Posts about: "Fuel Pumps" [Posts: 151 Pages: 8]

The pumps are load shed during (and possibly before) engine start. Available power is presumably somewhat limited when on APU only (two generators not four) and especially when you're electrically cranking the engines. When on ground and with engines stopped, aircon, IFE, and galley probably takes priority over pumping fuel to nowhere.

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.

The Airbus manuals imply or clearly state that centre pumps are inhibited when the flaps are extended, so both engines draw from the wing/main tanks. I haven't seen anything clearly matching in the Boeing manuals.

May I ask where this information of load shedding comes from please
In my experience the APU supplies enough power to run all systems. Hydraulic pumps, fuel pumps etc

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 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.

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.

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.

It's exactly the same on the B777 - the centre fuel pump switches go on before start if the FUEL IN CENTER EICAS message is displayed. The switches go off again when the FUEL LOW CENTER message is displayed. On the ground, the B777 needs two power sources for both centre tank pumps to operate, so one pump is normally shed until after engine start. The centre tank pumps output about three times the pressure of the main tank pumps. Fuel is fed from the centre tank until the centre tank pumps are selected off.

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.

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.

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.

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?

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?

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.

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.

Thanks for the clarifications

My question is then: what is the minimum loss of thrust in both engines (perhaps more relevantly expressed as a % in fuel flow reduction from expected) that could produce the profile we saw. I appreciate this is a figure with many variables including timing and rate of loss.

The reason I think this question is relevant is because we pretty much have 2 prevailing theories at this point:

1. A failure, or reduction of thrust (below idle, indicated by loss of AC generators), that somehow impacted both engines, within 20s of rotation (explaining the RAT and gear orientation)
2. Somehow a loss of all AC power, leading somehow to a reduction of thrust or failure of engines (both engines impacted identically is assumed in this scenario since all AC is lost), and was of course below the minimum thrust needed to fly with gear down at this weight and temperature

I agree that if it is completely infeasible that loss of all AC power could do anything but cause thrust reduction of X where thrust minus X is not enough, even with gear down in high temperature and significant weight at the critical moment of takeoff to cause the profile we saw, then theory 2 is invalidated. I would love to invalidate any of the theories here but I do think some specific calculations, simulations or test data is needed