shoey1976
4th Sep 2008, 14:00
BBC COPY
The British Airways Boeing 777 that crashed at Heathrow in January was PROBABLY brought down by ice in its fuel system according to the latest findings of a report by the Air Accident Investigation Branch.
The pilots of the plane managed to get it down safely, and 136 passengers and 16 crew escaped without serious injury.
The AAIB now believes the flow of fuel dropped shortly before the engines on the plane lost power -- at 720 feet above ground, less than a minute before touchdown and that ice could have clogged the fuel system.
But the investigators say they still don't know how the ice could have formed. Water is naturally present in aviation fuel -- the investigators believe there may have been around 5 litres within this aircraft's fuel load. But the report says levels of water recovered the fuel after the crash were very low for a Boeing 777.
They dismiss the suggestion that the fuel itself froze or became 'waxy' as icing occurred.
The interim report says the plane flew through unusually cold air over Siberia while en route from Bejing to Heathrow. The fuel temperature fell to minus 34 degrees centigrade. But jet fuel should not freeze until it is at less than minus 57 degrees centigrade, and the report says the temperatures involved were not "unique".
The investigation into the crash of flight BA038 continues with testing at Rolls Royce in Derby, and Seattle in the US, home of Boeing.
Water in aviation fuel can be dissolved at the molecular level, or simply float as free water, suspended in the fuel. As the fuel gets colder tiny droplets can form and freeze.
The mystery facing investigators is why this might have happened on an apparantly fully-functioning aircraft.
Water in the fuel is controlled by draining it regularly out of the fuel tanks -- and on the Boeing 777 a so-called 'scavange system' pumps it out.
Ice can form when the fuel temperature drops to around -1 to -3 degrees centigrade. Generally the ice crystals simply float and drift in the fuel without causing harm.
Only when the temperature falls further does the ice stick together. Within the fuel system a heat exchanger is used to increase the fuel temperature, but its possible the blockage might have occurred before this point.
The investigation team have build a test rig and introduced pre-prepared ice into the fuel system to see if it would clog up. But the amounts they had to put in to make this happen were far greater than is normal.
Despite that the scenarios being considered by the AAIB are based on the idea that the ice formed gradually in the system and was released as the plane prepared for landing.
But the report makes three safety recommendations -- that the US Federal Aviation Administration and the European Aviation Safety Agency introduce interim measures to reduce the risk of ice forming on the Boeing 777 powered by Trent 800 engines.
The other recommendations are that the agencies should consider the implications for other aircraft types, and review the requirements for new engines.
This accident remains an enormous for the investigation team. But their reported stresses the rareness of this crash.
"The accident flight was unique", it says, "in that this has been the only recorded case of a restricted fuel flow affecting the engine performance to the extent of causing HP pump cavitation" - the damage found within the pumps that alerted the investigators to the loss of fuel pressure.
The report goes on: "this is the first such event in 6.5 million glihht hours and places the probability of the failure as being 'remote'."
The British Airways Boeing 777 that crashed at Heathrow in January was PROBABLY brought down by ice in its fuel system according to the latest findings of a report by the Air Accident Investigation Branch.
The pilots of the plane managed to get it down safely, and 136 passengers and 16 crew escaped without serious injury.
The AAIB now believes the flow of fuel dropped shortly before the engines on the plane lost power -- at 720 feet above ground, less than a minute before touchdown and that ice could have clogged the fuel system.
But the investigators say they still don't know how the ice could have formed. Water is naturally present in aviation fuel -- the investigators believe there may have been around 5 litres within this aircraft's fuel load. But the report says levels of water recovered the fuel after the crash were very low for a Boeing 777.
They dismiss the suggestion that the fuel itself froze or became 'waxy' as icing occurred.
The interim report says the plane flew through unusually cold air over Siberia while en route from Bejing to Heathrow. The fuel temperature fell to minus 34 degrees centigrade. But jet fuel should not freeze until it is at less than minus 57 degrees centigrade, and the report says the temperatures involved were not "unique".
The investigation into the crash of flight BA038 continues with testing at Rolls Royce in Derby, and Seattle in the US, home of Boeing.
Water in aviation fuel can be dissolved at the molecular level, or simply float as free water, suspended in the fuel. As the fuel gets colder tiny droplets can form and freeze.
The mystery facing investigators is why this might have happened on an apparantly fully-functioning aircraft.
Water in the fuel is controlled by draining it regularly out of the fuel tanks -- and on the Boeing 777 a so-called 'scavange system' pumps it out.
Ice can form when the fuel temperature drops to around -1 to -3 degrees centigrade. Generally the ice crystals simply float and drift in the fuel without causing harm.
Only when the temperature falls further does the ice stick together. Within the fuel system a heat exchanger is used to increase the fuel temperature, but its possible the blockage might have occurred before this point.
The investigation team have build a test rig and introduced pre-prepared ice into the fuel system to see if it would clog up. But the amounts they had to put in to make this happen were far greater than is normal.
Despite that the scenarios being considered by the AAIB are based on the idea that the ice formed gradually in the system and was released as the plane prepared for landing.
But the report makes three safety recommendations -- that the US Federal Aviation Administration and the European Aviation Safety Agency introduce interim measures to reduce the risk of ice forming on the Boeing 777 powered by Trent 800 engines.
The other recommendations are that the agencies should consider the implications for other aircraft types, and review the requirements for new engines.
This accident remains an enormous for the investigation team. But their reported stresses the rareness of this crash.
"The accident flight was unique", it says, "in that this has been the only recorded case of a restricted fuel flow affecting the engine performance to the extent of causing HP pump cavitation" - the damage found within the pumps that alerted the investigators to the loss of fuel pressure.
The report goes on: "this is the first such event in 6.5 million glihht hours and places the probability of the failure as being 'remote'."