1. Possible interim safety measures suggested by Boeing
I could not find it expressly referred to elsewhere in this thread but interim safety measures as reported last week to be 'in the pipeline' were:
"Boeing's [Nick] West said the initial instructions will be to periodically vary altitude when fuel in the main tank is below minus 10 degrees Celsius, and to advance the throttle to maximum for 10 seconds before the final descent when fuel has been below that temperature for more than three hours, clearing out any water buildup.
The instructions cover 220 777s powered by Rolls-Royce Trent 800 engines in service with 11 airlines worldwide, Boeing said. For 777 aircraft on the ground in freezing conditions, fuel pumps must be run at maximum for one minute each to prevent water buildup, the planemaker said.
In the U.S., the FAA will send a directive within two days covering 56 777s operated by AMR Corp.'s American Airlines and Delta Air Lines Inc., agency spokeswoman Alison Duquette said. The carriers will have 10 days to comply, she said."
see
http://www.bloomberg.com/apps/news?p...HA6Kw&refer=uk (although seems to be mistaken reference to "frozen kerosene").
2. Earlier FAA leaked memo
The apparently leaked FAA memorandum dated 24 January 2008 disclosed back in February 2008 (see
New BA 777 info - Airline Pilot Central Forums) seems to have been largely consistent with information only revealed in the latest AAIB interim report (eg see the fluctuating right hand engine P30 data). It seems that the reference in the memo to:
"• Ice in the fuel somehow limiting the fuel flow to the engines. A maintenance message indicating excessive water in the center tank was set during taxi on the two previous flight legs, although it cleared itself both times. The airplane was being operated in a high humidity, cold environment, conducive to ice formation."
was also correct in part - perhaps though the maintenance messages of excessive water must have been discounted for the time being by the AAIB (and therefore not mentioned in the interim report) as a result of the sumping on both 14 and 15 January 2008 - ie presumably prior to each of the two previous "flight legs" – which in revealing no excessive water allow any earlier excessive water messages to be set aside as erroneous or of no import? Would it be sensible to recommend that such a message is brought to the pilots' attention as an EICAS advisory message, given sump checks will not necessarily indicate for some time after an uplift of warm fuel into a cold centre tank what water may have been hidden away there as ice? Better to learn before the flight than after the flight when nothing that can do about it? What advice is there if a pilot searches out the relevant page in the course of a long cold flight and discovers such a message this week?
3. Any way for melted ice to go from centre tank to each fuel manifold, not via main tanks?
Given the possibility of centre tank ice from the volume of water implied by any excessive water message (ie potentially more than 627 litres – see
http://www.pprune.org/3879461-post229.html - although in up to 79 tons of fuel) and the icing issues hinted at by the ongoing reporting requirements and proposed design changes in respect of the fuel scavenge pumps in related 777 series centre tanks (and for at least one operator apparently reported on 200ER), is there any other way in which ice from the centre tank might end up as a restriction in each feed line from the main tank without such ice causing water to be found at more than 40ppm in the main tanks themselves after the accident?
When the AAIB state (page 18) that "a low pressure in the fuel manifold would have led to air being drawn from the centre tank, via the jettison and override pump check valves", would this permit melted ice from the centre tank to pass directly to each fuel manifold (not via the main tanks)? If so, what would the pressure differential between each fuel manifold and the centre tank have to be to open the check valves? Is there a way for such differential to have occurred during the latter part of this flight and without a warning message, given the boost pumps were ON (implying it seems some positive pressure in each main tank feed line relative to the vented main tanks in the absence of any EICAS advisory of main tank FUEL PUMP low pressure) and the OJ pumps were OFF?
4. Icing scenarios
Icing even in the low flow scenarios outlined in the interim report certainly seems tricky to explain convincingly, unless the scenario also involves either:
- a lot more water in the flow available to accrete (taking into account back-to-back cold-soaked legs, centre tank unsumped water, etc) through the system, and/or
- an accretion "sweet spot" in each boost pump or a particular section of manifold common to each side of the system and no other spot to attract icing, so that the very low concentration of water in the slow fuel flow prior to its consumption by the engines can be sure to accumulate in sufficient volume to make the required restriction (and be dislodged as appropriate).