Posts about: "Fuel Pump (Engine Driven)" [Posts: 23 Pages: 2]

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adfad
2025-07-01T09:32:00
permalink Post: 11914147
Originally Posted by Someone Somewhere
...Once both engines are running and the four VFSGs are online, I would not expect any load shedding and certainly not of flight loads like fuel pumps...
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.

Originally Posted by MaybeItIs
...\xa7 25.903(b) includes the words: "in at least one configuration,"

It doesn't, that I can see, state that that configuration must be used during takeoff, though common sense would say it should.
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.

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.

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MaybeItIs
2025-07-01T12:20:00
permalink Post: 11914234
Originally Posted by adfad
Originally Posted by MaybeItIs
...\xa7 25.903(b) includes the words: "in at least one configuration,"

It doesn't, that I can see, state that that configuration must be used during takeoff, though common sense would say it should.
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 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.
Sorry, you missed the point I was trying to make. \xa7 25.903(b) does say that the fuel system must be able to operate in an isolated, two-sided mode (for a twin engined jet), such that nothing on one side, such as bad fuel, will adversely affect the other engine. Of course, during Takeoff, both sides drawing fuel from a Centre Tank containing a lot of contaminants (e.g. Fuel Bug matter, water) is a scenario that could bring down the plane. We are all aware of that. But the point I was trying to make is that although \xa7 25.903(b) requires "at least one configuration" that separates both systems entirely (such as Left engine drawing from Left Main Tank, and Right from Right) which can be configured, the Rule doesn't appear to make that compulsory for Takeoff.

A lot of other posters here have stated that according to FCOM instructions, the normal, accepted 787 Takeoff configuration is "Both sides draw from centre" if the Centre tanks have enough fuel in them. I think (maybe wrongly) that this (prior few posts) is the first time this exact point has been raised. I hope I'm correct there. If not, my humble apologies.

The great thing about this forum and sadly, this tragic accident, is that it's drawing a few previously little-known worms out of the woodwork.

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