AAIB BA38 B777 Initial Report Update 23 January 2008

[Stoic]

How many separate fuel temperature sensors?

NSEU

In post #258 I asked <<How many separate/totally independent fuel tank temperature sensors are fitted in the 777 tanks?>>

You have stated

The fuel temperature sensor is in the left tank,

Is that correct? Is there only one (common) fuel temperature sensor for the fuel to both engines?

Regards

Stoic

[M.Mouse]

I personally haven't seen any official statement on that until now.

That is because there hasn't been. I have never knowingly posted inaccurate information. They landed with in excess of 10 tonnes of fuel and the low fuel procedure was not carried out at any time during that fateful flight.

[r75]

Ref post#370, the 74s I refer to have been in a hangar for at least 24 hours, sorry if my post misled you. As an aside I for one do not envy the AAIB guys on this one, best to wait and see what they say next.

[Oldlae]

r75
I don't think you misled anyone who has personal knowledge of water checks, your observation was made at the very best condition of determining how much water might be in the tanks. If a water check was carried out in sub-zero temps and there was water in the tanks which might have frozen perhaps it would have not been possible to drain any fluid as the ice would have been at the base of the tanks. When the
G-MMMY engines were started there was evidently enough liquid fuel being supplied to the engines not water.

[Yankee Whisky]

L/D 777 gear down with landing flap setting (?)

QUOTE
Green Dot.....interesting post.

In the descent from cruise the engines would have been at Low Idle ( Gnd idle ) until any Flap/Slat or Gear selection, then this triggers the higher Flight idle speed. END QUOTE

With flaps and gear employed, what would be the L/D of the 777 ?
Assuming no power ,could it be 18:1?
At 18:1, 600 feet would have given 10,800 feet horizontal travel (a tad over two miles). I would not have been happy trying this, though!
I use 18:1 because I read somewhere that a widebody aircraft (I forgot the type) would do that in CLEAN configuration. Putting down gear and flaps would, therefore, require some power to be applied (engine spool-up time consideration etc). How much power would be required to give a L/D of 18?

[NSEU]

After the fuel goes through the fuel filter element, it goes to the hydromechanical unit (HMU) (RR = FMU). The HMU meters the fuel. The fuel goes from the HMU to the fuel flow transmitter and then back to the HMU. The HMU distributes the fuel into the fuel supply manifolds. The fuel supply manifolds send the fuel to the fuel nozzles.

The fuel feed on the RR is slightly different...

tank pumps ~ LP side of engine pump~fuel-cooled oil cooler ~ LP Fuel Filter ~ HP side of engine pump ~ Fuel Metering Unit (with fuel return to HP engine pump) ~ FF Transmitter ~ HP Fuel Filter ~ Fuel Manifold/Spray Nozzles.

Is there only one (common) fuel temperature sensor for the fuel to both engines?

I don't think you could call it "common"... The crew only know the temperature of the fuel in the left tank. Only God knows what the temperature is in the right and centre tanks On the 747-400, it is only known what the fuel temperature is in one of (up to) 9 tanks. Boeing, I'm sure did their tests and found that the left tank is always coldest. Whether this is true for an aircraft flying east to west (with the left wing in sunshine and the right wing in the shadow of the fuselage), I can't say... I don't know if this is negligible or not (compared to, say, ram air rise).

[FE Hoppy]

Thanks NSEU I've been trying to get hold of an engine fuel sytem schematic.

I'm going for LP fuel filter blockage. Is there a fuel filter bypass? and if so is there any indication?

[Stoic]

Common feature

Thanks NSEU

Quote:
Is there only one (common) fuel temperature sensor for the fuel to both engines?
I don't think you could call it "common"... The crew only know the temperature of the fuel in the left tank. Only God knows what the temperature is in the right and centre tanks On the 747-400, it is only known what the fuel temperature is in one of (up to) 9 tanks. Boeing, I'm sure did their tests and found that the left tank is always coldest. Whether this is true for an aircraft flying east to west (with the left wing in sunshine and the right wing in the shadow of the fuselage), I can't say... I don't know if this is negligible or not (compared to, say, ram air rise).

The 747-400 was therefore, as I recall, the same as the 747 Classic which I used to fly in the last century.

It appears, therefore, that if there were to be a failure or mal-function of the left tank fuel temperature sensor or gauge, both engines could be affected.

In an ETOPS aircraft, would it not be more sensible to have completely separate temperature monitoring systems for each fuel tank/engine system (disregarding the centre tank)?

Regards

Stoic

[GlueBall]

Boeing's take on low fuel temps . . .

"Properties of fuel at very low temperatures.
The fuel freezing point is the temperature at which wax crystals, which form in the fuel as it cools, completely disappear when the fuel is rewarmed. (This should not be confused with the fuel becoming cloudy upon cooling, which results when water dissolved in the fuel freezes, forming a suspension of very fine ice crystals. Airplane fuel and engine systems are designed to handle water ice crystals safely.)
The Jet A fuel specification limits the freezing point to a maximum of –40°C; the Jet A-1 limit is –47°C maximum. In Russia, the fuels are TS-1 and RT, which have a maximum freezing point of –50°C. (Note: Because specifications may vary by country, operators should ensure that they are using the appropriate fuel procurement specification for the fuel being dispensed.)
The maximum freezing point for some jet fuels can vary by the geographical region in which the fuel is refined or uplifted. Test methods for determining the fuel freezing point also introduce variability; reproducibility is approximately 2.5°C.
Some operators in the United States measure the actual freezing point of delivered Jet A fuel at the time of dispatch. Data show that the freezing point of delivered Jet A fuel is approximately 3°C lower than the specification maximum of –40°C. Table 1 shows the results of a study completed at several airports in the United States to verify the actual freezing point of Jet A fuel as delivered to the airplane. (An airline must verify the freezing point of the loaded fuel at dispatch if the airline uses a value other than the maximum specification.)


However, the fuel freezing point is not what dictates fuel flow to the boost pumps. The critical condition of cold fuel in an airplane fuel tank, in terms of flight safety, is its propensity to flow toward and into the boost pump inlets. Pumpability, or flowability, depends on the pour point of the fuel, defined as the lowest temperature at which the fuel still flows before setting up into a semirigid state. Generally, the pour point is approximately 6°C lower than the fuel freezing point. However, the exact relationship between freezing point and pour point depends on the source of the crude oil and the refining processes.
Because jet fuel is a mixture of many different hydrocarbon molecules, each with its own freezing point, jet fuel does not become solid at one temperature as water does. As fuel is cooled, the hydrocarbon components with the highest freezing points solidify first, forming wax crystals. Further cooling causes hydrocarbons with lower freezing points to solidify. Thus, as the fuel cools, it changes from a homogenous liquid to a liquid containing a few hydrocarbon (wax) crystals, to a slush of fuel and hydrocarbon crystals, and finally to a near-solid block of hydrocarbon wax. Because the freezing point is defined as the temperature at which the last wax crystal melts, the freezing point of jet fuel is well above the temperature at which it completely solidifies (fig. 4).
Refueling airplanes at different stations creates a blend of fuels in the tanks, each with a unique freezing point. The resulting fuel freezing point in each tank can vary widely. The flight crew must operate with caution and not automatically assume that the freezing point of the uplifted fuel is the actual freezing point of the fuel on board. Boeing published a procedure for estimating the freezing points of blends of Jet A and Jet A-1 fuel in service letter 747-SL-28-68 (Nov. 4, 1991).


If the freezing point of the fuel on board cannot be determined using the published procedure, Boeing suggests using the highest freezing point of the fuel used in the last three fuel uplifts. For example, if Jet A-1 fuel was used for two uplifts and Jet A fuel was used for one uplift, then a –40°C freezing point would be used for the current refueling. If Jet A-1 fuel was used in three consecutive refuelings, then a –47°C freezing point may be used for the current refueling. In the 747- 400 and 777, if the fuel freezing point is projected to be critical for the next flight segment, Boeing advises the transfer of wing tank fuel to the center wing tank before refueling. This makes it possible to use the freezing point of the fuel being uplifted for that flight segment.
Fuel systems and temperature measurement.
On the 747-400, the engine indication and crew alerting system (EICAS) displays the fuel temperature continuously on the upper CRT display except during jettison operations, when it is replaced by the FUEL TO REMAIN indication. 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.
When the fuel temperature on the 747-400 reaches –37°C, a FUEL TEMP LOW message is activated, and the fuel temperature displayed on the EICAS changes color from white to amber. The 747-400 system automatically defaults to the limit associated with the highest freezing point of fuel approved for use on the 747, which is –37°C for Jet A fuel. 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. (Instructions for this procedure are contained in the master minimum equipment list.)
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.


Fuel temperature on the 777 is displayed in white on the primary EICAS in the lower right corner. If the fuel temperature reaches the established minimum, the indication turns amber in color and the FUEL TEMP LOW advisory message is displayed (fig. 5). The 777 system automatically defaults to the limit associated with the highest freezing point of fuel approved for use on the 777, which is –37°C for Jet A fuel. However, the EICAS message can be set to other values. For example, if Jet A-1 fuel is used, the message can be set to –44°C (fig. 6).
On the 777, the fuel temperature can be entered in two ways: as the minimum fuel temperature or fuel freezing point. Both options provide an indication at 3°C above the fuel freezing point. Fuel temperature is not displayed during fuel jettison.
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. At 3°C above the fuel freezing point, the probe in the no. 3 tank signals a FUEL TEMP LO message display in the flight deck. To establish when the message should be displayed, the flight crew can enter the freezing point of the fuel being carried or select the type of fuel being carried. When the crew does not enter a value or specify the type of fuel, the system defaults to Jet A fuel, which has a freezing point of –40°C; a message displays at –37°C.


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.
Factors affecting fuel temperature.
Factors that affect fuel temperature are the size and shape of the fuel tanks, fuel management, and long-range operations at high altitudes.
The size and shape of the tanks significantly affect how quickly the fuel temperature is affected by wing skin temperatures. A tank with a high surface-to-volume ratio transfers heat through the wing surfaces at a higher rate than a tank with a low surface-to-volume ratio. Thus, fuel temperature is affected at different rates depending on the airplane model and tank design. For example, because the 747-400 outboard main tanks are long and narrow and have about half the total fuel volume of the 777 main tanks, the surface-to-volume ratio on the 747-400 main tanks is much higher. This means that heat transfer through the wing surfaces is greater on the 747-400, and the fuel temperature changes faster than it does on the 777. On the MD-11, the outboard compartments of tank nos. 1 and 3 have the highest surface-to-volume ratio. The next highest ratio is that of the horizontal stabilizer tank. These tanks are the most critical for fuel flowability at low temperatures on the MD-11.


Fuel is managed differently on the 747-400, 777, and MD-11, but in all cases, the wing main fuel tanks are the last to deplete. On some models, fuel in tanks with high surface-to-volume ratios is held until near the end of a flight. Whether a tank is full or partially depleted of fuel alters the rate at which the fuel temperature changes.
During long-range operations at high altitudes, fuel tank temperatures can approach the freezing point of fuel. On long flights, the fuel temperature tends to adjust to the temperature of the aerodynamic boundary layer over the wing skin. This boundary layer temperature is slightly lower than the TAT because theoretical TAT is not achieved. Initially, TAT is much lower than the fuel probe temperature because of the thermal lag of the fuel. Thermal analysis of the 747-400, 777, and MD-11 airplanes shows that the fuel tank temperature is driven more by TAT than airplane configuration.
Operations and procedures with low fuel temperatures.
In flight, a temperature differential must be maintained between the observed temperature indication and the freezing point of the fuel. For the 747-400, 777, and MD 11, the observed fuel temperature must remain at least 3°C above the specified freezing point. (The actual fuel freezing point may be used if known.)
When fuel temperature decreases to 3°C above the freezing point, a message of FUEL TEMP LOW displays in the 747-400 and 777 flight decks; the message FUEL TEMP LO is displayed in the MD-11 flight deck. If this condition is reached, the flight crew must take action, as described below, to increase the TAT to avoid further fuel cooling.


In consultation with airline dispatch and air traffic control, the flight crew decides on a plan of action. If possible, the action should include changing the flight plan to where warmer air can be expected. Another action is to descend to a lower altitude. The required descent would be within 3,000 to 5,000 ft of optimum altitude. In more severe cases, a descent to 25,000 ft might be required. Recent experience on polar routes has shown that the temperature may be higher at higher altitudes, in which case a climb may be warranted. The flight crew also may increase airplane speed; an increase of 0.01 Mach results in a TAT increase of 0.5° to 0.7°C. (It should be noted that any of these techniques increases fuel consumption, possibly to the point at which refueling becomes necessary.)
It takes approximately 15 min to 1 hr for a change in TAT to affect the fuel temperature. The rate of cooling of the fuel is approximately 3°C/h. A maximum of 12°C/h is possible under the most extreme cold conditions.
A minimum in-flight fuel temperature advisory message provides a margin of safety under all atmospheric and operational conditions to ensure that the fuel will continue to flow to the boost pump inlets. Besides the 3°C margin between the advisory message temperature and fuel freezing point, there typically is a 6°C difference between the freezing point and pour point of fuels, which provides an additional margin. 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.
However, flight crews on polar routes must be knowledgeable about fuel freezing points. Flight crews also must be cognizant of the en route fuel temperature and the possible need for corrective action to ensure continued safe, routine polar operations.


Operational aids for flight planning.
Boeing has developed the Fuel Temperature Prediction Program (FTPP) for the 777 and is developing FTPPs for other airplane models. The FTPP assists operators in addressing fuel freezing point concerns during the flight planning process. The program intended to interface with or be incorporated into an airline's flight planning system. The FTPP for the 777 has been calibrated with flight-test data obtained by Boeing and several airlines. The data are based on fuel temperature indicated by the fuel tank temperature probe. Details on FTPPs are available to airlines through Boeing Field Service representatives.
Measuring the actual freezing point of the fuel being uplifted can be a valuable step in the flight planning process for flights during which fuel freezing point is a concern. In general, actual fuel freezing points tend to be about 3°C below the specification maximum requirement. Details on measuring the freezing point when fuel is being uplifted are available to airlines through Boeing Field Service representatives."

[Stoic]

Only one common fuel temperature source for both 777 engines

Boeing's take:

The 777 has a fuel temperature probe located between ribs 9 and 10 of the left main tank.

Thanks Glueball. There is only one source of fuel temperature on the 777.

My question therefore still stands. Is it sensible to have the same single fuel temperature sensing system for both fuel/engine systems on an ETOPS aircraft?

Regards

Stoic

[TwinJock]

In the 747- 400 and 777, if the fuel freezing point is projected to be critical for the next flight segment, Boeing advises the transfer of wing tank fuel to the center wing tank before refueling. This makes it possible to use the freezing point of the fuel being uplifted for that flight segment.

To the technos out there - how is the fuel transfered from the Main to the Centre tank on the 777?

[woodpecker]

To the technos out there - how is the fuel transfered from the Main to the Centre tank on the 777?

Main to Centre tank,?? That sounds like the same tank to me!!

[Swedish Steve]

To the technos out there - how is the fuel transfered from the Main to the Centre tank on the 777?

Same as on all airliners. You open the refuel panel. Select Defuel. Open the refuel valve for the Centre tank. Then go to the flight deck and turn on the booster pumps in the wing tanks, and open the crossfeed valve. Some aircraft have a manual defuel valve like B737. (On the B737-200 you have to open the right engine LP fuel cock).
But can only be done on the ground with the refuel panel open.

Trouble with a B777 is that the refuel panel is a long way up, so you need a big set of steps.

I do it regularly on a A319. It arrives with about 3000kg of fuel in the inners at M20degC. I transfer it into the centre tank before refuelling. Then the wing tanks are filled with fuel at about P5degC, and the frost melts. Saves deicing.

[Milt]

GlueBall

Thanks for an excellent coverage of the freezing characteristics of Jet Fuels but it contains an assumtion which I believe to be a probable significant error.

Whilst the ICAO standard atmosphere defines the temperature at the tropopause (what is it 37,500 feet?) and above as -57 C this is only an average. As a TP with experience in tropical high altitude trials I have seen generally lower temps than standard at airliner cruising altitudes which then continue to go as low as -90 C up to about 50,000 feet. It is my understanding that the warmest upper air temperatures are over the poles. This phenomonen caused the Australian Air Force to develop its own Tropical Atmosphere standard.

I believe the phenomonen is generally world wide and hence the problems with low fuel temperatures may well be worst equitorially than polar.

[ChristiaanJ]

Hi Milt,

The ISA tropopause is at 11,000m, i.e., about 36,100ft.
Never too old to learn... I thought there was an international (ICAO or otherwise) Tropical SA, but obviously not!

Phenomenon well known... it was one of the things that allowed Concorde to get up to 60,000ft on the Barbados run, while on the New York run 57,000ft was generally the limit.

Another nasty in the tropics is the sharp temperature gradients you can run into, which tends to play havoc with a basic Mach Hold.

A bit out of topic here, since BA038 did certainly not pass through the tropics....

[NSEU]

I'm going for LP fuel filter blockage. Is there a fuel filter bypass? and if so is there any indication?

There is a fuel filter bypass on the Low Pressure Fuel filter. A differential pressure switch on the LP Fuel Filter will sense an impending blockage and will trigger an EICAS message before bypass occurs.

Rgds.
NSEU

[CONF iture]

M.Mouse, you may have direct access to still confidential information, but in that case I doubt you’re allowed to publicly comment, but you may also report what you’ve been told, or not, from more or less direct privileged source … !?
Anyway, if you tell me “You have never posted anything that was not factually correct” I’d like to take advantage of it:

1- Would you tell me if the earlier mentioned BOEING COLD FUEL MANAGEMENT PROCEDURE was carried out in PEK

if the fuel freezing point is projected to be critical for the next flight segment, Boeing advises the transfer of wing tank fuel to the center wing tank before refueling. This makes it possible to use the freezing point of the fuel being uplifted for that flight segment

2- Would you know if BA038 had to hold anytime before landing ?

[swiss_swiss]

FE Hoppy - mind telling me what you do for a living? I hope its not as a licenced aircraft engineer or pilot?

I refrained from commenting on the fuel/oil heat exchanger theory earlier in this thread but the question about the fuel filter is basic stuff.

of course there is a bypass mechanism for the fuel filter and a differential pressure system that indicates a clogged filter.

[phil gollin]

Quote :-

.......... of course there is a bypass mechanism for the fuel filter and a differential pressure system that indicates a clogged filter.

unquote

So, IF THE PRESENT "ASSUMPTION" IS CORRECT - that the problem revolved around slush/ice in the fuel then the bypass should have opened and thee differential pressure system given an alarm ???

Or could the slush/ice overwhelm the filter and bypass ?

???

[Stoic]

Please answer my question!

Is it sensible to have the same single fuel temperature sensing system for both fuel/engine systems on an ETOPS aircraft?

Surely some technically savvy 777 person can have an opinion?

Regards

S