Cockpit blackout

Rajmund Kozma was worried about what would happen if a Total Glass Cockpit Blackout, i.e. a simultaneous blackout of all electronic display units (CRT or LCD displays), were to occur during a flight. This is an understandable concern as there have been some such incidents.

Airbus, the supplier of Finnair’s fleet, has recorded 50 incidents in which three or more of the six main display units (DUs) have blacked out simultaneously durintg the last 25 years. The blackout of all six display units has occurred only seven times.

In an aircraft, the likelihood of a major electronic defect is in the range of 10-5, or one failure per 100,000 flight hours. In most incidents where all display units have blacked out, the reason has been a double defect – two defects happening at the same time. As the likelihood of such a defect should be extremely small, the electric system needed modification. The conclusion was that the main reason was the fault tolerance of one of the main buses (AC 1). In May 2007, Airbus published Service Bulletin SB A320-24-1120,9 which included a request to make a modification (37317). The modification improved the ability of the aircraft’s electric system to adjust to any failures in the AC 1 bus coupler.

Another significant point is that none of these 50 incidents led to a disaster. In most blackout cases, only half of the display units have blacked out. In this case, one of the pilots has all display units available. If all display units black out, the plane can be flown to the closest airport by using the so-called standby instruments on the captain’s side of the cockpit. The standby instruments consists of an artificial horizon, airspeed and altitude indicators as well as a fluid compass. The instruments get its data from different sensors than the main displays do and the required electricity comes directly from batteries.

An aircraft could depart to a commercial flight even if one of the six display units did not function. With the cockpit switches, data related to the current flight phase can be fed to the remaining display units.

The cause of the blackout of several display units is likely related to power supply. In a two-engine aircraft, both engines have generators which can supply power independently to all systems in the aircraft. The electric system is roughly divided into two separate systems. In case of a defect, one generator can feed power to both sides. However, if there is a short-circuit or a similar situation, the other side (or at least the single defective bus) must be kept separated. In this case, some monitoring instruments are left without power. Nevertheless, as for their power supply, all devices using AC and DC current have been divided logically so that the flight can be continued safely. For this reason, an aircraft always has at least two of all essential devices. If there is a defect in both generators, power supply can be ensured with the so-called APU (Auxiliary Power Unit), which is located in the tail cone of the aircraft. Even if the APU was out of order, there is still two options: a generator can be extended from the fuselage, generating energy in a windmill-like manner. In addition, some devices can be powered directly by batteries. Depending on the type of the aircraft, there are two or three sets of main batteries.

Consequently, the likelihood of being a passenger in an aircraft experiencing a total cockpit blackout is very, very small. And even if such blackout occurred, it would hardly cause a disaster.

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