Description

This article provides general guidance on electrical fires and other electrical malfunctions in transport-category aircraft The information herein does not supersede type-specific guidance in a company's Standard Operating Procedures (SOPs), Flight Operations Manual (FOM), Quick Reference Handbook (QRH), or other official publications.

Electrical Component Failure

In modern, highly automated aircraft, most electrical component failures do not cause the loss of electrical busses and the equipment powered by those busses. Typically, systems are designed so that if one generator fails, another generator picks up the load automatically. The QRH procedure may be as simple as disconnecting the failed generator from its constant-speed drive (CSD). Other malfunctions involve simple resets. The first step for a failed transformer-rectifier unit (TRU) may be to place the TRU switch to the OFF position, wait a few seconds, and then return it to the ON position.

When disconnecting a failed generator or resetting a failed TRU or inverter, it's always good crew resource management (CRM) to have the other pilot confirm that you have your hand on the correct switch. This avoids creating further problems by turning off an operating component.

System design usually isolates faults automatically, so that a malfunctioning generator or other component cannot damage other equipment with voltage interruptions or spikes. In previous generations of aircraft, isolating a faulty electrical bus could require a flight engineer to pull circuit breakers or even physically remove current limiters from electrical panels.

However, even in the newest aircraft, some component failures such as failed power distribution assemblies can render equipment inoperable. QRH procedures will normally list the inoperable equipment associated with a given failure and describe the impact on aircraft performance. Although items such as thrust reversers and ground spoilers might be hydraulically powered, they are usually electrically controlled. Therefore, an electrical malfunction can cause loss of those items. This, in turn, can impact performance such as landing distance, and the QRH will include relevant performance data.

In these situations, crews will need information from multiple places within the QRH. The initial malfunction might direct crews to a list of affected equipment, then to an aircraft performance table, and then to a modified landing checklist. The flight deck of an emergency aircraft is not the place to read these procedures for the first time. Pilots should study their QRH during initial training and review it periodically.

In the event of serious electrical malfunctions, an auxiliary power unit (APU) can sometimes power the entire electrical system, and the QRH will provide relevant guidance. Some APUs have altitude and temperature limits, so the crew may have to descend before starting the APU and using its generator. After loss of a single engine-driven generator, crews should consider starting the APU in flight as a precaution. In this situation, the APU generator can serve as backup in case another generator fails.

If a modern transport aircraft loses all main generators, it goes into an emergency electrical configuration, and a ram air turbine (RAT) provides emergency power. A RAT consists of a small propeller or turbine that deploys into the relative wind and drives a small generator. In most electrical systems, the loss of essential AC and/or DC busses will trigger the RAT to deploy. RAT generators typically have a much lower capacity than engine-driven generators, so they will power only the most critical equipment. Pilots should know what equipment will work when the aircraft is powered only by the RAT. The list is usually short: perhaps one radio, one electrically powered hydraulic pump, standby flight instruments, partial flight deck lighting, and flap control. (Many, if not most, aircraft have a mechanical means of emergency landing gear extension.)

RATs often have airspeed limits. For example, the Embraer 170 RAT requires an airspeed of 130 knots to power the AC essential bus. To help ensure adequate airspeed, RAT deployment in the E170 limits flaps to a lesser position than normally used for landing. RAT operation is usually quite loud, and simulator training should help pilots prepare for the distraction and startle factor associated with RAT deployment. 

When a RAT deploys, it needs a few seconds to spin up and begin producing power. To avoid complete power loss during the brief spinup period, aircraft batteries can power essential DC busses for a short time. Some aircraft are equipped with DC-powered inverters that can supply limited AC power during battery-only operation. Pilots should be aware that without any source of charging power such as a RAT generator, battery life can be very short--as little as ten minutes.

Emergencies that trigger the RAT, such as failure of all engines with the resulting loss of the engine-driven generators, are rare. Apart from engine failures, other malfunctions can cause loss of main generators. For example, an Airbus notice from March 2025 described events when A320-family aircraft lost generators due to worn components found inside their CSDs. The article described how worn CSDs can cause frequency regulation problems, and the article discussed preventive maintenance that could detect the problem in advance. In one case study, an A319 crew lost both generators but managed to reset them.

Battery thermal runaway can create an emergency. QRH procedures for a battery overheat often require landing at the nearest suitable airport. The U.S. Federal Aviation Administration (FAA) defines thermal runaway as "rapid self-sustained heating of a battery cell driven by a chemical reaction of the materials within the cell where energy is released in the form of light or heat." FAA Advisory Circular 120-80B, "Firefighting of General and High-Energy In-Flight Fires," says thermal runaway "is generally evidenced by a sharp increase in temperature and pressure and a drop in cell voltage."

QRH procedures for some electrical malfunctions may direct crew members to open or reset circuit breakers. Modern aircraft have electronic circuit breakers in addition to the standard pull-type thermal circuit breakers. Electronic breakers are controlled through the aircraft's multifunction control and display unit (MCDU), so flight crews should familiarize themselves with how to access the relevant MCDU pages.

Pilot paging through MCDU functions. (SKYbrary photo by Thomas Young.)

In many cases, minor electrical problems at the gate, such as nuisance engine indicating and crew alerting system (EICAS) messages, can be cleared by powering down the aircraft, waiting a few minutes, then re-powering the aircraft. This should be done under QRH guidance or instructions from maintenance.

Electrical Fire/Smoke in the Cabin or Flight Deck

FAA Advisory Circular 120-80B, referenced above, says indications of hidden electrical fires include:

• Abnormal operation or disassociated component failures.
• Tripped circuit breakers.
• Hot spots on the floor, sidewall, ceiling, or other panels.
• Fumes,
• Visual sighting of smoke.

In some cases, crews may have no direct fire indication. A 2016 incident involving an Embraer 190 on a flight from Boston to Toronto provides a case in point. The autopilot disengaged, three of the five electronic flight displays went blank, and multiple electrical malfunction EICAS messages appeared. Eventually, the crew restored electrical power and landed safety. The report by the Transportation Safety Board of Canada (TSB) said an examination revealed substantial fire damage in an avionics compartment. The report said the fire resulted from an unidentified fluid that had come into contact with an alternating current bus bar. "Within 36 seconds of the initial fault, power was lost to all main bus bars, and as a result, the smoke detector in the recirculation bay and the recirculation fans lost power," the report said. Due to the loss of the smoke detector, the pilots received no fire warning.

Some QRH procedures for smoke of unknown origin inside the aircraft assume that the source is an electrical fire. For this reason, QRH guidance for smoke in the cabin and the guidance for an electrical fire often look similar. These procedures can involve a lengthy process of powering down an electrical bus, observing whether the smoke stops, and re-powering the bus if it is not the smoke source. Then the procedure calls for powering down another bus, and so on. 

In this situation, as in any emergency, situational awareness and crew judgment become important. If a suitable airport is close by, an immediate landing could be a better option than continuing with a long checklist or QRH procedure.

If a suitable airport is not close, the crew must make every effort to identify the fire source. Following an electrical fire checklist or QRH procedure to the end requires close attention to each step in a moment when stress level and workload are high. Here again, this is not the time for a first reading of the procedure. Pilots should review the procedures for electrical fire and smoke elimination now and then, as these procedures can be complex. In simulator training, pilots have been known to lose their place in the procedure and take steps that are either inappropriate or redundant. Familiarity with the QRH can reduce this hazard.

The first step in an electrical fire checklist is typically a memory item: don oxygen mask/smoke goggles. Donning the mask can be disorienting if pilots have not practiced it. If a pilot wears glasses, the glasses can be knocked off. This makes a dangerous situation worse if a pilot must search for glasses in a smoke-filled flight deck. Even if a pilot does not wear glasses, the mask can limit peripheral vision. In addition, the use of interphone and radios through an overhead speaker, coupled with the hiss of oxygen, can become a barrier to communication. For all these reasons it's a good idea for pilots to practice donning the mask and to get more comfortable wearing it. The cruise phase of a long flight can be a good time for conscientious crew members to rehearse this step.

In the event of smoke or fumes in the aircraft, do not delay donning the mask. Delay can result in incapacitation. If the smoke and fume elimination procedure is not a required memory item at your operator, consider making it a personal memory item for yourself. Smoke can make it difficult to read a checklist or QRH. Know your flight deck's switchology well enough to find items such as the pressurization dump switch by feel.

When an electrical fire is identified and located, make sure flight attendants or other crew members fight the fire immediately and aggressively. Appendix D of AC 120-80B lists the time that various crews had from the first indication of fire to the time when the fire became catastrophically uncontrollable. The times range from seven to 26 minutes.

Know your aircraft. As suggested in the above-mentioned AC, crews should understand the volume of overhead, above-ceiling space in an aircraft to combat fires in that area. Know what panels and hatches can be opened to spray extinguishing agent into underfloor and sidewall compartments, as well.

For more detail, please see the SkyBrary article on electrical fires.

Further Reading

• U.S. Federal Aviation Administration (FAA) Advisory Circular 120-80B, "Firefighting of General and High-Energy In-Flight Fires," March 2023.
• U.S. Federal Aviation Administration (FAA) Advisory Circular 25-16, "Electrical Fault and Fire Prevention and Protection," April 1991.