HotDog

Thanks for that NSEU, glad to say that in 33 years of flying, I never encountered an iceberg. Pity Titanic didn't have wings. Interesting username you have. Neutron Single Event Upset?

Yellow Shark

It is not the norm at all. In fact it is very much the exception to the norm. Typically, most systems will have more than one channel (think of it as a primary channel and a back up channel) on separate cards. Typically there would be some health checking between the channels with the primary staying in control of the system unless it is "less healthy" than the second channel. In most cases, the software contained in each channel is identical. In rare cases, the software will be different (referred to as Multi Version Dissimilar Software in RTCA/DO-178B - the guidance to which the vast majority of modern aeronautical software is compliant).

infrequentflyer789

On the first, I don't know. Porus chinese walls I could believe, ditching triplex development completely I would find slightly suprising. I don't know the details though.

As to the second question - yes. Although it may be better stated that normal would be separate teams (which may be in same company). Typically the teams would be in different sites / locations etc. (regardless of company). The geographical separation probably doesn't buy you as much these days.

The technique is known as multi-version or N-version development. The assumptions (eg. that separate development reduces common-mode failure risk) that underly it have been questioned - look up Knight Leveson experiment.


All of which, though interesting, is of no relevance to BA38, as the AAIB have already established that flight data shows all the software working correctly right through to opening of the fuel valves. The only possibility for software failure now is if it was a type of failure that caused the engines not to respond as commanded and simultaneously caused all systems to send fake data to QAR and FDR that everything was normal. Simultaneous (or nearly) flying saucer ingestion is more probable.

grebllaw123d

Swedish Steve,

You say that the CTR tank water scavenge pumps are ALWAYS running.

But you also state that their "power supply" comes from the tank booster pumps outlets.

My question is then - how can the water scavenge pumps run if there is no fuel in the CTR tank?

I suppose that they will shut down automatically, when booster pumps stop producing pressure due lack of fuel (at approx. 900 kg) - you see what I mean?

TheShadow

It would appear that the evidence and opinion is becoming increasingly supportive of the center-tank having been the logical source of the obstruction causing the double-engine stagnation and pump cavitation (as pointed out in detail at post 111)
.

It has become a plain and simple Occam's Razor exercise

All that would appear necessary to identify now is the exact physics, timing and mechanics of the event.

scanscanscan

Correct me again if I am wrong please.....Engine failure drill (or possibly thought engine failure)
The drill is.....after Fuel Control Switch to cut off action (possibly not done) comes....open fuel cross feed...start APU..
Question ...Any evidence on the tapes a crew member did this?
Presently it seems people think a fuel cross feed was open and the APU was in the first stages of starting.
If the engines were giving power above flight idle to impact...why did the APU
start without human assistance.
If it did begin to start automaticly it seems it started with the landing gear
collapsed and one engine fuel cock open.
It also appears the increasing pitch attitude to near stall AOA if that occured was not helpfull to fuel feed problems.
Could a B777 pilot (Hand SOLO maybe) explain the logic of how this all could have occured.

gas path

With fuel in the CWT and the (CWT) boost pumps running (this fuel is ALWAYS used first due to the fact that the pump pressure is 36psi as opposed to 12 psi for the wing tank pumps) the motive flow water scavenge pumps draw fluid from the bottom of the tank and pass it into the inlet side of its respective boost pump. As Swedish Steve points out these scavenge 'pumps' are small diameter pipes certainly less than 1/2 inch bore.
Each wing tank boost pump also has a fuel scavenge motive flow pump BUT this line (same or similar diameter!) has a float operated valve in the CWT AND a float operated valve in its respective wing tank. When the fuel is used up (down to about 900kgs) in the CWT the float valve will open and fuel will attempt to move into the wing tanks, BUT at this point because the wing tanks are full the float operated valve cannot open. When the fuel level in the wing tanks has dropped to a predetermined level the float valves open and the fuel is scavenged from the CWT into the wing tanks.

Swedish Steve

You say that the CTR tank water scavenge pumps are ALWAYS running.

But you also state that their "power supply" comes from the tank booster pumps outlets.

My question is then - how can the water scavenge pumps run if there is no fuel in the CTR tank?

I suppose that they will shut down automatically, when booster pumps stop producing pressure due lack of fuel (at approx. 900 kg) - you see what I mean?

Yes, took a peek in the AMM. It is not very clear but the Centre tank scavenge pump and the Centre tank Water scavenge pump are linked. It looks as though with the Centre Tank scavenge system running, liquid is sucked through the centre tank water scavenge lines as well. The centre tank scavenge jet pump motive power comes from the wing booster pumps.

JamesCam

Re drop in fuel pressure - If there was an obstruction in the fuel supply between the tanks and the HP pump(s) the pressure in the pipework between the obstructon and the HP pump inlet would drop to hugely negative levels due to the suction caused by the HP pumps. Could this not cause rapid freezing of the fuel, especially if it had a higher than normal water content?

I accept that this doesn't explain both engines beinging affected at similar times etc, but I would be interested on peoples views on the possibility of fast freezing as described above.

My first post, so now crouching for the flames! I'm a PPL rotary pilot who has learnt pretty well everything about jetliner fuel systems from reading this facinating thread.

James

nlarbale

From my reading of the AAIB report, they tested the wing tanks, but due to contamination with foam, etc, the center tanks were not tested. Also, due to the center tank breach by MLG, does it not seem possible that any ice may have melted and drained from the center tank by the time the AAIB had a look, or any remaining water mixed with foam etc so as to make any analysis difficult at best

SyEng

We’re talking about the centre tank, not the wing tanks.

No, it would be fine.

Quote:

Machaca

Thanks for the pics. Got a system schematic?

No, but I can’t imagine the 777 tank environment is so different to the types I’ve worked on. Secondly, in many years experience, I’ve never come across a single reference to fuel stratification as an issue for aircraft fuel systems.

First, several folks here seem to be assuming that all the aircraft systems were working as designed and that, e.g. “the CT would have emptied early on” or that the water would have been scavenged into the wing tanks. This approach doesn’t make too much sense. If it did, the corollary is that this aircraft systems were operating as the designers intended, and 777s would be dropping out of the sky every other day. There must have been failures (not necessarily equipment failures).

My inferred design philosophy for the CT transfer system is:
-CT boost pumps operate until the inlets uncover. Inlets are positioned forward in the tank to avoid picking up large quantities of water in climb/cruise attitude.
-Wing tank boost pumps now take over supply to engine feed.
-CT (fuel) scavenge jet pumps with motive flow from wing tank boost pumps empty the CT at a leisurely pace. Any water picked up is safely mixed with wing tank fuel before being fed to the engines.
-CT water scavenge jet pumps get motive flow from the CT boost pumps, so only operated when CT boost pumps do.

Now, here are the 2 functional failures necessary to support my theory (post 216):

1) Failure to scavenge effectively CT water.
2) Engine feed source switches from wings to CT during approach.

Failure 1) possible contributory factors:
1a) Water remains frozen during turnarounds precluding effective drains operation (See NSEU post 292).
1b) Water remains frozen during most of flight precluding effective water scavenge operation.
1c) FOD

Failure 2) possible contributory factors:
1a) The most nose-down attitude of the whole flight (including descent) occurs when landing flaps are selected. This I think is likely true of many civil types. Perhaps someone can confirm for 777. So any residual liquid in the CT moves forward at this point.
1b) CT boost pumps remain running throughout flight (I imagine this should generate a warning).
1b) CT boost pumps switch back on uncommanded when pick-ups become covered (is there any way (including failures) that this can happen in the 777 system?).
1c CT boost pumps switch back on by crew action.

Please remember that the CT was breached and contaminated by firefighter’s foam and hydraulic fluid after the landing. It is not clear from the AAIB report that they even tested for water in the CT. It sounds like it may have been a pointless exercise.

It is conceivable that in the final seconds of the approach, with the increase in pitch, the CT boost pump inlets uncovered again allowing engine feed to resume from the wings, helpfully flushing evidence from the feed lines but not in time for the engines to spool up enough to make a great difference to the outcome.

Like I said before, I’ve not seen anything here or from the AAIB that rules out this theory. I’m open to offers, though.

steamchicken

In the B777-200, the center tank message is shown at approximately 14.5 gallons.

14 *decimal* 5. Does anyone else think the "138 gallons H20" thing is a missing decimal point? Otherwise the difference between the -200 and the -200ER is just shy a factor of 10; and I doubt the -200ER's centre tank is 10 times as big as the -200's.

borghha

If this were to produce any effect, the fuel scavenge valves in the wing tanks would have to open or to be open, as an earlier poster said. But at what wing tank fuel level do these valves open? would the amount left in the tanks of this flight come anywhere near that level?

and where are the fuel scavenge line outlets in the wing tanks? is there any chance of ice or slush coming from them partially obstructing the boost pump inlet screens in the wings?

SyEng

The effect of CT boost pumps switching on with inlets covered is to feed CT fuel (or water) direct to the engines. (as I understand the system... in the absence of schematics etc.)

borghha

Yes sorry SyEng, I was thinking of the CT fuel scavenge pumps being triggered... but I understood this to happen only if the main tanks are low on fuel, triggering an EICAS message too and leaving a trace on the FDR. I was trying to understand how low on fuel they have to be and to compare this to the actual fuel on board.

SyEng

Borgha,

See Swedish Steve's post 304 above. I imagine the function of the wing tank float valve on the CT fuel scavenge outlet is to prevent transfer into a full tank, i.e. prevent transfer causing wing tank overflow. If that is so, then the float valve coud be set fairly high up in the tank.

Chris Scott

Quote from SyEng [Today, 17:04]:
The most nose-down attitude of the whole flight (including descent) occurs when landing flaps are selected. This I think is likely true of many civil types. Perhaps someone can confirm for 777. So any residual liquid in the CT moves forward at this point.
[Unquote]

Haven't flown the B777, but doubt that would be the case. In the absence of a B777 pilot on the forum at the moment (?) I'm going to stick my neck out.

The lowest pitch attitude is likely to have been in the early part of the descent from FL380, when still at or near the cruise Mach, when it is likely to have been zero or less. If you think about it, passenger and trolley-dolly comfort in the cruise dictate a deck angle of something between zero and 3 degrees nose-up.

To descend at idle thrust and maintain speed (the standard technique on jets), the change of pitch angle is going to be about 3 - 5 degrees down, initially. This suggests a pitch attitude of minus-something in the descent, i.e., below the horizon.

With the aeroplane fully configured for landing, and at final approach speed, the attitude is definitely nose-up, even with full flap selected. Prior to that, on the glide-slope with intermediate flap, the speed is usually higher, resulting in a similar pitch attitude. There is, admittedly, a short-term pitch-down as each increase in flap is made, before the speed decays to the new value; but at no time would the actual pitch be below zero on the approach on a large modern jet.

Hope this helps.

SyEng

Thanks Chris,

In the "large amount of water in the tank" scenario, it would still be frozen at TOD. Agree it would be interesting to hear from 777 pilots on pitch attitudes. The answers to the pitch attitude question will be on the FDR of course.

Chris Scott

Okay SyEng, I like the way you argue your hypotheses.

There is another point worth mentioning, perhaps, again quoting the AAIB:
During the descent, from FL400, the A/C entered the hold at Lambourne at FL110; it remained in the hold for approximately 5 minutes, during which time it descended to FL90.

Entering the hold at FL110 is a completely different regime from the early part of the descent that I was dealing with above. The aeroplane will be on speeds now, not Mach. Entering the hold, the IAS would be back to about 210 - 240 kts, to comply with holding-pattern rules.

Also, although they continued to descend, they only averaged 400 ft/min during those 5 minutes. This is minimal - not much different from level flight. The pitch attitude to fly at (say) 220 kts would have been of the order of 5 - 8 degrees nose-up. This is as much or more than on final approach.

Additionally, there is every probability that significant thrust would have been used, unless they entered the hold at a higher speed than optimum, and were slowing down.

I wish the AAIB had told us whether thrust above idle was used in the hold, or later on the intermediate approach. It normally is at LHR.

You will draw your own inferences. A B777 pilot could tell us how much fuel-temperature rise normally takes place between FL400 and FL90 in a typical descent, with about 12 tonnes of fuel remaining in the wing tanks. Would it have risen from the -34C quoted to above 0C?

But as for your postulated large block of ice in the centre tank, who can tell?

r011ingthunder

SyEng said:
Now this is the best thought out argument I've seen, looking forward to seeing how this develops.