Hi folks, 

 

sometimes I am in doubt about my English... 

The intro document on the NGX reads on page 121: 

 

 

'Center Wing Tank fuel quantity measuring system capacitance

error modeled in the Center Wing Tank. The quantity will vary

accurately during climbs/descents. This is an airplane

Idiosyncratic behavior.'

 

Actually I don't get it. IMU the capacity of the Center Wing Tank seemingly varies depending on the altitude. 

However, at FL0 and FL250 I could 'load' it with 13063kgs of fuel. What's going on?? 

 

(First I thought the amount of fuel in the middle would vary during climbs and descends, but my initial load of 2000kgs, centre fuel pumps off, was quite stable...) 

 

:Confused:

 

:unsure:

Not altitude. Attitude. if you have 15 degrees nose up pitch, the same amount of fuel in the tank will read a different amount than if you have a 3 degrees nose down pitch. So you could load it with 3000kg, but the gauge will show 2955 or 3065 depending on how high or low your nose is.

 

If it is full to the brim and can't hold anymore this effect will be less obvious.

 

One way to test this would be to put 1000kg (or other nice, round number) in the centre tank, but leave the fuel pumps for that tank off. (There will be a warning about fuel config, but don't worry).

 

During climb, the 1000kg of unused fuel in the centre tank will show up on the gauge as a different number than if you are decending (pitch attitude). No fuel is going in or out of the tank during your climb or decent, it's just that the way the fuel is 'read' by the system means that it sees more, or less, fuel than is actually there depending on the aircraft pitch attitude.

 

I often hold about 100kg in the centre tank and notice it vary by around 20kg comparing from FL380 in level flight, and decending past FL200 in idle decent. I havn't actually tested it on climb, because I'm usually either using fuel from the centre during climb, or have nothing in there to start with.

 

Not altitude. Attitude.

 

Ok, then my first comprehension was right. 

Ah, and you might see it only without fuel quantity sum, but with three round gauges... 

Will test it, have to as WJA1222 from CYYZ to KLGA today... 

Ok, I did WJA1205 from KLGA to CYYZ on FL300 and filled 500kgs into the centre wing tank, pumps off.
Routing GAYEL J95 BUF V36 LINNG LINNG4:

1453z 500kgs T/O rwy04
1504z 491kgs passing GAYEL FL260
1505z 487kgs T/C FL300
1514z 468kgs FL300 over BUFFY
1519z 458kgs FL300 over CFB VOR
1527z 440kgs FL300 over VIEEW
1533z 425kgs FL300 over KELIE
1537z 412kgs T/D 15nm to BUF
1548z 391kgs 7800ft over LINNG
1557z 371kgs LND rwy 33R
1600z 371kgs O/B gate B11

This does not prove the attitude assumption...
It would be comprehensible that different aircraft attitudes influence the meassurement, but simply goes down as if the tank is drained slowly... (Wasn't it said that the SR71 leaks while sitting on the ground? Here it is the other way round...)

So, back to the original question: Which error is modelled in 737NGX?

You have less than 2000kgs in the left wing tank, this has opened the fuel scavenge system wich drained fuel from the center tank.

You will confirm that by checking the umbalance between the wing tanks.

To test it you must take off with fulll wing tanks (or more than 2000kgs on the left wing)

Or, if you feel better, load how much fuel you want but turn off the wing forward pump/s and the center tank pumps.

By turning off the left wing forward pump you will disable the fuel scavenge system

Hi Andrea, 

 

all completely true what you describe, I just tested it in short test flight. but what is the modelled error? 

 

 

'Center Wing Tank fuel quantity measuring system capacitance
error modeled in the Center Wing Tank. The quantity will vary
accurately during climbs/descents. This is an airplane
Idiosyncratic behavior.'

The real plane uses the center wing box to store fuel, that's the center tank, the box is almost a squared box and the  fluid will move inside with aircraft movements.

The fuel quantity uses capacitors and compensators to calculate the correct fuel quantity.

The wing tanks have a lot of capacitors (if I remember 11 + 1 compensator), the center tank uses only one capacitor and one compensator.

As for your car, if the pitch and roll varies the fluid will level inside, this will increase level in one side, reducing on the other side. fuel capacitor is in a position wich will reduce the errors, and the hysterisis of the system will help to have not too much errors. However a difference in values should be present at different angles.

NGX simulates that difference. But, to see it you must not use the center tank fuel, and change pitch and roll and mantaining that condition for minutes, this will let the system to stabilize.

Also the low pressure are pitch dependent, and if I remember the NGX simulates this also.

I never tried this on the NGX personally as this is something you will never see as inflight you will burn the ctr tank fuel before the wing one.

But, to see it you must not use the center tank fuel, and change pitch and roll and mantaining that condition for minutes, this will let the system to stabilize.

 

Well, I had 600kgs in the centre fuel tank, more than 2000kgs in the wing tanks and climbed out of CYYZ to FL350 *without* any change in the 600kgs on the MFD (display without sum, ie. the round gauges) and this takes while... 

 

Also 30° pitch down and up (no passengers were on board) didn't change the value at all. 

 

:mellow:

Edited April 7, 201313 yr by 19AB67

Hm, is 600kgs not enough due to the centre tanks geometry? 

Repeated steep and longer climbs and descends with 10000kgs, to no avail...

 

I still don't get it, except the scavenging, but that does not seem to be an error... 

I think you expect a big error, as fuel is vital for the aircraft there are strict limittions about the possible errors, it is something less than 3% depending on the loaded fuel.

If pmdg says that it is simulated, trust them, it is, you need only to find it.

But as told before, don't expect the disappearing of thousand ok kilograms of fuel, the value could also increase.

Use the values on the FMC setup page (wich uses more precision) and compare with the EICAS values. I think you will find a difference, if not, don't waste time in searching it.

 

I never tried this on the NGX personally as this is something you will never see as inflight you will burn the ctr tank fuel before the wing one.

In the real aircraft the center tank shows different readings between phases of flight - when the quantity is low. With say 500kgs, we can expect a center low pressure light to show during takeoff or during braking.

 

With a relatively full center tank, the howgozit will show initially an increasing excess peaking at around +200 kgs and ending up back at 0kgs once the center tank runs dry.

I think you expect a big error, as fuel is vital for the aircraft there are strict limittions about the possible errors, it is something less than 3% depending on the loaded fuel.

 

No, I am a physicist, I am used to very big and very small numbers. 

 

 

In the real aircraft the center tank shows different readings between phases of flight - when the quantity is low. With say 500kgs, we can expect a center low pressure light to show during takeoff or during braking.

 

With a relatively full center tank, the howgozit will show initially an increasing excess peaking at around +200 kgs and ending up back at 0kgs once the center tank runs dry.

 

 

Still I haven't seen it in the sim, who does? When? Are my settings right? 

 

Still I haven't seen it in the sim, who does? When? Are my settings right?

Personally speaking, I wouldn't expect PMDG to be aware of this undocumented characteristic or model such subtleties.

 

Personally speaking, I wouldn't expect PMDG to be aware of this undocumented characteristic or model such subtleties.

 

Hi Vagabondo, 

 

don't underestimate PMDG... the very indication of the 'error' I got from the 737NGX introduction document, delivered in the package, page 121... as indicated in the beginning of my thread. 

 

 

'Center Wing Tank fuel quantity measuring system capacitance
error modeled in the Center Wing Tank. The quantity will vary
accurately during climbs/descents. This is an airplane
Idiosyncratic behavior.'

 

All we found out that on the one side the centre tank fuel indication might vary according to the aircraft's attitude (not seen yet), and I learned that the centre tank gets drained with centre tank pumps off (but this is not the error). 

Personally speaking, I wouldn't expect PMDG to be aware of this undocumented characteristic or model such subtleties.

PMDG themselves wrote that they have this behavior in their sim, among many other quirks, such as doors sealing on takeoff and windows overheating on a hot day.

 

As for myself, I have seen the center tank quantity fluctuate on climb out. This is most easily seen when shutting off the center for take off when quantity is less than 3000 lb.