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Slick9

Real world ops question...

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morning all,

 

quick real world ops question regarding pre flight prep.  I used to start off my pre-flight routine by getting the FMC set up, run through Postion, Perf Initializations, etc.  Now with the 777 i've been using the groud ops to load fuel and this means the final weight and the CG are not available for awhile.  The more I think about it, this probably mimics real world ops pretty well as the final passenger and baggage (and possibly fuel) will not be known till just before the doors are closed. So my question is - in real world ops does the pilot setting the FMC/CDU up wait until all info is complete before finishing the setup, or do they have predictive figures based on historical or someother information that they use, and then tweak if the actual figures are different?

 

thnx!!

 

Richard

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my guess is they fill in all the info the fmc need based on the flight preparation doc then just prior to door closing when they receive final loadsheet they just check the figures and modifiy the fmc figures if needed ?

I think it's kind of procedures that are very airline variants, a real airline pilot could answer this

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thnx gents, looks like kyle's tutorial recommends entering the perf data as currently known and then ammending it as the final figures become neccessary..  It's sort of the same as the Runway assignment question, select a runway now and risk having to change it later or wait till you're assigned a runway and then enter the info in the CDU. 

 

 

Richard

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Our airline leaves ZFW and CG empty untill a loadsheet is received (by ACARS usually).

 

Capt reads the data from the loadsheet and enters it in the FMC.....FO writes the data on the flight plan and checks FMC entries are correct.

Then the Take performance calculation is done and entered in the FMC.

 

This way you dont run the risk of having the wrong data (preflight estimate) in there and taking off like that.

But if you develop a proper procedure to prevent that, then you can also just enter the preflight estimates.

We however dont even put the take off flap setting into the FMC until we have done the performance calculation.....again so you dont have the wrong estimated flap setting in there.

 

Entering the loadsheet data into the FMC really only takes a few seconds.

Everything else can be programmed without performance data (SID/Route/STAR/FIX page/Vnav page/Nav data page etc)......so I personally dont see the need to enter estimates.

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I'm full in with Rob on this... I'm always cautious of the danger due to changed focus as in your are doing the same flow over and over and it's habit but suddenly something unexpected happens and routine becomes a trap.  Set a manual fuel load, then get diverted and forget to go back and correct the estimate.

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Hey Rob,

 

T

 


This way you dont run the risk of having the wrong data (preflight estimate) in there and taking off like that.

But if you develop a proper procedure to prevent that, then you can also just enter the preflight estimates.

We however dont even put the take off flap setting into the FMC until we have done the performance calculation.....again so you dont have the wrong estimated flap setting in there.

 

This is exactly what generated my inquiry in the first place.  I was preflighting for a long haul KIAD-FAJS, and it took over an hour to load the required block fuel.  I had gone ahead and filled in all the CDU perf info, planning on updating it once I had the final fuel figure. Well I completely forgot to do so, luckily during the rush hour traffic jam to the departure runway I was looking over the perf numbers and I caught the mistake.  So I think I'll stick with my new technique which is to go in and enter the perf numbers once I have the final figures.

 

Richard

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Good points. Didn't even think of that for the tutorial. My angle there was generally not overloading people to keep them "on schedule" and not getting behind with a bunch of tasks just before departure.

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Hi Richard,

In the airline I fly for (and I'm on the single aisle Airbus not 777 but I know they have very similar procedures) we use Late Closeout Procedures (LCP). I don't know how many other airlines use it (I think American do and probably some of the other larger carriers but I'm not sure) but, as ever in flying, there're lots of different ways to 'solve' the 'problem' you're talking about.

Fairly soon after we arrive at the aircraft we're given a provisional loadsheet, this is generated by load control (and it is version controlled) based on figures from the commercial department. It contains all the information a normal loadsheet would and we use it to enter all the figures into the FMGC. The flightplan fuel calculations are usually based on the same weight (but from a few hours ago). When requesting our performance calculations we add a ton (might be 2 tons for the big boys, I'm not sure) to the Take Off Weight (TOW) and enter the performace data based on that higher weight.

Then the doors are closed and we push back and taxi out to the runway.

During that taxi out we expect/must receive a 'Final' loadsheet by ACARS (except in Italy where they don't like this procedure so we have to receive the final loadsheet before we release the brakes but load control are aware of this and it's rarely a problem). Once the loadsheet is received we must 'Acknowledge' it in ACARS (rumour has it it doesn't actually do anything but we're meant to push the button anyway).

There is a concept of a 'Compliant' final loadsheet which means that the final weight and trim is within set tolerances (for me, it's +1 and -2 tons of weight and +/- 2% in trim, with additional complications with the Airbus fwd CG calcs but I digress, I suspect it's different for the 777) of the provisional and hence there is no requirement to change any of the figures used for take off.
 
If a loadsheet is not compliant then the figures need to be changed before departure. As there are 2 (often 3 on the 777) crew it's not actually that big a deal to change them on the taxi out (one guy gets the new data and enters it in the FMGC then the other guy hands control to him while he checks it's all correct).

Our Before Take Off checks (and I believe it is in our 777 electronic checklist) include the item "Loadsheet". This refers to receiving the final loadsheet and making any alterations as required. This should procedurally catch any omissions in this process.

In my experience the commercial department is pretty good at giving us a worst case estimate for any given flight and 95% of the time the loadsheet is 'compliant' (usually with a slight weight decrease). 4% of the time the only change is the trim (which is obviously a very quick and easy change, move the trim wheel) and very rarely is there actually a big change that requires the reentry/recalculation of all the data.

In flight, passing FL100 we usually update the weights (unless the flight is really short, <1hr, then it's not required but most guys do it anyway) so the FMGC predictions are as accurate as possible.

The whole process sounds unweildy and in Sim World is quite difficult to do in a smooth and efficient manner, mainly because there's only one of you, in real life it's not a big deal. In the (relatively few) tubeliner flights I do, I tend to just issue myself with a 'Final' loadsheet (as I know exactly what's loaded on the aircraft) and enter everything in from the get go.
 

Just another take on one of the ways it's done in the real world,

 

Hope this helps,

 

Ian

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In flight, passing FL100 we usually update the weights

 

Hope this helps,

 

Ian

 

hi ian thanks you for this nice explanantion, could you explain what you mean by updating the weights after FL100, are you saying you're entering a new ZFW in the FMC ?

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Slick9

 

What a very interesting question to pose and revealing replies. Well done.

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hi ian thanks you for this nice explanantion, could you explain what you mean by updating the weights after FL100, are you saying you're entering a new ZFW in the FMC ?

Yep, carefully, also MACZFW these days but that might be an Airbus thing...

 

Ian

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How close are the loadsheet values compared to the aircraft's actual weight? I guess we'll never know because we don't really weigh the planes, but I wonder if the calculated values are a few hundred pounds off or up to a thousand or so?

 

I use one number for the FS ACTION (say 430.0) but use another for the FMC PERF INIT (say 431.2) to give some differences between actual and calculated weight.

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The actual weight and moment of an aircraft is measured with load cells, just like your highway department weighs 18-wheelers. That data is part of the aircraft certification and has to be redone with any modifications.

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With the exception of the 747, which can weigh itself if the optional Weight and Balance System is installed. The system uses strain gauges in the landing gear struts to calculate weight and CG position.

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Neat! But then, the sensors can only tell you the total aircraft weight, right? It doesn't know what of the total makes up the ZFW and cargo. Even then, why do we enter an independently calculated ZFW? I can imagine having that info to verify the plane's sensors.

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It doesn't know what of the total makes up the ZFW and cargo.

 

I can somewhat see your confusion here, but it's stated a bit awkwardly. Cargo is part of your ZFW. Fuel isn't. The sensors calculate the total weight and CG. The fuel tank sensors can get a decent reading of what amount of fuel is in there. GW - Fuel = ZFW. That's all you really need.

 

 

 


Even then, why do we enter an independently calculated ZFW? I can imagine having that info to verify the plane's sensors.

 

Trust but verify. Always.

 

Plus, sensors aren't error free. Remember that we have tons of that to deal with in aviation - IAS and CAS is only one of those issues. CAS exists because we realize that there are errors in collecting the data to determine our airspeed.

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I can somewhat see your confusion here, but it's stated a bit awkwardly.
Sorry, English isn't my language but you got my meaning. But yes, it almost seems like the real FMC could have a "ZFW autofill shortcut" like PMDG  B), then let the pilot verify against the loadsheet, of course.

 

Still though, while the cargo+baggage are weighted the pax and their carry-ons are only averaged or estimated. At 300 pax, assuming average weigh of 180lb*300 = 54000lb, and if the pax are closer to 170lb then it's 51000lb. Operationally it makes little difference of course (3000lb out of, say, 555000lb is less than 1%!), my point is only that the loadsheet's figure can be different than actual weight.

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Still though, while the cargo+baggage are weighted the pax and their carry-ons are only averaged or estimated. At 300 pax, assuming average weigh of 180lb*300 = 54000lb, and if the pax are closer to 170lb then it's 51000lb. Operationally it makes little difference of course (3000lb out of, say, 555000lb is less than 1%!), my point is only that the loadsheet's figure can be different than actual weight.

 

Yep, which is the entire reason I have a video about Weight and Balance. Too many simmers try and get into the gritty details of flight numbers because "the dispatch says X" when in reality, that dispatch is using planned numbers and not actual.

 

Still, the ZFW shortcut is already provided because the pilot simply confirms the GW, keeping in mind that he or she should also be verifying the other numbers.

 

It's like this:

X + Y = Z

 

If you verify X, you must also verify Y, but you should also verify that X + Y = Z. Either way, confirming Y (the ZFW, as an example) is only confirming one aspect of the equation that brings you to Z.

 

If you verify Z, you must also verify that Y and X bring you to Z (primarily because X - the fuel - changes over time). Confirming Z is only confirming one aspect of the equation that is made up of other variables.

 

However...since all of the variables are related (ZFW + Fuel = GW), confirming one value is essentially confirming the others (though you should confirm each individually). Moreover, the stress sensors are calculating the plane as it sits at that moment, which is the GW. In my mind, since the sensors are calculating the GW, then you should be confirming that value on the LSKs of the CDU, if that makes any sense.

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