Fuel quantity measurement, 738

[KristianNorway]

Hello guys

I have a tech-question here that bothers me, so I hope that there is a knowledgeable 737-800 technician here that can offer some insight into the problem.

On all flights on the 738 until now and on different specific planes I have noticed the same thing. During inflight fuel checks the FMS indicates that we will have a larger amount of fuel at our destination when we still have fuel in the centre tanks, than when we do during the same fuel checks after the centre tank fuel is used.

On a typical sector of 4 hrs. the difference will be around 300 kgs.

It would seem that the measurement equipment in the centre fuel tank is less accurate than the main fuel tanks, causing a correction when the centre tank is empty.

Would you know the reason for this?

[Vortechs Jenerator]

I'm not a 73 guy but I'll just say it's unlikely to be a fuel quantity measuring issue. I'd guess a generic FMS software glitch. Hopefully a guy with the ticket will turn up soon.

You could get your tech service department (or engineering) to query it with Boeing.

[KristianNorway]

Here is one theory:

When airborne at cruise level the wing main tanks are far more exposed to the cold than what is the case for the center tanks. With differing temperature of the fuel one will necessarily have different specific gravity. If the totalizer calculates a mean specific gravity between the two (the cold wing tank fuel and the comparatively warm center tank fuel), and the center tank fuel is emptied, a too low specific gravity calculation basis will represent the wing tank fuel measurement and indicate too little fuel. At least until the totalizer has recalculated the actual - higher - specific gravity in the wing tanks.

I thought of this when my captain showed me how to check the specific gravity of the fuel - which we need to specify in our tech log I believe - if the fuelling agent hadn't specified it, by entering the FMS via the BITE. I had to use the SG value from the center fuel tank because the fuel in the wing tanks was mixed with cold fuel and would indicate the wrong number.

If this is the case it shows less fuel carried upon landing, but measures correct CG and hence correct amount of fuel after a while.

Could this be a plausible explanation?

[CY333]

I also have noticed difference on fuel indications due to s.g, especially afer the ac has been flying and you need to do a quick refuelling on t.u but also the water drain issue is a major.
Not doing your waterdrain before the first flight of the day can lead to issues.
WA320 with the FQI also when it comes to indications.

[HAWK21M]

The Size of the CTR tank is responsible for this condition,as area wise the fuel remaining is spread over a wider area.common on B757s too.
regds
MEL

[KristianNorway]

I'm sorry, but could you explain for a young pilot how that would affect the readings? :)