An A320 aircraft was flying in a turbulent area when sparks were observed in the cabin above row 5, followed by the loss of the forward cabin lights. The flight crew noticed that a Circuit Breaker (C/B) tripped on the 123VU panel at the rear of the cockpit.

When on ground, maintenance personnel reengaged the tripped C/B, which caused two additional C/Bs to trip. A maintenance personnel in the cabin also reported a spark in the cabin above row 5.

Ceiling panels were removed, revealing damage with signs of burn on thermal blankets close to frame 28 (FR28). Several wires of a power distribution route were burnt and melted. A P-clamp maintaining the wires appeared broken with significant heat damage and signs of metal melting. (fig.1).

Investigation indicated that the wiring of the power distribution route was previously modified during installation of a Supplemental Type Certificate (STC) modification. Additional wires were included to the wire bundle without modifying the size of the P-clamps accordingly. This resulted in an inappropriate P-clamp installation in several locations along th epower distribution route, where P-clamps were found not fully closed. This caused the wires to be in contact with the metallic part of the clamps. 

Additional damaged wire insulation was detected at the level of the incorrectly installed P-clamp above seat row 2. This damage was most probably caused by chafing against the P-clamp metallic part , either during flight due to vibration, or during the installation of the STC modification (fig.2). A similar chafing against the metallic part of the P-clamp is most probably the cause of the short circuit that occurs at frame FR28.

(fig.2)  Wires were added to the power distribution route without adapting the P-clamp dimension, causing chafing against the metallic part of the clamp (picture provided by the Operator)

Analysis of the damaged wire indicates that an overheat started, most probably caused by intermittent short-circuit in the chafing area. The overheat degraded the insulation of the various wires around the chafing zone, until the first C/B tripped due to a continuous short circuit.

When the maintenance personnel reset the tripped C/B, damage propagated to other wires, causing additional short circuits that caused the tripping of two additional C/Bs.

The short circuits ejected hot melted parts into the nearby area, causing burn damage. However, the fire-resistant properties of the surrounding materials prevented a sustained fire.

The EASA Certification Specification for Large Aeroplanes CS-25 defines the Electrical Wiring Interconnection System (EWIS) as “Any wire, wiring device, or combination of these, including termination devices, installed in any area of the airplane for the purpose of transmitting electrical energy, including data and signals, between two or more intended termination points.”

EWIS components include:

In the late nineties, two major accidents (TWA800 and SWR111) involving onboard fire raised the attention of the industry on EWIS. Investigations identified poor EWIS condition as either the root cause or a significant contributor to these fires. It was the industry‘s duty to respond, in order to prevent these types of accidents occurring again.

In 1998, the FAA created the Ageing Transport Systems Rulemaking Advisory Committee (ATSRAC), whose members included significant industry stakeholders, including Airbus. The committee's role was to evaluate the condition of EWIS on active commercial aircraft, and offer industry guidance for enhancements.

The assessment revealed many issues, regardless of the aircraft manufacturer or model, including wiring deterioration, corrosion, incorrect installation or repairs, and contamination from metal shavings, dust and various fluids.

In response, the committee highlighted the need to increase EWIS awareness through dedicated training and a dedicated subpart-H chapter within Certification Specification 25 for Large Aeroplanes published in 2007. This provided general guidance to design EWIS from functional architecture to physical installation. It also established guidelines for aircraft manufacturers to create a manual that collects standard practices for electrical wiring. The committee also amended the Instructions for Continued Airworthiness (ICA), to introduce the Enhanced Zonal Analysis Procedure (EZAP). This improved scheduled maintenance, so that EWIS discrepancies can be detected on time, before creating further damage.

Airbus followed the ATSRAC recommendations to create a manual gathering standard practices for electrical wiring. The Electrical Standard Practices Manual (ESPM) was created. The existing information on EWIS from the AMM was transferred to the ESPM, and the new information from the ATSRAC recommendations were also included. On A350 aircraft, this document is called the Electrical Standard Practices (ESP), and is included in the Line Maintenance chapter 20. On A220 aircraft, it is called the Electrical/Electronic Components - Standard Practices Publication (ESPP). 

In the early 2000s, Airbus created the “Electrical Wiring Interconnection Systems course” to help operators better understand and maintain their aircraft electrical installation. This course is customized to take into account all Airbus aircraft. It includes practical and hands-on sessions to train technicians on how to correctly evaluate the wiring system, and effectively use the applicable documentation.

In 2001, the tasks resulting from the EZAP were included into the Maintenance Planning Documents (MPD) by updating the Zonal Inspection Program (ZIP) section. New specific EWIS cleaning and inspection MPD tasks were also introduced. All the EWIS related tasks of the MPD are easily identified by the term “EWIS” at the end of each task description.

Here are the most common discrepancies that may be found on in-service aircraft, and their associated risks. Detecting them early will prevent further damage.

Aircraft are designed so that their wiring bundles are properly held in place, and sufficiently separated from each other, from other systems, and from the aircraft structure to prevent chafing. However, during an aircraft service life, wiring installation may be degraded or modified, causing wires to be affected by unwanted rubbing against other aircraft parts. In this situation, chafing damage may occur to the wire insulation and possibly to its electrical conductor.

Damaged cable insulation caused by chafing creates a significant risk of electrical arcing or short-circuiting with nearby components (fig.3), and with other wires if their own insulation was also degraded. A fire may ignite if arcing occurs in a flammable environment.

If the electrical conductor of the cable is also damaged by chafing, the resulting reduction of the wire cross section can cause generation of heat. The electrical continuity of the cable being maintained, this type of damage cannot be detected by protection devices (no C/B trippage).

The generated heat can ultimately melt the electrical conductor, causing the wire rupture, or damage to the nearby objects, increasing the risk of fumes, smoke, hotspot or flame.

(fig.3) Risks associated with cable chafing

Incorrect cable installation, or damage to the wiring installation, can cause cable chafing, arcing or short circuits. 

(fig.4) Typical incorrect or damaged cable installations

The torquing value to be applied to an electrical terminal block provided in the maintenance documentation is determined to maintain the connection lugs sufficiently in contact with each other to ensure a good electrical conductivity.

An insufficient torquing of a terminal block may prevent a correct contact between the connection lugs, forcing the current to go through a smaller section than the normal one. This causes an increase of the temperature of the block that can cause damage (fig.5). The electrical continuity of the circuit being maintained, this type of problem cannot be detected by protection devices (no C/B trippage).

On lower power distribution networks, thermal damage may remain localized to the connection point. However, for high power circuits, the temperature can increase up to nearly 1 000 °C. Under these circumstances, thermal degradation is likely to propagate toward nearby components and insulation materials, potentially causing additional damage, fumes or flames.

(fig.5) Insufficient torquing of a terminal block results in heat generation

Correct crimping is essential to ensure proper connection between the conductor of a cable and its terminal lug.

An incorrect crimping of a cable inside its terminal lug causes a reduction of the surface of contact between the electrical conductor and the lug (fig.6). This results in heat generation which is likely to propagate toward nearby components and insulation materials, potentially causing additional damage, fumes or flames. The electrical continuity of the circuit being maintained, this type of problem can’t be detected by protection devices (no C/B trippage).

(fig.6) Incorrect crimping can result in heat generation

Contamination of EWIS components can be the cause of significant damage. It must not be disregarded. There are two types of contamination: solid and liquid.

Solid contamination including metallic chips and abrasive dust (fig.7) can cause cable chafing and connector wear when combined with vibrations. Accumulation of dirt can also create a flammable environment that, when combined with overheating or arcing, can create fumes or flames.

Contamination by fluids (moisture, oil, hydraulic fluid, fuel, etc…) can also be a threat. Connectors are particularly sensitive to contamination by liquids because it can cause short circuits or corrosion. Corrosive liquid can cause damage to the cable insulation.

(fig.7) Examples of solid (metallic chips and dust) and liquid contamination on EWIS components

Prevention of damage to EWIS components during maintenance relies on several essential considerations.

Any personnel interacting with EWIS components must be familiar with the risks associated with EWIS discrepancies so they can identify and report them.

Compliance with ESPM/ESP/ESPP procedures ensures safe EWIS installation and maintenance.

ESPM, ESP and ESPP are continuously updated based on the in-service experience. Therefore, it is essential to use the latest version of the document.

Maintenance personnel must be careful when performing maintenance tasks near EWIS components. They must make sure not to cause damage to nearby EWIS components or contaminate them. Operators should refer to the ESPM/ESP/ESPP recommendations for protecting and stowing EWIS components during maintenance. 

If an EWIS component is damaged or contaminated during a maintenance task, maintenance personnel are required to check the ESPM/ESP/ESPP for the necessary repair or cleaning procedures. 

C/Bs are here to protect the wiring and aircraft system. Troubleshooting must be performed before resetting the C/B. A reset of a C/B without troubleshooting may cause additional damage, as in the example at the beginning of this article.

High-pressure spraying equipment must not be used to clean areas with electrical items like harnesses, sensors and connectors. 

Operators must follow ESPM, ESP, ESPP recommendations to clean EWIS components. If the use of solvent is required, operators should check that it is in the ESPM/ESP/ESPP list of recommended cleaning materials.

In the case study described at the beginning of this article, we saw that STC modifications may have an impact on EWIS components. 

In-service feedback on EWIS is essential.