Originally posted by Darren Wheeler
I thought they had modified the plane so that the fuel was only fed to one desginated engine in case of problems.
It won't have been a 'modification' per-se, just the fuel configuration used for the ferry flight.
The 747-400 has eight fuel tanks, three in each wing, a centre tank in the belly, and a tank in the vertical part of the tail. Various fuel configurations are used depending on the fuel load. The wings always have fuel, if more fuel is required, the centre tank is loaded, and finally if the flight is really long, the tail tank too. It's then consumed in reverse order (tail tank is emptied first - actually fuel is taken by the engines from the centre tank, as fuel is used, the tail tank drains into the centre tank).
With less than 130000 lbs, or about 59 Tonnes, of fuel on board, all the fuel remaining is in the wing tanks, and the 747-400 feeds fuel to the engines in a configuration known as 'TANK/ENG'. In this configuration, the centre and tail tanks are empty, and each engine is fed directly from a separate fuel tank in the wing (no cross-feeding).
The fuel required for the ferry flight would be significantly less than 59 Tonnes. (The total fuel loaded would likely be less than 20 Tonnes.)
Therefore the aircraft would by default operate in TANK/ENG config, which would ensure that the biofuel tank would only feed one engine.
So I'm guessing one of the wing tanks will either have been drained of regular Jet-A and refilled with the biofuel mix, or the biofuel additive will have been blended in the correct proportions with the existing Jet-A in the tank for the flight.
Given that WOW is going for maintenance at KLM Engineering, the tanks will be drained for inspection during the maintenance process in any case, and it's likely that the biofuelled engine was already due for exchange with a freshly overhauled engine too.
It was probably a good use of a circumstance to generate some publicity for the airline and provide fuel researchers with some valuable statistics. I'm certain that the engine data will have been recorded and closely monitored during the flight, looking for things such as differences in fuel flow for a particular power setting, differences in exhaust gas temperatures, etc.
Oh, and as far as being an 'aircraft down', some of you will have noticed that LAX 7/8 were planned to switch to A346 on Friday, so this has been planned for some time (and one of the Airbuses on maintenance in Manila came back on Wednesday, I think).
Mike