Boeing 777 - LHR short landing.

[Agile]

The report went into considerable detail on the question of fuel temperature but seems to be saying the measurements were well within the acceptable parameters.

Tony

 

[blondini]

AFAIK there is only one fuel temperature probe which is in one of the wing tanks on a 777. It was abnormally cold and who knows what could be happening elsewhere in the fuel system with a sustained ambient temperature well below the freezing temperature of the fuel...

 

[Softus]

I was interested in this accident from the outset when, within 24 hours, it became clear that they did not suspect any serious design problem with the plane because they did not take the usual precaution of grounding the fleet.

Please list at least three incidents and at least two examples of aircraft being grounded in those incidents. Alternatively, please explain what you mean by "usual".

My intitial thought was that it may well have been a crew operational error and I wondered if they had allowed the engines speed to reduce to far to enable them to spool up fast enough when power was required to maintain the glide path.

Could you explain how that's possible when the aircraft was not under control of the crew during that flight phase?

Unfortunately, that further interim report lacks enough technical depth to enable me to come to any conclusion or even make any guesses.

Nonsense. The reports even states the conclusion, being a restriction in the fuel supply lines.

It seems to imply that the plane was in full ILS mode and the crew were leaving the systems to themselves perhaps without watching the performance as closely as I would have expected them to.

It doesn't imply anything of the sort.

I presume the HP pumps are fed by LP pumps

Why?

What evidence does cavitation leave in the outlet ports?

Cavitation evidence. I thought everyone knew that.

 

[Softus]

I think the extremely low temperatures encountered during the flight could become more significant.
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...the crew accepted an ATC request to climb to 10,600m where the ambient temperature was approximately -65C...
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Could the fuel temperature in some part of the fuel system have fallen below that indicated to the crew and eventually lead to a partial blockage?

Good observations. There's something to be concerned about when the ambient temperature is below the fuel freezing point.

I can't help but remember the Challenger O ring failure and NASA's "nah, it'll be alright mate" attitude before the launch.

 

[Softus]

I have removed a post which served no iuseful purpose.

Was that during a break you took from shooting fish in a barrel?

 

[blondini]

Anyone interested in this should review the AAIB special bulletin 1/2008 as it appears to have been amended since first sight and contains a lot more technical detail than the brief update. I don't recall seeing some rather significant details in previous links. http://www.aaib.dft.gov.uk/cms_resources/S1-2008 G-YMMM.pdf

There are also two links within the previous link posted by empip that make for interesting reading.
http://www.iasa.com.au/folders/Safety_Issues/RiskManagement/cold_fuel.htm
http://www.iasa.com.au/folders/Safety_Issues/others/The_BA038_LikelyCause.html
In particular I recall that the practice of Continous Descent Approach was somehow criticised for increasing risk in the event of engine failure during approach. I can't remember the logic behind that, but CDA comes up again in at least one of those links. There is something along the lines of prolonged periods of reduced engine power during CDA reducing the efficiency of fuel warming systems, and also the suggestion that during CDA any loss of engine thrust control could go unnoticed until the crucial point where more power is called for.

 

[megawatt]

Fuel is heated by the engine Fuel Cooled Oil Coolers (FCOC's) ... Primarily there to cool the engine oil but a neat side effect is that the process also heats the fuel

During a prolonged flight idle descent the engine temps would be lower than a powered approach and the fuel temperature not increased quite so much ... This is a standard pattern though for long final approaches i.e. where a circuit pattern isn't necessary and the aircraft has been cleared direct to finals and it would be odd for the system not to cope with it, particularly on such a state-of-the-art aircraft.

Softus wrote:

Good observations. There's something to be concerned about when the ambient temperature is below the fuel freezing point.

Not really Softus as aircraft regularly fly in such conditions, that's why they have FCOC's and, don't forget, that aviation turbine fuel does contain anti-icing inhibitors to ensure it doesn't freeze solid (maintains a mushy flow).

You never know though

Softus: Why are your replies to Agile's posts so hostile?

MW

 

[megawatt]

AFAIK there is only one fuel temperature probe which is in one of the wing tanks on a 777. It was abnormally cold and who knows what could be happening elsewhere in the fuel system with a sustained ambient temperature well below the freezing temperature of the fuel...

Whilst I'm not familiar with the 777 specs in detail it would be highly unusual to just have a single fuel temp probe as they usually work in groups using discrimination logic to error check and average a value ... Actually, it's highly unusual nowadays to have one of anything on an aircraft

Wing tanks would be a good choice for them though as the fuel will be at its coldest in these areas.

MW

 

[blondini]

AFAIK there is only one fuel temperature probe which is in one of the wing tanks on a 777. It was abnormally cold and who knows what could be happening elsewhere in the fuel system with a sustained ambient temperature well below the freezing temperature of the fuel...

Whilst I'm not familiar with the 777 specs in detail it would be highly unusual to just have a single fuel temp probe as they usually work in groups using discrimination logic to error check and average a value ... Actually, it's highly unusual nowadays to have one of anything on an aircraft

Wing tanks would be a good choice for them though as the fuel will be at its coldest in these areas.

MW

The 777 has a fuel temperature probe located between ribs 9 and 10 of the left main tank. The probe is approximately 12.6 in from the lower wing skin and is located one rib over, approximately 40 in outboard, from the aft boost pump inlet. Because the left wing tank contains a single heat exchanger, its fuel can be slightly colder than that in the right wing tank, which contains two hydraulic heat exchangers.

If there is only one probe, it would make sense for it to have more than one sensing element. Maybe it would have been better to say there is only one temperature probe location.

I know aircraft are supposed to be able to cope with such cold conditions and there are management strategies to maintain fuel temperature, but I'm sure I recall reports that pilots said they had never seen temperatures so low as on that day.

 

[Agile]

Fuel is heated by the engine Fuel Cooled Oil Coolers (FCOC's) ... Primarily there to cool the engine oil but a neat side effect is that the process also heats the fuel
MW

Isn't there some degree of temperature control there so that the fuel is not overheated when the aircraft is in a long holding pattern over a Saudi desert airport?

I cannot immagine there is not at least one fuel temp sensor per tank. I would also have expected there to be seperate reading software for port and starboard!

Tony

 

[megawatt]

Isn't there some degree of temperature control there so that the fuel is not overheated when the aircraft is in a long holding pattern over a Saudi desert airport?

Many aircraft are fitted with Air Cooled Fuel Coolers (ACFC's) for this very purpose

I cannot immagine there is not at least one fuel temp sensor per tank. I would also have expected there to be seperate reading software for port and starboard!

Me neither, it makes no sense at all only sampling fuel temperature in a single tank ... It could leak (for instance) and be isolated from the system by the crew ... Far more usual to disperse sensors across the system.

MW

 

[blondini]

The temperature probes in the 747-400, 777, and MD-11 are located where the bulk of the fuel is coldest. However, 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.

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. Because the probes are located near the bottom of the tank, the temperature reading is representative of the critical fuel temperature in the tank.

 

[megawatt]

No problem with any of that, totally sensible IMO ... Probes does suggest more than 1 though Blondini, where does it say that a probe is only fitted to a single tank as that would be really odd?

MW

 

[blondini]

... where does it say that a probe is only fitted to a single tank as that would be really odd?

MW

My quote was from [http://www.iasa.com.au/folders/Safety_Issues/RiskManagement/cold_fuel.htm

It also says

On the MD-11, a fuel temperature probe is located in the outboard compartment of tank no. 3 and another is in the horizontal stabilizer tank

And of the 747-400

The temperature signal originates from a single resistance-type temperature probe located within the no. 1 main tank. The temperature probe is mounted on the rear spar approximately 8.5 in off the bottom of the tank and approximately 40 in outboard of the aft boost pump inlet.

 

[megawatt]

I'm not sure that the report is saying there is only a single sensor, it depends how you read it.

With any luck someone will know the detail and post