Electrical Problems: Guidance for Controllers

Electrical Problems: Guidance for Controllers


This article provides guidance for controllers on what to expect and how to act when dealing with aircraft experiencing the consequences of electrical problems and related aircraft systems' malfunctions. There are some considerations which will enable the controller, not only to provide as much support as possible to the aircraft concerned, but also to maintain the safety of other aircraft in the vicinity and of the ATC service provision in general.

Useful to Know

Depending on the severity of the electrical failure(s) the consequences could be various, ranging from isolated system or subsystem malfunctions and navigational problems to failures having adverse effects on the aircraft's handling and performance. Historically, the electrical failures often result from interconnection breakdown between aircraft systems. For example, a problem with one system could lead to a bus bar failure potentially resulting in a complete or partial failure of an airplane's avionics system.

An electrical problem may be the first indication of a fire.

Modern jet transport aircraft are designed and equipped with at least three AC generators (alternators) of equivalent capacity, one of which will be powered by the Auxiliary Power Unit (APU). There will also be other methods of generating AC power such as a hydraulically powered generator or a ram air generator and the ultimate backup of DC power from at least one main battery. If one of the principal (engine-powered) generators fails, the other generator(s) supply power to the main AC bus bars. In case of failure of more than one of the main generators or their associated motive power (or a single failure following an Acceptable Deferred Defect dispatch with an inoperative main generator or APU), it may be possible to use a hydraulic system to activate a hydraulic motor-driven emergency generator or to deploy Ram Air Turbine. Different aircraft manufacturers use different sources for back up AC power, e.g back up generators on the Boeing 777 and Ram Air Turbines (RAT) on Airbus types. In a worst case scenario, where these emergency/back up generators fail and the main battery, which has a declared endurance based on specified maximum electrical loading, is depleted, the aircraft becomes electrically unpowered.

It is important to emphasize that a serious electrical problem can, under specific circumstances, constitute a high-risk scenario.

Anticipated Impact on Crew

A wide range of practical problems could arise following onboard electrical failure(s). Depending on the type of failure(s), whether it includes loss of all generators (alternators) and battery power only available (power supply reduced to emergency level), some possible effects on crew are:

  • Increased workload. Crew determining the nature and the severity of the problem.
  • Turning off non-critical electrical items (such as second radio, passenger cabin lighting and recirculation fans and other non essential electrical systems) in order to isolate and identify the source of the problem and / or to reduce the electrical load.
  • A decision to land at the nearest/most suitable airport.

The worst case related scenario is an on board fire in flight which is caused by an electrical fault and cannot be contained readily by the crew.

What to Expect

  • Navigation problems. Commercial aircraft are equipped with stand by instruments which are either mechanical or independently powered. In general, these instruments provide attitude, altitude and airspeed information and have limited or no navigation capability.
  • Communication loss if the malfunctions affect the radio equipment.
  • Loss of Transponder temporarily or completely if it is necessary to reduce electrical load or a failure has occurred on the channnel powering the in use transponder.
  • Limited readback. Expect crews to minimise the readbacks and possibly to acknowledge ATC instructions by keying the microphone.
  • Level changes to maintain VMC.
  • Manual gear extension.
  • Approach and landing without landing lights

What to Provide

Best practice embedded in the ASSIST principle could be followed: (A - Acknowledge; S - Separate, S - Silence; I - Inform, S - Support, T - Time)

  • A - acknowledge the emergency, provide position information and suitable vectors if navigational problems are reported.
  • S - separate the aircraft as necessary, expect the aircraft to request level change in order to maintain VMC.
  • S - silence the non-urgent calls (as required) and use separate frequency where possible.
  • I - inform the airport emergency fire rescue services and all concerned parties according to local procedures; as tower controller expect airport authorities to execute their emergency plan; inform the supervisor and other sectors/units concerned.
  • S - support the flight by providing any information requested and necessary such as next suitable aerodrome, type of approach, runway length and aerodrome details, etc.
  • T - provide time for the crew to assess and deal with the emergency, don’t press with non urgent matters.


When informed about possible/actual electrical failure, be ready to:

  • Inform the pilot about nearest suitable aerodrome and provide radar vectors as necessary.
  • Provide airfield and weather information .
  • Coordinate and arrange (if applicable) type of approach desired by the crew.
  • It is important to avoid a go-around because of ATC action, whether by direct or indirect cause.
  • If practicable keep the flight clear of IMC.
  • Arrange for Ground Power on arrival (GPU).

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