737 NG Fuel system

[Becker Junior]

What is the reason for the fuel temperature to be taken from tank 1?

[DeltaT]

Taking an utter guess, probably under the assumption that the fuel temp will be more or less the same in all tanks so only one temp probe is required, and they went with tank 1.
Imagine all the information you would need to display to show all the individual tank temps, which at the end of the day isn't really high on the priority list to monitor I would have thought. (well, taking into account its a 737 and not airbourne long enough for long haul fuel iceing considerations).
Just a thought.

[Flight Detent]

Hi BJ,

I believe that the #1 hydraulic system is cooled in the #1 main tank, and with the higher load, will experience the highest heat transfer.

As a sidenote, our B737NGs have fuel tank temperature sensing for all 3 wing tanks, #1, CTR, and #2.
I only mention that only to show that there is a very big variation in 737NGs out there!

Cheers...FD...:)

[greenslopes]

When facing north in the southern hemisphere the left wing has the longest period of sunshine, System "B" has the greatest # of services and it's heat exchanger is in the Right wing so go figure!

[BelArgUSA]

For those of you that are Boeing virgins...
xxx
The nº 1 tank, in all previous Boeing airplanes, has been the tank where the fuel temperature probe was located... in the 707, the 720, the 727, your 737, and in 747... I would expect it to be in nº 1 tank as well,, in the 757, the 767, the 777 and the 787... Shall we inquire about the 797... ?
xxx
Those of you "superior" 737 NG pilots (NG, is it "no good" ?) ... even your sliding cockpit windows are the same as... 707, 720 and 727. I would even guess that your "ESSential PWR" items list is the same as... my 727 that are now retired in the deserts...
xxx
When you go from one type of Boeing, to another type of Boeing, you notice that many things remain "same"... I went from the 727 to the 707 in my early days, and if I had to study hard initially, for the 727, I found to be quite easy when I went to the 707... it was a "repeat" of acquired knowledge... Even for the 747, the airplane is just a big 707 with the 727 accessories...
xxx
So when you ask yourselves "why" in the 737, just ask yourselves "how" the other Boeings were.
xxx
And I had a few years on DC-8s... could not sleep in the classroom.
All was different from Boeings...!
xxx
:D
Happy contrails

[stilton]

BelArg,

Always enjoy your interesting and informative posts. Wish you the best on your upcoming retirement and trust we will still enjoy your regular contributions after that date.

[BOAC]

The nº 1 tank, in all previous Boeing airplanes, - I take it from that you don't have an answer? I cannot find one, but normally 'historic' decisions have some reasoning behind them.

Aside from which, I believe that with the increased range and altitudes nowadays, Flight Detent's system should be mandatory on all applicable new aircraft. Yes, even the 737 NG - 7 hours at 410 is enough!

[ampclamp]

I'm sure mr boeing had a reason way back when but for me its historic.
Makes it easier for us wire jerkers to find.

[BelArgUSA]

Location of fuel temperature probe (Boeing airplanes) is in the "coldest" tank.
Coldest (sorry, I use that designation) by its location, and absence of heat exchangers.
Such as hydraulic system heat exchanger/coolers.
And obviously, a CWT is always much warmer...
xxx
If fuel temperature probe inoperative, assume it is same as TAT or RAT.
In 747, the selector reads fuel in tank nº 1, or at each engine...
Fuel gets cold and you worry for 7 hrs cruise at 410... learn PanAm's technique.
On long flights, they kept fuel residual in the 747 CWT.
I recall F/Es generally keeping up to 5 tonnes of "warm fuel" in the CWT.
If need be, or at end of cruise, that fuel was transferred to "cold tanks".
xxx
If your Boeing aircraft type cannot do tank to tank transfer, you still can do CWT fuel to engine.
:8
Happy contrails

[john_tullamarine]

I recall F/Es generally keeping up to 5 tonnes of "warm fuel" in the CWT.

Hopefully that was the AFM procedure ? else the fatigue life might be a concern after a lengthy period of hours .. ?

[BelArgUSA]

I understand your objection.
Back then, I did not see such procedure in the AFM or AOM...
It was a PanAm "expert old timer F/E - in-house - procedure".
Heard that TWA, and NWA did the same...
xxx
So there is a potential structural fatigue for keeping 5 tonnes in CWT since not in AFM.
And I would be telling you "well... sorry, that is the way they did it..."
However I got something else to justify that there is nothing contrary to the AFM.
xxx
Many 747-100 had a "second career" when converted to 747-100SF cargo...
They just located a side cargo door between L-4 and L-5...
And reinforced the floor as required for the pallet weights.
Then...
It was found that these 100SF were very "nose light" or "tail heavy" when empty.
Boeing solution was to recommend leaving... 5 tonnes in the CWT as ballast fuel.
The fuel (despite being ballast) could be used if need be.
So, as far as I am concerned, 5 tonnes in CWT for ballast or warm fuel is structurally similar.
Even with absence of any note or warning in "the bible"... QED...
xxx
JT, I know PanAm and BOAC were competitors on the Atlantic.
But I am certain that some old BOAC F/Es had a few techniques that were "not published"...
And who knows, Air France and Lufty did same. We spied on each other.
I recall mentioning it to my Sabena buddies over a beer in Brussels.
xxx
Example of another procedure I often did...
Old 747 sometimes have leading edge flaps failing to extend (bleed air).
It is due (supposedly) to cold soaked actuators.
If it happens, requires alternate (electric) extension of LE flaps. Time consuming.
So with that in mind, on long flights, I always operate wing TAI 1 hour before top of descent.
Ever since doing so, LE flaps (warm...?) never fail to extend as required.
That also is not "in the book" - but old timers know how to...
:ok:
Happy contrails

[galaxy flyer]

BelArgUSA

Now that WAI idea is a goodie, in my present plane we have had problems extending the LED after departure from wet/high humidity locations and long sectors at high levels (>430). Might try it if the conditions seem to require it.

GF

[john_tullamarine]

"expert old timer F/E - in-house - procedure".

First I declare my biassed preference to 3-crew ops with a real off-the-hangar-floor F/E .. albeit that 2-crew is great fun operating.

There would only be a problem if the AFM had a fuel usage requirement which was being subverted. If such didn't exist, then it is reasonable to presume that the fuel usage was not critical.

Re freighter conversions, and on the same sort of topic, when Ansett converted the Electras, they picked up an increase in MZFW by imposing a minimum fuel load in the outboard tanks for bending relief.

Love the WAI trick ..

[Flight Detent]

...The fuel captured in the CWT is fine if the actual ZFW and the maximum ZFW will allow it.

[john_tullamarine]

On the surface, yes .. but, if the fatigue calcs are based on a specific fuel usage sequence then there may be some devil in the detail.

[BelArgUSA]

You probably know this JT...
Loading fuel, structural considerations. And here, about the 747 classics.
We deal of course with MZFW in one hand.
Then the location of that fuel in the airplane... CWT and wing tanks.
Yes, there is definitely a sequence for fuel use.
Structural considerations (again) and CG location because of swept wings.
xxx
We load 747s wings first. Main tanks, then reserve tanks nº 1 and 4.
Then it is filling the CWT... some even have an additional FWD AUX tank.
Then we finally fill reserve tanks nº 2 and 3 (near the tip of wings).
xxx
Taxi and takeoff is done main tanks to their respective engines.
As soon as flaps up, reserve tanks nº 2 and 3 are emptied (reduced VMO limits).
Then we empty the CWT to achieve an aft CG (takes hours at times).
Finally, our reserve tanks nº 1 and 4 are emptied at top of descent.
We generally land with say, 3 tonnes remaining in each main tank.
xxx
A structural limit that is not often mentioned by pilots is WBW wing bending weight.
I ran across that limitation on some Learjets, in lieu of MZFW.
It included ZFW + any fuel located in the CWT...
xxx
In a 747, you could load a fuselage with "extra payload" in cabin in lieu of fuel in the CWT...
Of course, you would never be able to jettison fuel to achieve max landing weight.
Or, obviously, throw the "extra payload" out of the cabin through the doors.
Makes me laugh. I call this "experimental increase of airplane payloads".
xxx
We are quite far of the original 737 fuel system questions...
:}
Happy contrails

[spannersatKL]

Be interested to know how you can transfer fuel from the CWT to any other tank in flight? I know on the Classic 74 it is possible to run the override jetttison pumps in the CWT, opening the jettison transfer valve pressurises the refuel / jettison manifold but you would need to be on the ground to select the refuel valve of the individual tanks open? There is of course gravity transfer from the reserves to their associated main tank but this would be colder fuel the tanks being located outboard of the outboard engines.
It is possible to feed the cross feed manifold from the wing tank jettison pumps via the CWT jettison transfer valve but I cant see how to get the fuel in to any other tank in flight. (Can be done on the ground in a number of ways).

[BOAC]

No airborne tank to tank transfer on 737s but Flight Detent's system is a good second best.

[KristianNorway]

I have read it was a remnant from the 737-200. As stated above, the heat exchanger in hydraulic system A had less capacity than the one in hydraulic system B, and hydraulic system A's heat exchangers are coupled to the left main tank (tank 1). But then.. what is the reason?
The underlying reason for this (and now this is my theory), is that it's because of the hydraulic hiding in system A. The landing gear is coupled to the hydraulic system A and you see the hydraulic quantity indicators showing less for system A when airborne than on ground because part of the hydraulic fluid is trapped in the landing gear system.
With less heated hydraulic fluid in system A there is less capacity to heat the fuel.