Posts by user "Someone Somewhere" [Posts: 44 Total up-votes: 79 Pages: 3]

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Someone Somewhere
2025-07-01T10:19:00
permalink Post: 11914164
Originally Posted by adfad
We know (from the 248-day bug) that full AC power failure is possible and we see from the RAT and landing gear orientation that full AC power failure was likely within ~10 seconds of leaving the ground.
I believe that particular bug is fixed, though it's always possible there's other issues causing a total AC loss.

Not really relevant to what you quoted though, as the scenario in question requires:
  • Engines running on centre tank fuel during takeoff while the aircraft is operating normally
    • We don't know for certain if this is the case. It seems to be but it's not something that happens on other families.
  • Then, total AC failure stopping fuel boost pumps.
  • Engines suction feed from contaminated/full-of-water wing tanks.

I also don't see any evidence that engine driven fuel pumps alone must be able to handle this scenario: provide enough fuel flow for takeoff and climb, even while the pitch is rotating, even in a hot environment with significant weight, even while the gear is stuck down.

I know that the engine driven pumps have documented limitations and that the regulations allow for some limitations. I know that at least one of these limitation is high altitude and I _suspect_ that the design intends for this unlikely scenario (engine driven fuel pumps alone with no AC pumps) to guarantee enough fuel flow to get to an airport and land. I also suspect that the APU is expected to solve loss of all AC generators - and as we know, there wasn't enough time for it to start in this scenario.
The aircraft has two engines and should be able to climb out on one, plus it dropped like a rock . 'Significantly degraded' thrust isn't really compatible with what we saw. You'd also expect the engines to recover pretty quickly as it leveled off.

The limitations at high altitude are primarily air/volatiles degassing out of the fuel. That's not going to be much of an issue at sea level, even if the engines are a bit higher up during rotation.
APU is a nice-to-have; it's on the MEL. If you lose all four generators, it's because of some major carnage in the electrical software/hardware and chances of putting the APU on line even if it's operating are very slim.

Subjects: APU  Centre Tank  Fuel (All)  Fuel Pump (Engine Driven)  Fuel Pumps  Gear Retraction  Generators/Alternators  MEL  RAT (All)  RAT (Deployment)

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Someone Somewhere
2025-07-01T10:42:00
permalink Post: 11914172
Originally Posted by AirScotia
One of the things I've learned on this thread is that planes landing with the RAT deployed may be rare, but it does happen. The videos I've watched suggest that the engines were usually running as the plane landed, but of course the RAT can't be un-deployed in flight.

My question is: what caused the RAT to deploy on those flights? Presumably reports have to be submitted in those cases?
Many are maintenance or production test flights. Someone commented upthread that every Boeing widebody built gets the RAT deployed on its first flight, and I imagine some maintenance procedures require it too.

ASN has a section on electrical power incidents: https://asn.flightsafety.org/asndb/cat/ACSE

In particular try these:

https://assets.publishing.service.go...009_G-EZAC.pdf
https://asn.flightsafety.org/wikibase/233343
https://asn.flightsafety.org/wikibase/219748
https://asn.flightsafety.org/wikibase/34357

Subjects: RAT (All)  RAT (Deployment)

Someone Somewhere
2025-07-01T12:59:00
permalink Post: 11914257
Originally Posted by Sailvi767
On the 767, 757 and A330 anytime you are in single generator operations the aircraft is load shedding. The 787 with a totally different electrical system might function differently.
The manuals suggest the 787 has even more advanced load inhibition/load shedding, shedding/recovering individual loads as required for both operational and availability reasons.

Remember the 787 uses electrics for engine start, wing anti-ice, centre hydraulics, and cabin air compressors. There's some big electrical loads.

Centre tank boost pumps are probably comparatively small, but if you can conclusively say x is not required during ground engine start , why power it?

Subjects: Centre Tank  Fuel (All)  Fuel Pumps  Generators/Alternators

Someone Somewhere
2025-07-01T13:08:00
permalink Post: 11914265
Originally Posted by adfad
As an electronics and software engineer who has read the AD and related materials on the 248 day bug my understanding is that:
  1. The specific 248-day integer overflow was patched, and before the fix was rolled out, the AD required this system to by power cycled every 120 days to prevent overflow
  2. The PCU software still has the functional requirement to be able to command all AC GCUs to enter failsafe mode, this means that while the initial bug was fixed, the ability for this particular software system to command the same result is still a functional part of the architecture - presumably for safety management of the AC system
  3. This was not the first or last "software overflow error" issue in Boeing or even in the 787
Although I'm not qualified in aviation engineering I do believe from an engineering safety standpoint that this architecture creates a rare but entirely feasible scenario in which the aircraft would be without AC power for at least 30 seconds until the APU could restore it.
Similar failures have happened on 737s/A320s/A330s and others. I'm not denying it's possible. There's a reason it's a certification requirement for the engines not to be dependent on aircraft power. The APU is MELable and battery starts are not extremely reliable.

I do agree that the engine driven pumps should be able to provide fuel alone, the whole point of these pumps is to keep the plane flying within some limitations, high altitude is one of those limitations, I propose that there may be others based on the following:
  • Some more knowledgable people here have proposed or countered vapour lock, fuel contamination and automatic fuel cut-off theories to various degrees - even if these are not enough on their own, loss of electrical during rotation at high temperature could combine with these in a way we have not yet considered
  • Thrust is nonlinear, and while I'm not qualified to say how much loss of fuel flow or loss of thrust would be critical in this scenario we do know that it was a hot takeoff with significant weight and gear remaining down - I know others here have run sims but I don't think anyone has focused on specific thrust / fuel flow params
  • While electric fuel pumps might not be physically necessary for takeoff, my final point is: why are they required for takeoff? Is it not to mitigate cavitation, fuel sloshing at rotation, or any other kind of problem that might be relevant here?
Thrust is non-linear and complex. Reaction engines (i.e. fans, props) are generally most efficient at minimum power - lowest excess velocity. Turbine engines are generally most efficient at high power. These cancel out somewhere in the middle. With two engines at low power, you also don't have the drag from the dead engine or the drag from the rudder countering yaw.

Cavitating destroys pumps rapidly - someone upthread said replacing the fuel pump immediately is SOP if it has suction fed. Expect end of life in tens of hours rather than tens of thousands.

Some aircraft have switched to using jet/venturi pumps powered by returned fuel, like the A220. The electric boost pumps there are mainly for redundancy and are shut down in cruise; only one in each wing tank. Some A320s replace the centre override pumps with venturi transfer pumps.

Subjects: APU  Air Worthiness Directives  Fuel (All)  Fuel Contamination  Fuel Cutoff  Fuel Pumps

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