Aircraft Electrical Wiring Interconnection Systems (EWIS): A Comprehensive Guide

Posted on May 3, 2024 in Technology

EWIS

An EWIS (Electrical Wiring Interconnection System) encompasses all wires, wiring devices, and termination devices installed within an aircraft for the purpose of transmitting electrical energy. Examples of EWIS components include:

  • Wires and cables
  • Bus bars
  • Connectors
  • Electrical grounding and bonding devices
  • Electrical splices

Bonding and Grounding

Bonding and grounding are crucial aspects of aircraft electrical system maintenance. Inadequate bonding or grounding can lead to various issues, such as:

  • Incorrect operation of systems
  • Electromagnetic interference (EMI)
  • Electrostatic discharge
  • Personnel shock hazards
  • Damage from lightning strikes

Grounding

Grounding involves electrically connecting conductive objects to a conductive structure.

Bonding

Bonding refers to the electrical connection of two or more conducting objects (e.g., connection for refueling aircraft-truck).

Ground return connection: It’s essential to ensure an adequate connection with negligible voltage drop.

EWIS Inspection

EWIS inspections aim to identify potential faults caused by factors like aging, chafing, chemical contamination, and improper routing of wires and avionic components. Simple corrective actions can mitigate degradation and extend the functional life of the EWIS.

Key focus areas during EWIS inspections include:

  • Connectors
  • Clamping points
  • Terminations
  • Backshells
  • Damaged sleeving and conduits
  • Grounding points

Wire Identification

Identifying electrical wires and cables is essential for facilitating maintenance tasks and ensuring the safety of both the aircraft and maintenance personnel. Wires are typically identified with printed markings along their length, often consisting of a part number (P/N) followed by a CAGE (Commercial And Government Entity) code, which is a five-digit manufacturer identifier. This allows for easy identification of cable specifications and prevents the use of incompatible cables.

Placement of Identification Markings

Identification marks should be placed at the ends of the wire and at intervals of no more than 15 inches along its length. Wires shorter than 3 inches do not require identification, while those between 3 and 7 inches can be marked at a midpoint. Markings must be visible and can be printed horizontally or vertically.

There are two primary marking methods:

  1. Direct marking: Printing directly on the outer covering of the wire or cable.
  2. Indirect marking: Printing on a heat shrink sleeve, which is then installed over the wire or cable.

These markings should be present from end to end, at the ends, and at intervals no greater than 6 feet. Individual wires within a cable should be identified within 3 inches of their termination.

Types of Wire Markings

The preferred method is direct marking without damaging the insulation. Special sleeves are often used for Teflon-coated wires, shielded wiring, multi-conductor cables, and thermocouple wires.

Several methods can be used for direct marking, including hot stamp marking, inkjet printers, and laser jet printers. Markings should be legible and contrast with the color of the cable or sleeve. Hot stamp marking can damage thin insulation and potentially cause electrical arcs during operation.

If hot stamping is not feasible, flexible, transparent or opaque sleeves with associated color codes can be used. For high-temperature environments (over 400ºF), materials like silicone fiberglass are recommended. Sleeves are commonly used for bare shielded wire, thermocouple wire, coaxial cable, multi-conductor cable, and high-temperature wire. In some cases, identification tape can be used instead of sleeves.

Polyolefin sleeves are suitable for areas exposed to synthetic hydraulic fluids and other solvents. Sleeves can be secured using cable ties or heat shrinking.

Weighing

Before flight, pilots need to know the aircraft’s weight and center of gravity. To calculate this, they require information about the empty weight and center of gravity (empty weight CG).

Engine Starting and Operation

Several factors must be considered before starting an aircraft engine:

  • Position the aircraft into the wind to ensure proper airflow.
  • Ensure the area is clear to prevent property damage or personal injury from blast or jet exhaust.
  • Have a CO2 fire extinguisher readily available during starting procedures.

Turboprop Engine Starting

  1. Turn on the boost pumps and ensure the power lever is in the start position.
  2. Place the start switch in the “start” position to begin engine rotation.
  3. Turn on the ignition switch and monitor engine exhaust temperature, oil pressure, and temperature lights.
  4. Maintain the power lever at the start position until the specified minimum oil temperature is reached.
  5. Disconnect the power supply if used.

Turbofan Engine Starting

Before Starting:

  1. Remove all protective covers.
  2. Clear the run-up area of personnel, loose equipment, and foreign object debris (FOD).
  3. Check aircraft fuel sumps for water or ice, and inspect the engine intake for FOD.
  4. Visually inspect fan blades, compressor fan blades, Inlet Guide Vanes, etc.
  5. Verify the correct operation of engine controls, instruments, and warning lights.

Starting Procedure:

  1. Place the power levers to “idle” position.
  2. Turn on the fuel boost pumps switch to deliver fuel inlet pressure to the engine fuel pump.
  3. Turn on the engine start switch to initiate engine rotation and monitor oil pressure.
  4. Turn on the ignition switch and move the start lever to idle or start position.
  5. Observe the rise in EGT, fan rotation or N1 reading, and oil pressure to confirm engine start.
  6. Turn off the starter at proper speeds and ensure engine parameters remain within limits after stabilizing at idle.

Hot Start

A hot start occurs when the EGT exceeds limits due to excessive fuel or insufficient airflow.

  1. If EGT rises abnormally fast, quickly reduce fuel or power lever to cut off before the needle reaches the red zone.
  2. If the needle remains in the red zone for more than 16 seconds, the engine must be replaced.

False or Hung Start

  1. The engine starts but RPM remains low instead of increasing to normal starting RPM.
  2. This could indicate insufficient power to the starter, requiring engine shutdown.

Engine Will Not Start

Possible causes include lack of fuel, insufficient electrical power, or incorrect fuel mixture.

  1. If the engine fails to start within the prescribed time, turn it off and ensure fuel and ignition are off.
  2. For another attempt, perform dry motoring (rotating the compressor without fuel) for 15 seconds to remove accumulated fuel.
  3. If dry motoring is not possible, allow 30 seconds for fuel drainage before attempting another start.