737NG estimated arrivel fuel.

[Lucky Angel]

Since i am on this a/c about a year now. I noticed that about 200-300kgs of fuel dissapear from the FMC estimated arrival value once you start your descend. For example the other day i had an estimated arrival fuel of 3000kgs. Five(5) minutes into my descend this figure was showing 2700kgs.
Anyone knows why? Thanks.

[BOAC]

I don't know about "once you start your descend" but it has always been prudent to take about 200kg (735) to 300kg(733/4) off FMC predictions IN THE CRUISE and to add 2-3 minutes to FMC landing time. I have always done it, it always works, and I assume it is due to the FMC programme not actually reflecting how we really get directed/etc. I do it on the NG as well.

Of course, a few shortcuts and a bit of 'ace' handling can restore the predictions:D

[The Greaser]

Also if you actually compare what the FMC fuel totaliser gives you and compare it to actual fuel gauge readings, you will see that it is frequently 200kg or so too much. When doing fuel checks I generally ignore the FMC.

[CaptainSandL]

The gauges vary slightly with body angle particularly with small amounts in the centre tank.

Personally I always subtract 100kgs from FMC fuel qty on NG's and 200 on classics.

Dont forget you will burn some fuel using reverse thrust on landing and taxying in.

S & L

[Brenoch]

I seem to recall that it was almost always a discrepancy of about 100 kgs between fuel used on the engine display and the fuel used on the ACMS tech log page on the CDU. (NG)

[stator vane]

the excell version of the 737-800 operations manual, in the fuel system states that the accuracy is 2.5% of full indication. i don't have the book at hand to quote exact verbage, but i do remember the number. and what little i know about the operations of capacitors etc, (very little indeed) the body angle change could account for some of the change.

and then when you consider that with 1800 total in the tanks you will start getting low fuel quantity lights coming on, adding to the distraction of what already events might be causing a tightning of the bum hole, and then the final reserve figure staring you in the face, and then the possibility that everyone and his/her brother might be trying to get into the same alternate that you will be steering towards, moves me to think at the flight planning stage, "fuel is our friend" for sure.

[Carpathia]

Don't know about the 737, but in a 146, you "GAIN" fuel in the descent, usually landing with about 200kg more than the GNS predicts in the cruise. Reason being, the GNS predicates its estimate on arriving overhead the airport at crusing altitude and speed. As you bring back the power for the descent, you burn less fuel than if you had maintained a cruise power setting.

[The Greaser]

The 737 is much more sophisticated than that , it calculates the fuel burn based on the Vertical profile entered into the FMC, and assumes idle thrust descents to any altitude constraints entered into the FMC and then level flight until the next idle descent etc etc.

[jonny dangerous]

I am removing the initial part of my post, until I get my crap together....

JD

GREASER

I must take exception to part of your statement that reads "and assumes idle thrust to any altitude constraints...".
Our FMCs (U10.5) in the 700NG may be different from yours or possibly operated specified, who knows.

However upon carrying out an arrival with a "below profile" restriction such as 10000 ft and 250 knots at something greater than let's say, 40 miles from the airport, and then maybe a later 4000 ft and 200 knots on a downwind abeam point, the resultant descent profile will be a "power on" one.

You can spot them easily enough (Toronto CYYZ has lots) ahead of time by comparing the next waypoint's distance crossing altitude to the previous waypoint altitude constraint. If this shows a profile much flatter than 3:1, it will be power on. (i.e. [Alt change/distance]<3:1)

I personally take the AFDS out of VNAV when it has calculated a power on phase and go VS and descend at 500-1000 ft , diverging above the "path" and monitoring the VB (LSK 3R) on the DES page until the next waypoint reads closer to 3:1 and then go level change at the appropriate distance assuming speed requirements and wind direction. Sometimes I can sneak back some fuel from the miserly bugger (by staying up higher, and longer), or so I delude myself.

UPDATE

From Bulfer's FMC USER'S GUIDE (p160):" U10.3 option:Point to point descent is software option (OPC)...Only the first descent is computed at idle;subsequent paths may or may not be at idle..."

From FCTM(p3.17):"shallow vertical path segments may result in the autothrottle suppying partial power to maintain the target speed..."

So my apologies GREASER. Apparently our set up is operated specified following U10.3, according to Bulfer. Cheers,


JD

[stator vane]

page 12.10.3
march 29,2004
"accuracy is +/- 2.5% of fuel tank's maximum capacity."

as to the top of drop change, personally i have never watched it that closely. sometimes i land with more, sometimes with less. too many variables for me to keep track of.

at least i did find the 2.5 quote straight out of the boeing book.

in case it might matter, the number at the bottom of the page is:
D6-27370-8Q8-SBE


cheers;