Electrical Fires

Description

Electrical fires on aircraft originate from electrical components. An immident electrical fire often can be detected by indirect indications or alarm-system sensors before the fire ignites. It is also possible that electrical fires will remain hidden to the flight crew or cabin crew for a prolonged time. in those situations, the electrical fire may become apparent too late for crewmembers to take action to eliminate the threat.

Because modern commercial transport aircraft typically contain miles of wire inside electrical cables, looms and wiring harnesses (most of them in hard to reach places) the most frequent cause of electrical fires is some type of insulation issue, such as:

  • Worn insulation — Mainly due to ageing and exposure to heat, worn insulation may be more susceptible to catching fire from overheating; worn insulation also may fall off wires, exposing the metal conductor to short-circuiting risks;
  • Torn insulation — Tearing is typically due to improper cable installation that may lead to the cable being repeatedly under mechanical stress, ultimately resulting in the conductive metal being exposed;
  • Contaminated insulation — When soaked with oil from any source, insulation is also more likely to catch fire (e.g. from electrical overheating); and,
  • Breaks in insulation — Exposed metal conductors (wires touching) inside cables may result in a short circuit or an electrical arc or spark.

Imminent or smouldering/burning electrical fires may be indicated directly or indirectly by the following clues:

  • Over-voltage warning
  • Higher than normal electrical load
  • Popped circuit breaker
  • White smoke and acrid smell of hot or burning insulation; note, however, that these clues may also be caused by cable/wiring insulation being burned by some other type of fire.

Defences and Best Practices

Prevention is the best countermeasure against electrical fires, including these actions:

  • Proper aircraft maintenance - Cables should be checked for signs of ageing or damage at regular intervals and replaced as necessary.
  • Lead acid storage batteries containment — These batteries must be contained in an approved battery box with the lid vented to the outside of the aircraft to prevent explosive hydrogen gas from entering the aircraft.
  • Electrical component replacement — When replacing electical equipment (e.g. light bulbs), only regulator-approved components should be used. Also, the maintenance engineering should make sure the new component fixture is complete and intact.
  • Pre-flight inspection — A thorough inspection may reveal evidence (e.g., by sight or smell) of leaked fuel, oil, or hydraulic fluids.

When an in-flight electrical fire is suspected or indicated, common flight crew responses consider:

  • Emergency landing — Landing immediately at the nearest suitable aerodrome — per international best practices — is usually the best course of action if a fire is suspected, even if there are no visible flames.
  • Fire characteristics — Once ignited, electrical fires burn just like most any aircraft fire. One difference is that the heat from the energized electrical wiring or component that ignited the fire might sustain it by continued ignitition. Therefore, the common first step when electrical fire is suspected is to cut the power by turning off the master switch. This removes the ignition source, and if an imminent fire has not started to burn, may lead to elimination of ignition threat.
  • Troubleshooting — If electrical power is essential for the flight, some emergency checklists authorize the flight crew to attempt to identify and isolate the faulty circuit by:
    • Turning all individual electrical switches OFF.
    • Turning the master switch back ON.
    • Selecting electrical switches that were ON before the fire indication, one at a time, permitting a short time lapse after each switch is turned ON to check for signs of odor from fumes, smoke or sparks.

(Note that this procedure may have the effect of recreating the original problem. On the other hand, lack of electrical power would require a no-flap landing, manually lowering the landing gear, no normal communication capability and no navigation capability, which could increase risk factors, especially if flying at night or instrument meteorological conditions. See Further Reading below for current best practices in designing emergency checklists for smoke-fire-fumes events.)

  • Deploying extinguishers — If flames from an electrical fire have already started, cutting the power supply will not be enough, and immediate use of some sort of extinguishing equipment will be necessary. The crew's initial focus should be on aggressively extinguishing the fire with a readily available extinguisher. Water must not be used if the crew believes the fire is of electrical origin.
  • Declaring MAYDAY — If time permits, the flight crew should communicate with air traffic control (ATC) before powering down the electrical system for the following reasons:
  • Position-reporting beacon — By activating the emergency locator transmitter (ELT), which has its own self-contained power supply, the flight crew automatically will help guide the SAR teams to the location of a forced landing, should that become necessary.
  • Portable NAV/COM — On smaller aircraft, carrying a handheld radio transceiver, with integrated or separate handheld GPS navigation can be very helpful if the aircraft electrical system needs to remain off for optimal safety.
  • Evacuating smoke/fumes — Managing the smoke is another important part of dealing with an electrical fire. There is no universal solution. Venting the smoke may improve breathing and vision, but the process may also feed the flames oxygen. If the latter situation happens, the flight crew's best option is to close the vents. Current best practices for aircraft operators to update smoke-fire-fumes checklists on their fleets, as noted, also address the mitigation of toxic fumes.

Accidents and Incidents

Fire - Electrical Origin

Electrical Fumes

Related Articles

Further Reading

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