armchairpilot94116

ATW Daily News



777 Heathrow accident remains a mystery

Thursday May 22, 2008 Investigation into the Jan. 17 crash landing of a British Airways 777 on final approach to London Heathrow has not revealed an anomaly that could have caused the flow of fuel to the engines to be reduced as the pilots called for more power, according to the UK Air Accident Investigations Branch, although restricted fuel flow was certainly the cause of the nonfatal accident (ATWOnline, Feb. 20). In an update released yesterday, AAIB noted that the engine control system detected the reduced flow and "commanded the fuel metering valve to open fully." The valve responded, "but with no appreciable change in the fuel flow to either engine." Some kind of "restriction" in the fuel system between the fuel tanks and the pumps is suspected.
Examination of the aircraft and analysis of recorded data "have revealed no evidence of an aircraft or engine control system malfunction," nor is there any evidence of "a wake vortex encounter, a bird strike or core engine icing. There is no evidence of any anomalous behavior of any of the aircraft or engine systems that suggests electromagnetic interference. The fuel has been tested extensively; it is of good quality, in many respects exceeding the appropriate specification, and shows no evidence of contamination or excessive water. Detailed examination of the fuel system and pipe work has found no unusual deterioration or physical blockages."
Investigators are examining whether "particularly" cold temperatures along the flight path may have had something to do with the restriction, although the temperatures "were not exceptional" and tests revealed that the fuel onboard had a measured freezing temperature of -57ﾺC, well below the minimum recorded lowest fuel temperature of -34ﾺC.

soem dood

<tap tap tap>

"Is this thing on?"

Use a GC/MS analysis to characterize the [Chinese] fuel.

That is all.

barit1

The fuel system engineers will follow this train of thought, I trust, based on the difference between volume-unit and mass-unit measurements. If I follow that AAIB reading of the data correctly:

1) Both fuel metering valves opened per (initially) autothrottle, and later manual command. Thus I'd exonerate all upstream control hardware and software.

2) The flow metering valve is typically a volume metering device, calibrated such that when a fuel has known density and BTU content, then a calibrated mass flow (and thus BTU rate) is delivered to the burner.

3) However - fuel flow, measured typically by mass-flow devices (someone update me if this is not the case on 777s), in this instance shows little or no increase in mass flow.

The only conclusion I can draw is the properties of fuel reaching the burner were quite abnormal, likely a high volumetric percentage of air or vapor, with drastically reduced BTU delivery.

And the next question I must ask is: What was the indicated metering valve position, and the indicated mass flowmeter signal, during the latter part of descent before the throttle increase was commanded?

Bearcat

NigelOnDraft Bearcat... just a quick read of the AIIB bulletin would answer your question
Quote:
the big question is why did those engines fail....was the aircraft flown outside its temperature envelope ??
Quote:
The aircraft was operated within its certified flight envelope throughout the flight.

Thanks NoD.

GearDown&Locked

Would the oil temps that go into the heat exchange systems be recorded anywhere? What impact would it have in the fuel heating process if the oil wasn't warm enough?

If the overall engine temperature was bellow average and stayed that way throughout the reported idle descent, oil temps could have been also bellow expected, thus not helping the heat exchangers to do their thing in reheating the fuel properly, developing a "cold loop" between the engines and the fuel in the tanks.

arcniz

Well & good to identify spectra for the chemical constituents, but not necessarily a predictor of the behavior of the fuel when uncommon components and unfamiliar proportions are present (if happens to be the case in this instance - which is likely, given the inherent variability of fossil fuels and refinery processing methods.).

Consider the analogy of running a cat through a blender, then running spectra on the resulting goo to see if Sylvester was a good mouser.

There's also the matter of analyzing performance "at temperature" - for the very cold region of interest. Not so easy as putting a coin in a vending machine & having all the answers slide out the chute. Could turn into a full-scale research project to completely characterize the fuel in relevant circumstances. Probably is being done as the clock runs.

One lab test that would be very instructive would be to evaluate the relative frothing propensities of the fuel samples taken, as compared to "normal" JPxx.

Commercially available additives exist to virtually eliminate foaming in aviation fuels. Likewise, additives exist to greatly amplify fuel frothing and foaming behavior in airborne usage, typically as a method for limiting fire or explosive flame propagation in the event of tank ruptures & punctures - such as might arise from coming too close to a cannon during a military engagement. The possibility that high-grade fuel formulated for military use could become mixed with fuel for commercial use is not too far-fetched. Conceivably the mixing of military and commercial fuel batches might even be a common practice in some places when extreme cold-weather performance is known to be required.

Flight Safety

Surely early in the investigation, someone thought to subject some of the recovered fuel to controlled progressively colder temperatures, to see if it developed any unusual properties that might result in a restriction.

seekayess

Folks, let me apologise in ADVANCE if this one sounds dumb, but . . . .


Many, many eons ago, I was on a cross-country flight in a Gnat. After the entire 750 nm was done at about FL 400, with the minimum/no movement of the throttle needed, I commenced my descent, throttling back and a/b out.

As I finished my descent and put the u/c down, I had to now open the throttle.

Bloody hell! No response from the engine!!

Had to do a belly landing on the side of the runway (am cutting the story short!).

Investigations found that there was sufficient fuel on board.

One of the technical items in the engine was an AFRC -- Air Fuel Ratio Control. This, as it's name suggests, was supposed to automatically open/ close an aperture in the fuel flow to ensure that too much fuel did not go to the engines etc.

With no movement of the throttle for an almost one hour flight at FL 400, icing had taken place in this aperture. So, when I opened the throttle, the extra fuel required could not flow because of the icing, which ahd not had the chance to melt away due to the rapid descent.



I know technology has changed immensely in the last 40 years and this was a fighter aircraft (PS: I have ONLY flown fighters -- the reason for my apology in the first line!), maybe such a thing may not have happened.

. . . but, I thought why not post this one!!

ChristiaanJ

"...Detailed examination of the fuel system and pipe work has found no unusual deterioration or physical blockages."
IIRC there was a mention at one point of 'cavitation-type' damage to at least one of the fuel pumps. Am I wrong, or has that now been discarded as one of the probable causes?

CJ

soem dood

Thanks, arcniz --

Cat in blender analogy is a good one! So good, in fact, that I'll hijack it a bit -- what if we do in fact have 14 specimens of FelineFrosty™ aka cat-in-a-blender, and four were terrific mousers, nine were so-so, and one was an unabashed mouse-sympathizer.

While there is no guarantee that an analysis of the spectral signatures would establish a high correlation of any feature with mouse-aggression, IF (a speculative if) such a relationship did emerge, wither with or without a pre-existing hypothesis to explain it, surely that is probative, and at least could indicate a direction for future inquiry!

BTW -- T. gondii could account for odd cat-mouse interaction behavior if it were the mice we were looking at, but would likely not show up using such a crude method as GC/MS, so your point is somewhat reinforced in my mind -- physical tests that demonstrate properties important to the material's suitability as a fuel while in the conditions observed, are probably the best hope.

Thanks again!

misd-agin

Repeat after me, "their performance was heroic." That seems to be the common mantra against anyone wondering what happened during the flight portion of the accident.

Could someone please explain what actions they took that were heroic?

Oh, that's right, the investigation is still being completed. All we have are the tidbits the AAIB has given us, which didn't mention any heroic actions.

So I'll ask again, what were the heroic actions that folks keep bringing up?

arcniz

here's a stab at it:

By the result alone, without any further looking, about the crew of BA038 one can say:

In a very difficult situation, that occurred totally unforeseen...

with only seconds to make observations, reach conclusions and set the actual final plan for dealing with it...

they did their jobs as well as any persons could - and likely better than many would.

They kept their cool, collaborated to fly the aircraft for her last mile steady on the balance of a feather between too short and too slow, knowing full well their own lives and many others depended on the actions of that moment.

Their presence of mind and professional focus continued after the probably unexpected safe arrival, largely intact, on the earth. They did the job to render the damaged machine as safe as was possible with the electrical, engine, and fire controls available, monitored the evacuation, climbed out of the wreck and then likely started on the paperwork while waiting for a ride to the terminal. That was the heroic part.

Basil

seekayess,
I'd say yours is very reasonable comment.
Did you get a Green Endorsement for the Gnat incident?

misd-agin,
I think that the change of configuration demonstrated very quick thinking, involving, as it would, a rapid assessment of whether any immediate sink would be more than offset by the higher L/D ratio for the remainder of the flight. 'Heroic' ? Well, I think we're arguing semantics here. If they are heroes then the resulting attention and expectation will probably make their lives more difficult. Quick witted professionals? I'd say so.
I've certainly learnt from having weeks to consider actions which took place in seconds and, although having retired, I am unlikely to put any conclusions into practice, there just may be the occasion when the crew flying Basil to visit the kids, having read all this, will save the day

TheShadow

The theory I prefer for BA038 is related to the long-term cold-soak.

Forget the fuel having "frozen". Think moreso the cavitation damage to the fuel-pumps (and their restricted throughput/fuel starvation) having been caused by the treacle-like consistency of the fuel brought about by waxing (which occurs a lot earlier than the fuel's freezing point).

Haven't seen any arguments to the contrary, although there has been much reference to waxing. You have to extrapolate the waxing consequence to flows and that being related to the effect upon viscosity of a preponderance of wax globules in the fuel.

Paraffinization of oil has always been a problem for oil-pumping from great depths for ocean oil-rigs. It's likely to be little different for any oil-based fuel. Low temperature hydrocarbon metamorphosis in the presence of various catalysts (such as fuel additives and water) is a whole branch of organic chemistry.

Belgique

The Shadow
I'd tend to agree:

Microcrystalline wax:
is a petroleum based wax that has very different physical properties to paraffin wax. Micro wax is more dense (weighs more), has a different molecular structure (closer packed molecules), is thicker when liquid (higher viscosity), is more flexible (bends easier), has more adhesion factors (is sticky), tougher (scuff resistant), and has a higher melt point than paraffin wax.

Viscosity:
the Ability of wax to resist flow. The viscosity of wax is represented by a number that defines how a particular wax will flow at a certain temperature. The temperature and the viscosity of wax have opposing relationships. The higher the temperature, the lower the viscosity, and the lower the temperature, the higher the viscosity. This means that a hot wax has a low viscosity and will flow much better than a cold wax with a high viscosity.

It's also noteworthy that waxes have a differentiation between their congealing and freezing points and can be "precipitated out" of fuel. A jet fuel's "pour point" is taken to be 4 to 20 degrees below its freeze point. Fuel will not flow at all when it's below its pour-point. However, in contrast to "pourability", once the physical characteristics of a pumped fuel changes towards a higher viscosity, the pumpability of a particular system will very much depend upon whether the fuel is being pushed or pulled (i.e. sucked or blown).

Cavitation damage to pumps and a failure to throughput a programmed volume will be a logical consequence of a much higher viscosity (than allowed for in the system's design flows). Fuel is used as an hydraulic fluid in HP fuel system servo valves and the proper function of these servo-systems is dependent upon the fuel remaining in a design viscosity range.

The rate at which fuel temps decline (and the temp to which it will drop) is a function of:
a. airplane geometry
b. total (TAT) and ambient air temps
c. fuel management
d. exposure time
e. the fuel's initial temperature

(tinyurl.com/6jxpfn)

There's no guarantee that a fuel load will remain anywhere near a uniform temperature throughout its tankage.

777fly

There appears to be an impression amongst some that the environmental conditions experienced by BA038, prior to the accident, were unique or unusual. Rare, I agree, but not unusual. BA operate a daily service to Beijing, so presumably the PEK services flown in the days before and after operated through a similar air mass, as did dozens of 777's of other operators. I have flown 777s through almost identical flight profiles and temperatures several times, as have many others on hundreds of occasions, without a problem. The question has to be why this has not happened before and the task for the AAIB is to determine what made this particular flight unique. Given that all engine and fuel systems appear to have been functioning correctly and that the fuel was never near it's freeze point, the only variable factor, barring EMI, is likely to be the quality, composition and properties of the fuel uplifted in Beijing on that day. That may be stating the obvious, but the flight conditions that day were certainly not unique.

ChristiaanJ

777fly,
I agree that we hope the AAIB will manage to get all the pieces of the puzzle in place.

I doubt there will be one single clear-cut cause, or it would have happened far earlier in the life of the aircraft.
I'd rather think they will find that enough holes in the Swiss cheese lined up for the first time.

Rare weather conditions (not unique, but rare just the same), the aircraft maybe staying longer at FL410 than advisable (didn't others descend to FL240 and lower?), a continuous-descent approach (CDA) with no call for increased thrust until about 2 miles out, and then the fuel characteristics (I would not want to use the word 'quality' since from what we hear the fuel was 'within spec', but even so the exact fuel composition can vary considerably).

Maybe the AAIB will add another couple of holes to the Gruyère. An obvious one would have been a faulty fuel temp sensor, but I have seen no mention of that.


As to the discussion about 'heroes'....
'Heroes', to me, are people who do something incredibly brave and often incredibly stupid, with total disregard for their own lives, and bring it off.

To me, those two pilots were true professionals, who in those few seconds, thanks to their knowledge and experience, drew the right conclusions, did the right things, and turned what could have been a fireball into a remarkable short-field landing...
Nothing 'heroic' in my book, but true professionalism, for which I have a far deeper respect.

CJ

Pinkman

Soem Dood I know where you are coming from and I would hope that GC/Mass Spec would have been the first thing that was done for the post incident investigation. Well, possibly the next thing after the traditional Jet tests (Freeze point, Distillation Range, T90, Final Boiling point etc etc.)

If you had access to the results I am willing to bet that what it will show you is that while Chinese Jet fuel is compositionally a bit different to typical kerosene fractions in the west (hence the -52 deg FP) it met specification. The question in my mind long ago ceased to be what caused the fuel starvation, (a couple of us have repeated endlessly our prediction that it was due to fuel stratification caused by unusual temps leading to a mush of ice and wax at inlet) but whether existing Jet specs and automated test methods are fully protective of safety in all conditions.

The only way this will ever be proven is through the simulations now being done at Boeing and RR (challenging!). If it is ever established without doubt, it will lead to the biggest shake up in Jet fuel testing in two decades.

Pinkman

PickyPerkins

Belgique said:
Fuel full of cavitation bubbles isn't likely to work very well as a hydraulic fluid.

What exactly do these servo valves control?

UNCTUOUS

Picky Perkins said:
Just GOOGLE HP fuel system servo valves - and all will be clear.....