points along the Electrical Wire Interconnect Systems (EWIS) risk assessment process, it is important to remember the goal: to determine the risk level for each EWIS component including wire failure. As defined in many of the other articles on this site, risk is the combination of failure probability and failure severity. This article will review the different levels for of failure probability and failure severity to help you understand aircraft wire failure.

Failure Severity

Assessing failure severity can become a very burdensome process, especially with EWIS, as the failure effects are not localized. But once all of the consequences of a wire failure are identified, they will fall into one of five categories:

1. No Safety Effect
2. Minor
3. Major
4. Hazardous
5. Catastrophic

The following descriptions of the different severity levels are from the Federal Aviation Administration (FAA)’s AC 25.1701 (December 2007 Version).

1. No Safety Effect

These are failure conditions that would have no effect on safety; for example, failure conditions that would not affect the operational capability of the airplane or increase flightcrew workload.

2. Minor

These are failure conditions that would not significantly reduce airplane safety and would involve flightcrew actions that are well within the crew’s capabilities. Minor failure conditions may include:

• a slight reduction in safety margins or functional capabilities;
• a slight increase in flightcrew workload, such as routine flight plan changes; or
• some physical discomfort to passengers or cabin crew.

3. Major

These are failure conditions that would reduce the capability of the airplane or the ability of the flightcrew to cope with adverse operating conditions. For example, they could lead to:

• a significant reduction in safety margins or functional capabilities;
• a significant increase in flightcrew workload or in conditions impairing flightcrew efficiency;
• discomfort to the flightcrew; or
• physical distress to passengers or cabin crew, possibly including injuries.

4. Hazardous

These are failure conditions that would reduce the capability of the airplane or the ability of the flightcrew to cope with adverse operating conditions. For example, they could lead to:

• a large reduction in safety margins or functional capabilities;
• physical distress or excessive workload such that the flightcrew cannot be relied upon to perform their tasks accurately or completely; or
• serious or fatal injuries to a relatively small number of persons other than the flightcrew.

5. Catastrophic

These are failure conditions that would result in multiple fatalities and usually the loss of the airplane.

Failure Probability

The other side of the risk coin is failure probability. There are four standard levels associated with failure probability. These are:

1. Probable
2. Remote
3. Extremely Remote
4. Extremely Improbable

The following descriptions are from the FAA’s System Safety Handbook (December 2000 version)

1. Probable

• Qualitative: Anticipated to occur one or more times during the item’s/entire system’s operational life.
• Quantitative: Probability of occurrence per operational hour is greater than 10^-5.

2. Remote

• Qualitative: Unlikely to occur during each item’s total life. May occur several times in the life of an entire system or fleet.
• Quantitative: Probability of occurrence per operational hour is less than 10^-5, but greater than 10^-7.

3. Extremely Remote

• Qualitative: Not anticipated to occur during each item’s total life. May occur a few times in the life of an entire system or fleet.
• Quantitative: Probability of occurrence per operational hour is less than 10^-7, but greater than 10^-9.

4. Extremely Improbable

• Qualitative: So unlikely that it is not anticipated to occur during the entire operational life of an entire system or fleet.
• Quantitative: Probability of occurrence per operational hour is less than 10^-9

If you are interested in wire failure, you may want to read Lectromec’s Assessment of Mechanical Stress on Wire Insulation article.