Way back on March 3 I posted a theory for this accident. It looks better and better as the investigation advances.
Post #589 Page 30
http://www.pprune.org/forums/showthr...325095&page=30
1. There was no malfunction of any of the systems, mechanical or electrical.
2. The freezing point of the fuel had nothing to do with the cause of the accident.
3. There was no water ice or physical blockage in the fuel pathway to the HP pumps.
The short version ...
1. A cold spot developed in the fuel significantly below the temperature of the bulk of the fuel.
2. The viscosity of this cold fuel was higher than what the pumps can handle.
3. The cold spot in the fuel did not reach the pumps until the AC was on short finals.
4. The high viscosity made the pumps cavitate and the commanded thrust could not be maintained.
For those who did not understand my March post, the long version ...
Given a suitable input pressure, every pump subjected to increased RPMs will reach a point where cavitation occurs, regardless of liquid type or viscosity. A higher viscosity results in cavitation at a lower RPM. The operation of all pumps can be graphed and though the values may vary the form will be simular.
A positive displacement pump as is on the 777 has a predictable and certain output flow for any given pump speed. Displacement per rpm multiplied by the rpm gives output independent of pressure up to the point of cavitation.
Since fuel required tracts power produced, a positive displacement pump can be directly connected to the engine and geared to produce a varying output slightly above that needed at all engine speeds.
A smaller and lighter in weight pump can be used so long as the higher pump speed needed to produce the output does not push the pump into cavitation. In aviation, lighter is better, but in my opinion , the Boeing boys cut things a bit tight with the 777.
At the point of cavitation the percentage of design flow begins to drop and continues to worsen with additional pump speed.
The only standard for the viscosity of jet fuel I can find is based at minus 20C, where the maximum viscosity is stated to be 8 centistokes. The viscosity of most liquids increases as temperature drops. Many single component liquids, like pure water, have a linear function of viscosity to temperature up to the point of freezing. Liquids mixtures of hydrocarbons tend not to be linear. Ever hear of multi-viscosity motor oil?
Different formulations of jet fuel can have different viscosity profiles. All that is needed to meet standard is that it be below 8 centistokes at minus 20C. All that is known for certain is that below minus 20C the viscosity will be higher and the greater the difference of temperature from the standard the more uncertain actual viscosity of a particular fuel becomes as illustrated in the graphic below. Note that these viscosity changes apply across the full temperature range and have nothing to do with the point of freezing.
If my theory for BA038 is correct, a cold spot developed in the fuel, though the bulk of the fuel did not go below minus 34. (see my March post for particulars) This cold fuel was not mobile in the tank until lower flight levels. Once mobile, the cold fuel, maybe around minus 40C, began drifting toward the boost pump inlet. It arrived at HP pumps on short finals.
In the graphic below I have combined the concepts of the graphics above. One line represents the engine/pump rpm/thrust and the associated maximum viscosity for those values. Added are some possible temperatures that might be associated with a particular fuel viscosity.
From flight idle the engine responded to command with an increase in speed/thrust marked with green dots. Pumping minus 34C fuel there wasn't a problem. When the super cold fuel hit the pumps the viscosity increased above the maximum value and cavitation started. Starved of fuel the engines rolled back. The rollback to lower rpm improved the cavitation problem and when the slightly cavitating pump matched the needs for the thrust level, the situation stabilized at thrust levels of 1.03 & 1.02 EPR.
