AD
1. AD
AD pretty much as predicted last week.
As a stop gap measure to keep 777s flying (given regulatory imperative to avoid certain levels of risk for significant failure conditions hazardous to flight), it is definitely to be welcomed though it (almost inevitably due to its immediate nature) misses the opportunity to address some of the likely underlying contributory factors to "hazardous amounts" of ice accumulating in the "main tank fuel feed system".
I would highlight the AD does not reflect:
- the potential build-up of free water and ice in the 200-ER centre tank over two cold-soaked stages.
- the fuel in each main tank remaining cold soaked (together with its scavenge lines) on cold stopover.
- the timing of delivery of any centre tank water into each main tank fuel supply.
- the accretion of ice might not be of the "releasing" type in which case until it fully develops, full flow may not remove it or be affected by it.
In particular, while the in-flight run-up can happen 3 hours before TOD, any ongoing build-up of free water in the centre tank (not addressed by AD) may only be scavenged into the main tanks one hour or two before TOD as on BA038 - if run-up is done before fuel scavenge completed then arguably chain may not be broken unless require another run-up, say, 20 minutes after centre tank indicated as zero?
2. Location of "95%" restriction
If the restriction in each manifold is elsewhere than the "engine fuel" system, as could be suggested from the test rig results which showed significance of a time lag after the accelerations started but before the flow reduced, and operates as an effective restriction on 95% of the cross-section at a point in the "main tank fuel feed" system upstream of the engine, then this AD seems to miss the obvious risk that the restriction was never located in the "engine fuel" system! I assume though that (if I may try and be diplomatic) it is currently expedient to ignore that suggestion.
Ironically, pity the non-RR engined 777 crew and passengers - at least those flying in 777s equipped with RR should now face a reduced risk of icing-induced double rollbacks. Perhaps more risk-averse non-RR 777 operators though might see some advantage in being seen to implement the proposed AD on a voluntary basis, at least pending the next report?