Boeing 777 - LHR short landing.

[blondini]

The text in the report appears to have been lifted from an article in Aeromagazine published by Boeing themselves. http://www.boeing.com/commercial/aeromagazine/aero_16/polar_story.html

Where it says 'probes' it is referring to multiple aircraft types, then specifically for the 777 the wording implies that there is only one probe location.
For the 777-400 it states that there is only one probe location.
For the MD-11 it states that there are two probe locations.
No point in getting too tied up in how many temperature probes there may be on a 777 though. If you read the article it is accepted by Boeing that although generically the 'probes' are located in the coldest area(s), it is still possible for some fuel in the tank to become colder than the measure.

...some fuel may be colder than the fuel measured by the probes, such as the fuel that is in contact with the lower wing skin. This creates a temperature gradient in the fuel tank from the wing skin to the location of the probe.

Interesting that the design of the tanks pushes the very coldest fuel towards the pump inlets, although this would be a good thing until the fuel actually starts to wax.

As fuel travels to the boost pump inlets, the bottom, cold layer flows through small flapper valves located on solid tank ribs next to the bottom wing skin. These valves are used to control fuel slosh. Thus, the cold fuel tends to flow toward the boost pump inlets.

They don't see this as a problem because fuel temperature is conservatively managed with what they consider to be a wide enough margin for safety. Also in the event of temperature system failure they would simply use TAT.

When the fuel-temperature-sensing system is inoperative, the FUEL TEMP SYS message is displayed. The flight crew then is instructed to use total air temperature (TAT) as an indication of fuel temperature.

...the potential exists for fuel temperatures to approach the freezing point. However, current airplane systems and operating procedures provide confidence that fuel will continue to flow unobstructed to the engines in all plausible cold-weather conditions likely to be experienced on polar routes.

A review of the service history of transport airplane operations worldwide for the past 40 years does not show a single reported incident of restricted fuel flow because of low fuel tank temperatures. This service history affirms that the criteria used to establish the advisory message are adequate and conservative.

Well there's a first time for everything. On a day when (allegedly) all other aircraft flying through the same area were dropping to lower altitudes to avoid the exceptional cold, what was different about this one?

[/quote]

 

[megawatt]

A review of the service history of transport airplane operations worldwide for the past 40 years does not show a single reported incident of restricted fuel flow because of low fuel tank temperatures. This service history affirms that the criteria used to establish the advisory message are adequate and conservative.

Fuel properties in extremely low temperatures is extremely well understood and has been for many years (long before my personal involvement with aviation) and, in the 28 years or so that I have been working with and flying aircraft, I have never heard of any serious issues directly related to fuel flow problems attributed to low temperatures so, in my own experience, I would agree with Boeing

My money is still on the fuel flow management system ... Computers!

One thing I would add though ... It's highly unlikely that this will ever be admitted by anyone or disclosed in any report ... Just covertly remedied (if it hasn't been already)

MW

 

[blondini]

Maybe a computer freezing problem then?

 

[megawatt]

Indeed

 

[Softus]

I think the incident was caused by X, unless someone comes along who actually knows about X, in which case I'll claim that it's definitely Y. But I might change my mind later based on the wild presumption that everything that the AAIB writes is a lie, at which time I'll just say that I knew along it was Z.

 

[megawatt]

No positive contribution from Softus once again ... Yawn.

Any particular reason why you're acting like such a prat Softus or are you just bored?

MW

 

[kevnurse]

The fuel icing inhibitor doesn't prevent the fuel from freezing. It prevents the water particulate from icing up and blocking the engine filters. Therefore, the mimum fuel temperature in the tank must be greater than the fuel's waxing (freezing) point; normally in the order of minus 45C (Jet A1), thus ensuring that the tank booster pumps can push the fuel towards the engines. If some of that fuel is waxed when it gets to the engine, it will partially block the filter such that idling RPMs may be achievable but not cruise/approach power. By the time that they do the invest, the fuel will have warmed up to normal levels and the evidence is gone.

It would take a long time for the very low ambient temp to cool the fuel. But, IMHO, waxed fuel seems likely to be the cause because it is the only factor that is common to both engines. Megawatt, I understand your thoughts about engine management, but the aircraft would not get a certificate if both engines are managed by the same software or hardware. That would create a potential single point of catestrophic failure, which is not allowed.

 

[Softus]

D'ya know what? I actually really reeeelly think it's the A thing, not the Z one, on account of the special remote wiggle blocker servo.

Y'all might think that this is just conjecture, based on almost no information, but I actually have detailed knowledge of the 777 fuel systems and am an expert at gambling when there's a complete absence of scientific data.

 

[empip]

... Megawatt, I understand your thoughts about engine management, but the aircraft would not get a certificate if both engines are managed by the same software or hardware. That would create a potential single point of catestrophic failure, which is not allowed...

...Airbus employs a design diversity technique called multiversion programming or N-version programming [3]. In multiversion programming, several system implementations are prepared from the same set of requirements by different developers under the presumption that the designs prepared by each developer will be independent - that is, the probability that one implementation will fail on a particular set of inputs given that another implementation has failed on those inputs is equal to the probably of the implementation's failing alone. The various implementations are then assembled into a classical redundancy architecture in which they are run in parallel on the same inputs and their outputs are passed into a voter to check agreement. If a design fault is activated in one of the implementations, then, according to the theory, it is unlikely that the other implementations will also possess the fault and should continue to function. Clearly, the assurance that can be placed on multiversion programming rests on the assumption of design independence, and evidence exists that this assumption does not hold for all types of software systems [7].

The Boeing 777 FCS was not developed through multiversion programming but rather by employing diversity in the microprocessor architecture. Boeing compiled the software for the 777 FCS for multiple machine architectures and runs each version in tandem during system operation. This approach allows the 777 FCS to tolerate design faults in a specific microprocessor well as those introduced during compilation. It does not, however, provide any resilience to faults resulting from errors in the common source code from which the versions were built [13].

................Pip's underlining.
I am sure there was a kerfuffle about the 777 commonality - way back - but not sure...


Hopefully no stone unturned...

...A data analysis team, working with statisticians from
QINETIQ, are reviewing and analysing the recorded data from a large sample of flights on similar aircraft.
No individual parameter from the flight of G-YMMM has been identified to be outside previous operating experience.
The analysis is concentrating on identifying abnormal combinations of parameters...

Take a free trip down the Trent engine... Click for R-R demo I imagine the rpm encountered will be a surprise to some.
Amazing just what air cooling can achieve... Note the turbine temperatures. See how it is done...
Interesting enough to m/c the holes then they have to be measured - size and angle of penetration.. Not a cake walk.
http://www.eng.ox.ac.uk/turbo/andrewwheeler.html

A single Trent 892 burns approx' 4500 litres of fuel per hour in cruise, 16.5 Imp gals/min.
Now we have some idea of heat involved and amounts of fuel required by engine.

There is a decent discussion on Flight / Ground idle ( hi / lo idle) from about 'reply 5' here :-
http://www.airliners.net/aviation-forums/tech_ops/read.main/226152/1/#1

 

[Softus]

The Boeing 777 FCS was not developed through multiversion programming but rather by employing diversity in the microprocessor architecture. Boeing compiled the software for the 777 FCS for multiple machine architectures and runs each version in tandem during system operation. This approach allows the 777 FCS to tolerate design faults in a specific microprocessor well as those introduced during compilation. It does not, however, provide any resilience to faults resulting from errors in the common source code from which the versions were built [13].

Having worked on one of the tandem 7J7 FCSs, and on other 777 software systems, I have reason to believe that the above is some highly misleading b*llocks.

Anyway, the cause of the problem is clearly the B4 solenoid getting hypothermia.

 

[empip]

Are you saying that William S. Greenwell and J. Graham Alsbrooks are liars? Misinformed or quoted out of context ?

 

[Softus]

Are you saying that William S. Greenwell and J. Graham Alsbrooks are liars?

I'm not passing comment on the above two writers, since I haven't seen their original document.

Misinformed or quoted out of context ?

I couldn't [easily] see which document the text was lifted from, so who knows whether or not it was ever meant to be factual, or whether it meant something else in a different context.

However, in this context, it's inaccurate and misleading.

Anyway, it can't be the B4 solenoid after all, because there's a widely misunderstood noise-suppression capacitor that only operates when the 'gear is down, so it absolutely has to be the C9 anti-surge thigh-wrister.

 

[blondini]

Anyway, it can't be the B4 solenoid after all, because there's a widely misunderstood noise-suppression capacitor that only operates when the 'gear is down, so it absolutely has to be the C9 anti-surge thigh-wrister.

Ah, but the landing gear was down.

 

[empip]

I'm taking my ball away, so there !!

 

[Softus]

Ah, but the landing gear was down.

That's exactly my point. The capacitor was in use, as shown by the lack of Radio 1 being played on the cabin crew intercom.