Posts about: "Centre Tank" [Posts: 57 Pages: 3]

777/787 driver here.

With a block fuel of around 50 tons the engines will receive fuel from the center tank. Also during take off. Wing capacity is 16,5 tons per wing, thus 33 tons or more as block fuel requires center tank to be used.

Is it possilble that both engines were running from the centre tank which actually contained only a little fuel which ran out around V2 ?.
I know very little about the 787 so maybe complete tosh ????

This is contrary to what is published here and elsewhere.

https://kb.skyhightex.com/knowledge-...7-fuel-system/

Can you state the source of your information? I have no way of independently verifying what I've provided. According to that, however, the two center tank pumps are higher pressure than the L/R tank pumps and will override them if both are activated. So the center tank fuel is used first, then the L/R tank pumps. If no pumps are operating, the engines can suction fuel from their respective L or R tanks provided there's enough atmospheric pressure. The end result is still that an empty center tank cannot cause an engine shutdown absent some other malfunction.

Virtually everything in this post is wrong. The 787 fuel system is almost the same as the 767. The default setup is all fuel pumps on. Each wing tank sends fuel to the left or right respective manifold. The center (main) tank pumps send fuel to both manifolds and operate at a higher pressure than the wing tanks and will feed the manifolds for each side from its left or right pump before the wing tanks. If there is fuel in the center tank it is always burned first and directly supplies the manifolds to the engines. Once the center tank is empty the wing tanks then begin supplying fuel to their respective manifolds. If any pumps are inadvertently left off you will get a EICAS message and a light in the pump switch.

Most large aircraft including the 787 have overriding fuel pumps. The centre tank pump on each side delivers the highest pressure so they supply the fuel under normal conditions.

If/when the centre tank is fully used or the pump fails, the two wing tank pumps supply the on-side engine. This happens on every flight that takes off with more than ~34t of fuel (two wing tanks) and lands with less.

If neither on-side pump is operating, the pressure in the supply line drops below that of the tank and pulls open the suction check valve.

The only 'reconfiguration' the pilots can do is open the crossfeed valve, or turn off pumps.

1. The engines will run fine without the tank pumps - at least until FL200. That has been discussed in detail already.
2. The tank pumps are redundant. There are 2 (one per engine) in the center tank and one in each wing tank. Due to the pressure the fuel in the center tank will be used first.
3. There was more than 15t of fuel in the center tank
4. Any fuel in the system would be used up during taxi

777/787 driver here.

With a block fuel of around 50 tons the engines will receive fuel from the center tank. Also during take off. Wing capacity is 16,5 tons per wing, thus 33 tons or more as block fuel requires center tank to be used.

I understand that the 787 is able to feed both engines from a common centre tank on takeoff. Most other aircraft feed each engine independently from each side until a safe(r) stage of flight.

\xa7 25.953 Fuel system independence. Each fuel system must meet the requirements of \xa7 25.903(b) by\x97
(a) Allowing the supply of fuel to each engine through a system independent of each part of the system supplying fuel to any other engine; or
(b) Any other acceptable method.

\xa7 25.903 Engines.....
(b) Engine isolation. The powerplants must be arranged and isolated from each other to allow operation, in at least one configuration, so that the failure or malfunction of any engine, or of any system that can affect the engine, will not\x97
(1) Prevent the continued safe operation of the remaining engines; or
(2) Require immediate action by any crewmember for continued safe operation.

I'd be interested in any history or understanding as to why Boeing went with the common tank approach on takeoff?

Actually the engines are fed by all tanks during take off. (L engine: L wing tank + Center tank (if filled) / R engine: R wing tank + Center tank (if filled)).

Due to the fuel pressures of the feed pumps (that are all running) the center tank fuel is used first. In case the pump in the center tank fails or the center tank is empty the fuel from the wing will be used w/o any switch over taking place as the wing feed pumps are already running.

Thanks for the clarification.

Surely that's not quite right? If the center tank has fuel, both engines will be fed from the center tank. Only once/if the center tank doesn't have fuel, will the engines be fed from their respective wing tanks.

Well, thats what I wrote. Or not?

All pumps are running but the fuel from the center tank will be used first due to the higher fuel pressure delivered by the center tank feed pumps.

It is indeed, correct. CTR tank OJ pump output pressure is higher than the wing tank pumps. If there is fuel in the CTR tank, both engines will use it on takeoff.

Correct. If the flights involved needed less than 128,000 lbs of fuel the center tank would not be used. There is a scavenge system to insure the center tank remains empty when burning from the wing tanks. On the flight in question 128,000 lbs of fuel would in fact likely be close to the fuel required. The center tank might have been empty or contained a small amount of fuel. It\x92s quite possible the aircraft could have been transitioning from center tank to wing tank fuel at liftoff. With the design of the fuel system I can\x92t really see how that could be significant but I am sure it will be looked at in the investigation.

Block fuel should have been around 50t. Each wing holds 16.9t. That means at least around 16t would have been in the center fuel tank.

Sorry for the metric values ;-)

Never do math in public. You are correct. The center tank should have had a significant amount of fuel at takeoff. Probably at least 40,000 lbs.

With respect I don't agree. I've read every post in this and the previous locked thread, and vapour lock has rarely been mentioned. Could the lack of a/c in the cabin have contributed to the centre tank being warmer than usual for 43\xb0C outside temperatures and a two hour turnaround on hot tarmac? At 40\xb0C Jet-A can hold 800ppm of water, which has a higher vapour pressure than the fuel itself and expands 1,600 times when vaporising. Running through a very hot nacelle the fuel could have reached untypically high temperatures. Perhaps rotation reduced the head of fuel enough to make the difference. It's one of the few explanations that would account for nearly simultaneous flame-out.

Is this unlikely? Of course it is. But whatever caused the crash is unlikely.

DISCLAIMER: Poster (a) is one of the (apparently quite numerous) lawyers following this thread; (b) a long-time forum lurker and aviation enthusiast who loves studying FCOMs for fun (to each his own, I guess); (c) has followed and read this thread from the start.

What I cannot do is add new theories or uncover any new facts the actual experts have not already thought of. However, since summarizing and structuring information is one thing lawyers tend to regularly do (and sometimes even do well), here is my attempt at a useful contribution to this thread: an attempt to summarize the main theories discussed here since day one (which I think hasn't been done for quite some time) in the hope that a birds-eye view will be helpful to those who have not read everything since the beginning or might even trigger some new flash of inspiration for someone more knowledgable than me. I have focused on the cons since there does not seem to be enough evidence to come to any positive conclusion.

I shall try to be concise and to refrain from personal evaluations of my own. Of course, no disrespect whatsoever is intended towards all those who have contributed to this thread and to the individual theories, one or combinations of which may turn out to have led to this tragic outcome. That arguments can be made against every single theory that has been propagated seems to be the result of the highly improbable and unusual nature of this deplorable event and certainly not due to any lack of knowledge or reasoning skills in this forum.

DEAR MODS: If I have distorted anything or if, meaning well, should have achieved the opposite \x96 I guess you know where the delete button is\x85

Anyway, here goes:

A. Misconfiguration or wrong takeoff data
Widely refuted, since

• rotation, takeoff and initial climb seem normal;
• likely extreme errors would have been required to have such tragic effect (the fuel tanks should have been only about half full, so not close to MTOW);
• there is strong evidence that at least some flaps were extended for takeoff (post-crash photo, perhaps also visible in video from behind)

B. Flaps retracted post-takeoff instead of gear
Still brought up from time to time. However, widely disregarded due to

• the fact that with two working engines an inadvertent flap retraction should easily be recoverable, even with gear down;
• strong indications that hydraulic and electric power were lost (audible/visible indications of RAT extension, survivor statement, lack of engine noise, position of MLG bogies).

For a while, the forward tilt of the bogies as first part of the retraction cycle was seen as additional evidence that the gear had been selected up. However, it has been pointed out that the forward tilt and the opening of the gear doors occur almost simultaneously so that it seems unlikely that hydraulic power was lost in the split second between bogie tilt and gear door actuation. It is now assumed the forward tilt of the bogies was merely a consequence of the hydraulic power loss.
It should be pointed out that the question of "RAT in or out" was for a while the most contentious in this thread.

C. Low-altitude capture
Still argued, even if refuted by many since

• inconsistent with apparent loss of hydraulic/electric power;
• PF would have been flying manually (however, A/T reaction would have been unexpected for the PF);
• should have been recoverable (unless one assumes that the crew (a) remained unaware of the changed FMA annunciations although alerted by the unexpected FD commands; and (b) was so startled that an A/T thrust reduction was not noticed and corrected, even though the PF was apparently sufficiently alert not to follow the FD commands).

D. Loss of both engines at or shortly after rotation
Various possible reasons for this have been discussed:

I. Bird strike/FOD

• Would have to have occurred simultaneously due to lack of rudder/aileron input indicating symmetric thrust.
• No remains/traces on runway, no visual indications (flocks of birds, flames, structural engine damage).

II. Fuel-related III. Improper maintenance
Unclear which maintenance measures could possibly have been performed that would have resulted in simultaneous loss of both engines. No apparent relationships between malfunctions reported by previous passengers and essential systems.

IV. Large-scale electrical fault (e.g. due to water in E&E bay)
The engines will continue to run if electrical power is lost. FADECs are powered independently.

V. Shutdown of engines by TCMA
A parallel is drawn to the ANA incident. However, this would require not only a fault in the air/ground logic but also a sensed discrepancy between T/L position (not necessarily idle) and thrust output on both engines simultaneously.

VI. (Inadvertent) shutdown by flight crew VII. Malfunction/mishandling of the fuel cutoff switches (most recent)

It has been a week, and the recorders were recovered within two days. Contamination would have been found and almost certainly made public by now. A government statement yesterday asked people not to speculate on sensitive matters and whatever it was appears to need more time and deeper investigation. I am old and patient, so I have no problem with shut up and wait.

Is this a rhetorical question?

Runway performance seems to be still under some speculation but I thought it was fairly solidly shown that they rotated in about the normal position.

Two events that I see are rotate (g-forces/deck angle causing fuel sloshing) and weight-on-wheels going false due to lift-off.

The engines would be fed separately by the left and right pumps in the centre tank as there's >34t of fuel onboard; one pump per engine. I am not sure how physically separated they are in the tank.