Report cites loss of crew's displays and communications

Airbus has received four recommendations for modifications to its A320 series electrical systems following an incident in which an A319 crew lost voice communications and all the captain's electronic flight instrument system (EFIS) displays. In the absence of these systems the crew had to use the emergency undercarriage deployment system to prepare for landing.

The UK Air Accidents Investigation Branch (AAIB), looking into a previous incident, found extensive electrical malfunctions had occurred five times previously.

The first event the AAIB investigated occurred in October 2005 when a British Airways A319 crew lost both pilots' primary flight displays (PFD) and navigation displays (ND), plus the upper of two electronic centralised aircraft monitor (ECAM) screens.

The crew also lost all cockpit lighting and communications, and were left with only one operating ECAM. However, prompt use of the ECAM checklist quickly regained many of the lost services. Now the AAIB, reporting on a 15 September EasyJet A319 flight from Alicante in Spain to Bristol in the UK, says the crew lost fewer services but, unlike the BA crew, could not regain any of them.

The EasyJet problems were caused when an internal fault in the No 1 generator control unit (GCU) was interpreted by its monitoring system as an external fault in the No 1 generator line contactor, resulting in automatic isolation of one of the two main AC busbars.

The event happened in the cruise near Nantes, France. Before take-off, the No 1 engine-driven generator (EDG) had been selected off because of a fault, so the crew were operating with the No 2 EDG, plus the auxiliary power unit (APU) generator. The effect of the GCU fault was to open the No 1 bus tie connector automatically, which disconnected the APU generator supply from the No 1 main AC busbar that feeds the No 1 DC busbar.

This blacked out the captain's EFIS and multipurpose control and display unit, the upper ECAM display, disconnected the autopilot and autothrust, switched off the caption and integral illumination lights on the overhead panel and some on the centre pedestal, and caused the flight control law to change from normal to alternate, says the AAIB.

The captain, who was flying, had only the standby instruments operating on his side, so he handed control to the co-pilot, whose PFD and ND were operating. He tried to transmit a Mayday call, but none of the radios worked and the main transponder was not powered.

By carrying out the ECAM checklist, the captain successfully powered up the alternate transponder and entered the 7700 emergency code. He continued as flight-planned to Bristol, where the aircraft landed safely following deployment of the landing gear using the emergency system.

What Airbus has been told

The AAIB has recommended that Airbus should:

  • Revise the logic of the [A320 series] GCUs to prevent the monitoring system from incorrectly interpreting a fault within the GCU.
  • Design an automatic transfer of electrical feed to the AC essential busbar in the event of the loss of the No 1 main AC busbar.
  • Segregate more effectively the electrical supplies for radios.
  • Warn A320 series pilots that the risk of losing communications is higher than most realise. 

Read Kieran Daly's blog on the British Airways incident.


 

Source: Flight International

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