A water leak in the galley of a Qantas 747 jumbo jet knocked out electricity to a series of systems during a flight to Bangkok, leaving them just minutes to land on battery power.



Water flooding in the galley knocked out three of the four on-board power generators, including all AC-powered systems, as the plane was on descent 25 kilometres from Bangkok airport.



Air investigators say that if the flooding had occurred more than 30 minutes' flying time from an airport or had there been a delay in landing, it would have placed the plane — carrying 346 passengers and 19 crew — "at considerable risk" as more systems were likely to fail.



The leak had already knocked out power to radar, cabin pressurisation, autopilot, auto-throttle and some fuel pumps as the plane descended from 21,000 feet and passed through 10,000 feet.



The batteries, which were designed to last a minimum of 30 minutes before depletion, had been running for 21 minutes when the aircraft landed and taxied to the terminal building, investigators found.



The drama began when the galley flooded on the Qantas Boeing 747 due to ice formation that blocked a drain pipe after an anti-ice heater failed.



A build-up of water flowed forward and cascaded through a decompression panel into the aircraft's main equipment centre, leaking onto three of the jumbo's four generator control units, causing them to malfunction and shut down.



The 114-page Australian Transport Safety Bureau report released today into the January 7, 2008 incident said plane was reliant on its battery back-up for the last 15 minutes of flight.



"The limited battery power available restricted the amount of time that the aircraft's remaining functional instrumentation and communication systems were available to the crew, which necessitated an expedited descent and landing in order to reduce the risk of those systems failing," investigators said.



But as the plane descended, pilots abandoned responding to computer-error messages on a working screen and reported they did not have enough time to refer to aircraft manuals to diagnose problems.



Investigators found Qantas' quick-reference handbook for pilots did not contain information on how long the reserve batteries would last or what they should do to restore systems.



"The flight crew manuals did not contain information on means to extend the limited battery life or on managing the aircraft if the batteries were depleted," investigators said.



Pilots could only obtain pressure readings for the Number 4 engine, as the sensors to other engines were inoperable when the other three AC power units failed.



Despite it being Qantas policy for the pilots to declare an emergency landing - either a "Mayday" or "Pan Pan" - the pilot in command did not do so.



The captain believed there were enough remaining systems, including hydraulics and anti-skid brakes, and a clear runway in daylight in good weather, to make the plane manageable.



But investigators found that pilots should have declared an emergency so as not to potentially delay the landing.



The pilots managed to land the ailing craft safely without injuries, but when the engines were shut down, the cabin lighting cut out and the doors could not be opened because there was still partial pressure inside the cabin, until a pressurisation outflow valve was opened.



"In response to the event and the ATSB investigation, Qantas, Boeing and the US Federal Aviation Administration (FAA) have implemented a number of safety actions to prevent a recurrence," the safety bureau said.



"These actions include reinforcing protective 'dripshields' above electrical equipment, improving maintenance practices and pilot training and installing advanced standby flight instruments to Qantas 747 aircraft.



"Despite these safety actions, the ATSB found that the FAA's regulatory and guidance information does not fully address the potential harm to flight safety posed by liquid contamination of electrical system units in transport aircraft.



"As well, the information provided to 747-400 flight crews regarding standby power operations is limited. The ATSB has made recommendations to the US FAA and Boeing to address those safety issues.".



Responding to the investigators' report, Qantas today said the flight crew effectively managed what was "an extraordinarily rare set of circumstances".



Qantas "took an active approach to identify and address relevant aircraft design and other issues that created the potential for the event," a spokesman said.



The airline updated flight-crew training and information resources to include similar scenarios, he said.



"Boeing, as the aircraft manufacturer, and some regulatory agencies were also asked to take action on several safety issues," he said.

Meanwhile, A rumour broadcast that the Qantas A380 superjumbo that suffered a mid-air engine explosion last month had now been written-off due to structural damage and would be donated to a Singapore museum was "completely untrue", Qantas spokeswoman Olivia Wirth said yesterday.



"[The] aircraft is still in Singapore and will be repaired by Airbus before returning to the air."