Swedish Steve,
I sump fuel tanks on aircraft. We drain out about a litre into a clear plastic container, and you might see a globule at the bottom of the bottle, less than 1cc. Sometimes you find nothing. So a decilitre would be significant, and 5 litres cause for an investigation.
As I have sumped fuel tanks on aircraft many times and with similar findings as you discribe. Most of the time there was no visual indication of water what so ever. The odd every-now-and then somewhere around that 1 cc you mentioned. Never experienced more than that.
Indeed, finding 5 liters of free water during a transit check (certainly when sumping is performed on a daily basis) is reason for an investigation.
The estimated 5 liters of water in the fuel loaded at Beijing, according to the report, was divided into 3 liters of dissolved water and 2 liters of undissolved water (entrianed or free, spread evenly over the main tanks and the center tank), plus perhaps a maximum of 0.14 liter which may have entered via the fuel tank vent system during the flight.
If evenly spread, that would imply a maximum of approx. 0.7 liter per main tank and center tank regarding entrained or free water in the fuel as loaded in Beijing. Main tanks probably a bit more than 0.7ltr and the ctr tank a bit less when compared to the fuel quantity loaded in each tank.
When the aircraft's fuel pumps are activated, most of the dissolved and entrianed water/ice would have been well stirred and consumed during the flight, with the aircraft sumped twice the previous few days, how much water would have had time to actually settle at the bottom of the tanks to form a layer of ice? And if so, if that estimated small amount broke loose or melted into slush, wouldn't it have blocked the water and/or fuel scavenge pumps before it could have collected somewhere in the boost pumps or engine feed manifolds?
After take-off at Beijing the fuel temperature remained -2 deg. C until reaching initial cruise altitude, as measured by the probe in the LH main tank. With the center tank above the airconditioning packs would it be possible that the local fuel temperature was slightly warmer at the bottom of the center tank during this flight phase, melting any ice at the bottom into water which would have been a very small amount, if any at all, as estimated in the report?
If so, wouldn't most of it have been scavenged by the ctr tank water scavenge pumps, subsequently well stirred by the OJ pumps and have been consumed with engines set at climb power before doing any harm?
Another question is, if the fuel temperature in the above scenario remained below 0 deg. C, but with an almost empty center tank (situated above the airconditioning packs), what would the temperature be at the bottom of the tank with the 800kg of fuel remaining for approx. 5.5 hours before those 800kg were scavenged? Would the temperature have reached above 0 deg. C, melting any ice which would have been scavenged as a mix of fuel/slush/water but well stirred after passing the boost pumps before re-freezing again in the engine feed manifold? (ctr tank OJ pumps are off at this stage, therefore water scavenge pumps in the ctr tank are off) Or would the ice have melted at lower altitude after the ctr tank was empty and then scavenged? (max. 0.3 liter LH and 0.3 liter RH in the ctr tank) Any free water at the bottom of the main tanks would probably have remained frozen until touchdown and would not have moved much, even if it broke free, due to the dihedral of the wing lower surface. The picture taken of the aircraft with frost on the wing lower surface seconds before touchdown may be evidence to that.
I find it hard to believe such small amounts of free water/ice in the ctr tank, which has a flat lower surface, would have been distributed so evenly between LH and RH engine feed systems with an aircraft in motion to contribute to the cause of identical problems within seconds to both systems.
The 3 liters of dissolved water in the fuel, however, might have but the question remains, wouldn't we have experienced such events more frequently?
Green-dot