A plane carrying 76 passengers suffered a major electrical failure just after taking off from Heathrow Airport, a report has revealed.
The plane had reached 20,000ft after leaving Heathrow
The British Airways (BA) jet lost a number of important systems in what the Air Accidents Investigation Branch (AAIB) called a "serious incident".
Most of the affected systems were restored in about 90 seconds and the plane continued to Budapest.
Nine safety recommendations were made as a result of the incident in 2005.
The AAIB said it was unable to determine the cause of the incident due to a lack of evidence.
The report revealed the crew reported an audible "clunk" and the flight deck became very dark soon after take off from the west London airport on 22 October, 2005.
Both pilots' primary flight displays and navigation displays went blank, the VHF radio and intercom failed, most of the cockpit lighting went off and cabin lights went out momentarily, the report said.
Maintaining control the 53-year-old pilot used the horizon for reference as the standby instruments were "difficult to see in the poor light".
The 29-year-old co-pilot struggled to locate switches on the overhead panel due to the poor light and communications between the pilots were hampered by the loss of the flight interphone.
Thursday's report also identified that the flight crew had not received any formal training on how to operate this type of plane solely by using standby instruments.
The engineer in Budapest, who was not a BA employee, also failed to investigate the symptoms of the incident despite being told by the captain and it being in the air safety report (ASR).
The full extent of the incident was only realised when the original copy of the ASR arrived at Heathrow by post, the report revealed.