What You Can’t See Can Hurt You — How To Protect Wiring

What You Can't See Can Hurt You — How To Protect Wiring

Aircraft wiring is critical to safe flight as it supplies data/signals to critical avionics/flight equipment. The FAA has recognized the importance of wiring and created regulations for the Electrical Wiring Interconnection System (EWIS). Two key factors have contributed to this increased focus: the aircraft fleet is aging and repeated flights can affect the integrity of the wiring and advances in avionics means that more wiring harnesses are being loaded into old and new equipment.

On July 17, 1996, the center fuel tank of TWA Flight 800 exploded just 12 minutes after departing JFK International Airport. It crashed into the Atlantic Ocean with total loss of life.

In response, the FAA created the “Aging Transport Systems Rulemaking Advisory Committee” (ATSRAC) composed of airlines, OEMs, and regulators. The ATSRAC inspected 81 in-service aircraft and found 3,372 wiring/electrical distribution discrepancies that included deteriorated wiring, corrosion, wire chafing, wire arcing, improper bend radius, improper installation and repairs as well as contamination by metal shavings, dust, and flammable fluids.

Over time, wiring insulation becomes brittle and prone to cracking, which exposes the conductor and creates the potential for hazardous electrical system malfunctions, such as a short circuit or airframe arc. Rubbing and chafing can accelerate this and then condensation, fluid containment, metal shavings, dust and debris can bridge the gap between a wire conductor and an adjacent metal structure.

What is disturbing from the ATSRAC study is that all 81 inspected aircraft were airworthy, in-service and had received numerous prior inspections and progressive/preventive maintenance. Until this review, wiring had rarely received special maintenance or inspections, even though it was widely known that wiring discrepancies or failures caused delays, unscheduled landings, IFE system problems, and both non-fatal and fatal accidents.

Chafing Is the Leading Issue

According to a Piper Aircraft Special Airworthiness Information Bulletin (SAIB), “Several airplanes showed unacceptable wire separation from hydraulic lines and /or adjacent structure below the floor-mounted main power distribution circuit breaker panel. The inspections also showed early signs of chafing, which can lead to thermal stress and arcing in an area where flammable liquids are routed. These conditions could sustain an uncontrollable fire in an inaccessible area below the pressurized deck.”

The U.S. Navy (NAVAIR) has large datasets on this topic: it shows that as many as one million-man hours are spent annually in troubleshooting, isolating, locating and fixing wiring faults: chafing contributed to more than a third (37%) of all wiring failures on Navy aircraft from 1980-1999 (see Table 1).

According to a recent study called Growth Opportunities in the Aircraft Wire and Cable Market, “the increased number of orders for new aircraft across the globe and modernization of the existing aircraft are the key factors that are expected to drive the aircraft wire and cable market during the forecast period…. the aircraft wire and cable market is projected to grow at the highest CAGR during the forecast period.”

Wiring protection generally and chafing in particular is clearly a systemic issue that needs to be controlled. The aging transport fleet and the growth of avionics just make it more important.

Aging Fleet and Advanced Avionics

The safety and utility of commercial and military aircraft have been greatly improved by the integration of advanced avionics which help reduce pilot workload, increase situational awareness, and facilitate cockpit resource management.

These advances include: ADS-B, Digital (FBW) Fly by Wire Flight Controls, (AFCS) Automated Flight Control Systems with Coupled Navigation, FADEC (Full Authority Digital Engine Control System), and HUMS (Health and Usage Monitoring Systems) to name a few. All these advances have meant that harnesses have grown in both size and complexity.

Aircraft wiring harnesses can be quite large, 20-30 feet in length, weigh 50-75 lbs. with multiple branches or break outs. Routing harnesses of this size and complexity is time consuming and tedious since all of this wiring must pass through penetrations in metallic structure that are, on average, 3.5 inches in diameter. Harnesses are secured from point-to-point by MS cannon plugs typically with connector back shells for added strength and to eliminate tension on individual wires and are held in suspension by padded clamps mounted to provisions on the airframe.

Spring-Fast grommet edging by Device Technologies is made from spring steel encapsulated in an abrasion resistant polymer and snaps on without glue.
Spring-Fast grommet edging by Device Technologies is made from spring steel encapsulated in an abrasion resistant polymer and snaps on without glue.

An average harness may be routed through as many as 15-20 penetrations in the air frame structure and all of this must be performed without strain, abrasion or cracking of the insulation and protective braiding. When harnesses are routed and suspended in space they are very often in close proximity to adjacent structures, so using anti-chafe protection, a.k.a. grommet edging, is critical to protect the wiring integrity.

EWIS is Born

Armed with the wiring discrepancy data the FAA created regulations for EWIS or the Electrical Wiring Interconnection System. EWIS is the recognition by the FAA that wire and its associated components must be treated as a critical aircraft system.

The official definition of EWIS according to CFR.14 Part 25.1701 is: “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…. the term “wire” means bare and/or insulated wire used for the purpose of electrical energy transmission, grounding or bonding. This includes electrical cables, coaxial cables, ribbon cables, power feeders, and data-buses.”

Essentially EWIS recognizes that the wiring and associated components for a flap actuator are as critical to the safety of flight as the flap actuator itself.

EWIS – Separation and Anti-Chafe Requirements

The broad goal of EWIS is stated in CFR14.25.1707 (I): Each EWIS must be designed and installed so that there is adequate physical separation between it and other aircraft components and aircraft structure, and so that the EWIS is protected from sharp edges and corners, to minimize potential for abrasion/chafing, vibration damage, and other types of mechanical damage.

Wires suspended within the airframe are clamped every 14-18 inches and require a minimum .375” clearance between the wire/cables and the metallic structure. However, in reality the harnesses move over time, gust loads and maneuvering may cause them to shift out of their nominal position or sag/droop in some places while pulling and creating tension at others. This typically results in diminishing bend radii which can cause the wires to come in contact with the metallic structure.

Wire Anti-Chafe Protection 101

According to the FAA and EWIS Best Practices, “Grommets suitable for the environment must be used when the wire bundle passes through pressure bulkhead, firewall, and other openings in the structure. The grommet should cover the entire edge and come together at the top of the hole”

On many aircraft, wire anti-chafe protection has long been provided by the MS21266 nylon caterpillar grommet. These nylon strips have no natural retentive strength so have to be bonded to the penetrations with adhesive. Operationally, many of you will know this is a multi-step process. Bond preparation can take on average 20 minutes for contact cement or longer for an acrylic adhesive. Then the grommet must be applied and held in place with masking tape or clamps until the adhesive is fully cured.

Maintenance operations using this method are very challenging because wiring is often hidden deep in the skeleton and fabric of the plane. At most, only 15-20 percent of aircraft wiring can be readily accessed or visually inspected by maintenance engineers or inspectors. Even if the wiring can be accessed, the harnesses in place make the inspection and the bonding/taping/clamping operations far more difficult.

The use of adhesive requires H&S best practices as the adhesives most commonly used are Pliobond Contact Cement and 3M 1300L, both of which contain VOC/HAP, specifically the highly toxic MEK (Menthol Ethel Ketone). This requires ventilation or breathing apparatus depending on the situation.

Historically, this adhesive bond method has a high Cost of Poor Quality as one out of 10 required in process rework mostly due to dis-bondment and “egg-shaping” by the inflexible nylon grommet. This system relies on the reliability of the bond and the quality of the contact cement to protect aircraft.

Latest Technology – EWIS Ready

The alternative is a product like Spring-Fast grommet edging by Device Technologies, which snaps on without glue and is made from spring steel encapsulated in abrasion resistant polymer which gave it retentive strength, anti-chafe and dielectric qualities as well as half the total install cost. Many OEMs and MROs are switching to it.

EWIS has put increased emphasis on the wiring that every aircraft relies on. The data shown proves it is of critical concern. Every aircraft owner is required to make significant investments in safety and convenience upgrades. If you are adding ADS-B or in flight broad band WiFi or just doing standard maintenance, don’t forget to assess your EWIS to protect your investment and keep it flying safely. EWIS protection and compliance are critical components in safeguarding your wiring and your equipment.