PBL

overthewing asks
Modern FDRs record a few hundred parameters. However, as Mad (Flt) Scientist points out, the parameters *required* to be recorded are comparatively few. In other words, you need to know the specific FDR installation to know what's on it.

One interpretation of the question: if the FDR records a command signal, does it necessarily record the reaction, if there is no equivalent sensor or other similar feedback?

The answer: obviously it doesn't.

Next interpretation: Are there command signals recorded, such that the reaction of the kit to that signal is not recorded?

Answer: Undoubtedly so.

Rephrase: is it possible for the FDR to record an open-valve command signal without recording the sensed position of the valve?

Answer: sure it's *possible*, but whether anything like that happened is something that I would think the AAIB knows already.

As FullWings notes, actual fuel flow is so fundamental a flight parameter that one assumes that both command and reaction are recorded. However, one does needs to know how something is sensed and what the failure modes of sensing are in order to judge whether record is a reliable indication of reality.

PBL

snanceki

Re: dxzh post #384

IMHO I believe that it would be extremely unlikely that both engines could have rolled back within 8 secs of each other resulting from an independent (ice or whatever) blockage in each wing tank pick up.
However I believe that you may not be far off target.

Before posting an alternative I would appreciate an answer to the following question.

How does fuel get from the CWT to the engines?

a. Via the wing tanks (i.e.pumped from CWT into the wing tanks then out again using the wing tank pumps) OR
b. direct from the CWT to a rail / manifold etc common with the output from the wing tanks, with the higher boost pressure from the CWT pump giving it priority to supply the engines?

Anybody got a pic?

How many boost pumps (not water scavenge) are there in the CWT. 1 or 2 (1 LH and 1 RH)?

JFI is this a 777 specific or standard approach to engine fuel supply management?

sandbank

Post #395 from Worn Out Rubber is very interesting -as is Topslides's post 101.

Presumably the AP just disconnected itself at some point - maybe at an altitude of around 40 feet - when it felt it could render no further assistance?

borghha

The AAIB reports states clearly that there was no fuel contamination or water present in the fuel from the main tanks. This however does not exclude waxing, since it was likely to be gone at the time the examination started at a ground temp of + 10°C.

The cold soak this a/c went through was extreme and prolonged. Pilots on the earlier thread said they descended to lower FL. On the BA38 flight, TAT dropped to -45° and fuel temp -35°. But that is, fuel temp as senses by he fuel temp probe in one of the main tanks. It seems possible that somewhere down the fuel path, the actual fuel temp might have dropped further, as there it would not be protected by the inertia of the large volumes in the tanks. Pressure drops caused by suction at pump inlets would have made the temp drop even further. (Temp directly proportionate to pressure)

All of this could have caused local waxing and restriction, until an equilibrium was created during the cruise between fuel flow and temp drop somewhere along the path. at TOD, the thrust decreased and so did the flow, thereby increasing the restriction until a new equilibrium was created.

When more thrust was demanded, this was at first supplied to a certain extent thanks to the fuel volume in the lines between the restriction and the throttle valves, this creating a vacuum in the rest of the line, with possibly a very turbulent flow or even a shock wave and cavitation as a consequence. As the restriction prevented a higher flow than the one created by the last thrust reduction, thrust went back to that equilibrium, somewhat above flight idle. The time differential between engine 1 and 2 returning to idle could be explained by the difference in distance between the restriction and the valves.

Once on the ground, temp there would rapidly make the fuel in the restriction return to its non waxed state, leaving no trace? or would it leave a trace in the composition of the fuel??

It seems to me that serious thought should be given to prolonged 777 operations in very cold temperatures and the effects it has on the fuel temp along its path.

wilyflier

Was the autopilot an automatic disconnect, or did the pilot disconnect?

FullWings

Think about how long there was between the onset of the problem and touchdown; then take away the time for overcoming the initial shock and coming to some understanding of what has happened... Not much left, really.

We'll have to wait for a more detailed report to come out but, IMHO, having the AP engaged down to 200' or so (whether by decision or omission) may prove to have been beneficial.

That's an interesting assertion. I believe most people think about gliding performance with the aeroplane in a 'clean' configuration, where max. L/D and the stall speed are usually quite widely separated. This is not the case when using drag flap (30 in this scenario) combined with extended gear; you will find the best speed to fly is much closer to Vs than might be apparent on first inspection.

Vref is calculated using a fixed multiplier and checked against limits (Vmca, etc.) - it doesn't necessarily have anything to do with glide performance. Without access to the exact residual power settings and a simulator/modelling program to try it out on, I don't think it's possible to make a call on this one. Could have been better, could have been worse...

I've flown aircraft with best glide ratios ranging from about 6:1 to 60+:1. The common factor has been that the *usable* speed range has shrunk with reducing performance, to the point that an attempt to increase speed results in a higher rate of descent and not much else.

From the AAIB report:

I take it to mean that it disconnected itself, as opposed to "was disconnected", although I don't like to question the semantics of interim reports too closely as you can end up going round in circles over the exact meaning of a single word. Could easily have been taken out by the pilot.

Remember the 777 is a FBW aircraft and so the distinction between 'automatic' and 'manual' flight is subtly blurred. With the autopilot out, the envelope protection would have still been active (airspeed, angle of bank, etc.) and the automatics (even though nominally disconnected) would still be capable of making control inputs. The handling pilot would have been able to override these, albeit using higher-than-normal forces.

woodpecker

I seem to remember that the autopilot will continue to remain engaged with a reduction in speed (whether in say Alt Hold or in this case locked onto the glidepath) and continuing to trim the stabiliser for this new speed until the stall is approached when it stops trimming. No-one explained on the course whether the autopilot cuts out as a function of speed or due to the "out of trim situation".

Flying manually is exactly the same, the stabiliser can be trimmed until approaching the stall when further trimming is inhibited. Thereafter the fly by wire restricts (but can be overridden with excessive force) further rearward movement of the control column.

With regard to the fuel system and the possibility of any ice in the centre tank melting during the descent (and being scavenged to the wing tanks) my question to the fuel/liquid property experts is....

How would a small amount of water (now just above zero centigrade) react with being introduced into a wing tank containing about 6000kgs of fuel still well below zero. Could/would it form small ice pellets.

Wornout Rubber

Fullwings

See Post #115 from Misd-agin:

nearport

While not saying it is the cause and with all due respect "cavitation" IS fuel disruption to pumps, at least introduction of vapour or air. The report stated suction damage to pumps. http://en.wikipedia.org/wiki/Cavitation

So perhaps get off pilots' backs about flap settings and other procedures and think about the engineering/fluid dynamics issues that evidence of cavitation on pumps in the report suggests.

Cheers
Mike


"I think the previous poster who suggested fuel starvation is reading something into the report that isn't there. There's nothing I can see in this report that gives an indication of the cause"

Swedish Steve

Snanceki..
How does fuel get from the CWT to the engines?

a. Via the wing tanks (i.e.pumped from CWT into the wing tanks then out again using the wing tank pumps) OR
b. direct from the CWT to a rail / manifold etc common with the output from the wing tanks, with the higher boost pressure from the CWT pump giving it priority to supply the engines?

Anybody got a pic?

How many boost pumps (not water scavenge) are there in the CWT. 1 or 2 (1 LH and 1 RH)?

JFI is this a 777 specific or standard approach to engine fuel supply management?

The engine fuel feed line is split into two. Left and Right. These are connected by Crossfeed valves, two in parallel, normally closed.
There are two CWT Boost pumps. They feed into this line,one each side of the crossfeed valves. These pumps have a higher supply pressure than the wing tank boost pumps and so are the preferred supplier of engine fuel when they are running.

This system of fuel supply is normal in twin engined jet aircraft. The B737 B757 and B767 are basically identical. The A320 in nearly the same.

FullWings

WR,

He might not have wanted to recommend anything lower to line pilots. I'm sure the laws of physics change depending on who's sat in the seat.

I don't know what the figures are - I would be interested to know. What I am saying is that with all the LE & TE flap deployed (plus flaperon deflection), the wing profile becomes much more like the high-lift sections of old, where best performance occurs much closer to Vs. Given the amount of (deliberate) drag from the flap and the unavoidable bits from the gear, you may find that max. L/D occurs at a speed slower than 1.3/1.4Vs (or not... This is just a suggestion based on my experience with other aircraft.)

Also there was some thrust being produced by the engines: "above flight idle but below the commanded thrust" which complicates the situation somewhat. If you can get rid of a bit of drag by flying more slowly, that residual thrust will have more of an effect on the outcome. Food for thought...

Roybucket

Not a pilot - just an interested pax,

Could cavitation in pumps be caused by another source, say an ultrasonic presence or vaporised fuel due to abnormally fast pump speed rotation? Would this initially encourage spool up before starvation (witness on the ground did comment on excessive engine noise)

Go easy on me - very limited knowledge

Rgds

Chris Scott

Quote from ve3id [Feb24/03:30]:
Chris Scott said:
"PS: Isn't it remarkable that, 20 years on, the SECs may still be employing an Intel 80186 chip, now ancient history in the home-PC world? We were all buying PCs with 80286/80386 chips, even as the A320s were first going into service."
I couldn't let this go! NO, It is not remarkable, it is just good engineering. If a chip performs the designed task within spec it should stay in the design forever. The engineering world is not motivated by specsmanship and marketing like the consumer electronics world is. If you put a more modern chip in there, what benefit is it going to give you? Absolutely none, but what about the risk of mask errors introducing bugs that the original programmers did not test for because the new chip has circuits that were not even known back then? Very probable.
If you re-design with a new chip, you have to re-test all system components, and that costs a lot of money.

I'm just an ex-driver, so thank you for confirming my sentiment from an engineering perspective. "Remarkable" was not written in the pejorative sense of the word; more in admiration.

In the A320, the mother of civil digital FBW, the designers took a deliberate decision to go for well-proven COMMERCIAL chips, which were already being used in large numbers in a wide range of applications. For the SECs (spoiler-elevator computers), they chose the 80186. This, I am now told, was a close derivative of the 8086, which had brought PCs to offices and homes across the world, and whose bugs were well understood. Airbus and Sextant Avionique had the option of developing what they called "mathematically correct" micro-processors. Without wishing to sound complacent, their decision not to do so has so far stood the test of 20 years' service and (presumably) well over a million flight cycles.

If the missions of the SECs and ELACs (elevator-aileron computers) are not much more demanding in 2008 than in 1988, and replacement chips are still available, why indeed go for a more complex chip? As you put it, ...... It's not a matter of keeping up with the Joneses!

Despite all their differences, I expect the "Joneses" themselves would be in broad agreement.

snanceki

Thanks Sweedish Steve.
So the answer is b). In one sense I hoped as much.

I don't have time to explain my full proposed theory at the moment but I will ask another question if I may.

What prevents the CWT pumps from restarting due to aircraft attitude change once they have initially shut down due to the 9000kg limit being reached. Presumably the answer is nothing.
In which case a "slug" of "bad" fuel could be sent down the lines impacting both engines at a critical time the slight time difference being due to small differences in the LH to RH installation of pickups etc.
The key difference to dxzh post is that the fuel in the CWT is common to both engines. All we need to show is that this fuel was contaminated by high water content. Since the water scavenge pumps presumably shut down with the boost pumps water would have separated out and may well have been liquid due to the heat from the ambient as the aircraft descends, the A/C packs + of course reduced volume left in the tank.
Evidence of such an event may have been removed by a further attitude change causing the CWT pumps to shut down again and purge "good" fuel through the lines from the wing tanks...but too late to save the day. I believe somebody has already posted something similar a long way back in this thread.

Evidence of water in the CWT cannot be validated due to accident damage and contamination.

I'll try to write up the full sequence later today unless somebody shoots me down at this point.

Such a mechanism brings issues like the "spurious" maintenance water warnings, the extra cold temps, low ground temps etc into play and maybe explains why only this aircraft suffered such an event.

I wonder how long an engine takes (at idle) to use the fuel present in the lines from tank to engine? 5 secs, 10 secs, 30 secs, min before starting to spool down?

I'd still love to know if other 777's have shown evidence of cavitation. I suspect they have and that this is a red herring from the point of view of the accident.

FullWings

I suppose it's a question of alternatives. With the autopilot engaged, it will carry on down the LOC and GS, slowly losing speed; this is not necessarily a bad thing (as a temporary measure) and it frees up a bit of mental processing power to be used on something else. No need to try to explain what you're doing with the controls to the other pilots as they know how the AP will function in these circumstances.

If the first reaction to the problem is taking out the AP, what does the HP do? Lower the nose to point at the houses? Fly at what speed? At that point I think mental models would start diverging and the sight of an aiming point that is obviously a crash into buildings/roads would introduce unneeded fear/consternation into the scenario.

Another factor is that these guys had been woken up in another country some 15hrs before. I know from experience that I'm not the freshest flower in the bunch at the end of a long duty; all your skills/reaction times/thought processes are degraded and there is precious little you can do about it.

sispanys ria

To contaminate both engines with center tank fuel you will need to have either one pump running and an opened X-feed, or the 2 CT pumps running.
In case the crew didn't switch off those pumps, they are supposed to switch off automatically in low press situation. Would it be possible for those pumps to restart in case liquid fuel would become available ?

FullWings

Apart from the fact that they would have been physically turned off on the overhead panel about 1/3 the way into the flight. The scavenge pumps would have run later on to leave the CWT empty some time before TOD. Nice theory otherwise!

Swedish Steve

snanceki
What prevents the CWT pumps from restarting due to aircraft attitude change once they have initially shut down due to the 9000kg limit being reached. Presumably the answer is nothing.

When the 900kg message is shown to the pilots, they turn off the CWT pumps by operating the switches on the overhead. They will not restart with these switched off.

I wonder how long an engine takes (at idle) to use the fuel present in the lines from tank to engine? 5 secs, 10 secs, 30 secs, min before starting to spool down?

A lot longer. When you shut an engine down using the spar valves it runs for 30-50secs on the fuel in the pylon, so from the boost pimps it will probably run for about 2 minutes.

FullWings

These are optional, according to Boeing, and are currently fitted by some Far-East airlines.

PAXboy

To assist those who are still asking about fuel in the centre tank and how/when the autopilot disconnected ... I quote from the:
AAIB Bulletin S1/2008 SPECIAL
Published February 2008
I know that everyone participating in this thread has already read it but here are two items.

At 1,000 ft the aircraft was fully configured for the landing, with the landing gear down and flap 30 selected. The total fuel on board was indicating 10,500 kg, which was distributed almost equally between the left and right main fuel tanks, with a minor imbalance of about 300 kg. The fuel crossfeed valves indicated that they were closed and they had not been operated during the flight.
[edit]

The airspeed reduced as the autopilot attempted to maintain the ILS glide slope and by 200 ft the airspeed had reduced to about 108 kt. The autopilot disconnected at approximately 175 ft ...
[edit]

The recorded data indicates that there were no anomalies in the major aircraft systems. The autopilot and the autothrottle systems behaved correctly and the engine control systems were providing the correct commands prior to, during, and after, the reduction in thrust.