Still haven't found the answer, but how can i update the TOW on the takeoff page

Myself, I reset the ZFW and execute. Then I go through the rest of the takeoff pages, and reset

the flaps, CG, which will clear out the V speeds, and let you enter those.   Works fer me..

I guess that's what you are asking..  ?

You don't update the TOW directly.  You update the ZFW on the PERF INIT page and the FMC will compute the TOW for you.

So how does that work then? When will i update and why? Because as you know this is a simulation world and once i set my load in FS OPTIONS its final, unlike in your world of flying!

 

 

So how does that work then?

 

As Joe mentioned. "Update" the ZFW (using the same numbers will still force an update). TOW is recalculated.

 

 

 

When will i update and why?

 

As I mentioned in the other thread, unless you're pretty far off the mark, then there's probably no real point. Simmers get wrapped around the idea of the numbers matching perfectly, when it's not truly an issue unless the error is large. The heavier number would be more conservative anyway, in terms of vspeeds.

 

 

 

Because as you know this is a simulation world and once i set my load in FS OPTIONS its final, unlike in your world of flying!

 

Not sure I follow. Once you add that last passenger in the real world, that number is final, too. If you do find a way that passengers would suddenly change weight, though, you would probably make a killing at GNC.

Ahh i see so if my planned ZFW was 56.2 i will renter 56.2? But when will i do this?

Renter at what point? If you set you're fuel and payload before you start anything on the init ref, perf init etc then the numbers will be what you need.

 

So you calculate everything, I use PFPX so I'll use that as a base, random payload which gives me passengers and fwd & aft cargo, and then finally fuel. Load up the plane, enter the release fuel, enter the pax and cargo, then head over the init ref and going through the pages all the way up to take off. I cross check the Vspeeds given in the FMC to what Topcat calculated, I usually pick Topcat's numbers but they are never far away in distance anyway.

 

I know others that ignore the pax and cargo numbers from PFPX and simply take the ZFW, head over to the payload section and just enter the ZFW, which auto populates the pax and cargo numbers for you. Either, or; you're choice

Ahh i see so if my planned ZFW was 56.2 i will renter 56.2? But when will i do this?

 

Not sure what you're getting at here...why are you thinking this needs to be done?

 

If your planned ZFW is 56.2, then there's no reason to update this on the PERF INIT page at all, unless there's a gross error of some sort. Say you finished all of the page entries all the way through TAKEOFF and the TOW was 66.2. Say you taxi for a while and end up burning down to 66.0. 0.2 really isn't that big of a deal, and if you actually think about it, the difference just means your vspeeds are higher than what they need to be. This extra conservatism in the numbers is not a bad thing.

 

The only time I'd bother with updating it is if you pushed off the gate and then got told to go sit in the penalty box until a cell passed. In that time, say you killed an engine, but left one running to get a jump on the departures (instead of shutting down completely and running the APU), and burned 1.0 off. Then? Sure, I'd recalculate, but even if you didn't, you're still operating on conservative numbers. I'd be worried more about min fuel to depart versus my TOW and vspeeds (which is the only real reason TOW matters, as long as your TOW is below MTOW, and your performance can get you off the runway).

The ZFW, thrust limits and V-speeds are loaded into the FMC during the preflight either manually or via the uplink, normally it's uplinked.  At this point it's all based on planned data derived from the load planning system which takes into account how many passengers are booked and known cargo.

 

After we push we receive a load closeout message which among other things contains the actual ZFW, which is also uplinked into the FMC.  The FMC adds the fuel on board to the ZFW to compute the actual takeoff weight.  The actual takeoff weight is then compared to the planned takeoff weight.  If it is equal to or less than the planned weight you are good to go with no changes.  For example if you were planned for a takeoff weight of 135.0 and the actual weight ended up being 133.5 we would still use the thrust rating and V-speeds for 135.0.  If the actual weight ends up being above the planned weight then the V-speeds and possible the thrust rating will need to be changed based on the actual weight.  The same applies to the planned temperature, equal to or colder than planned is OK, hotter than planned requires changes.

 

This is a somewhat simplified explanation but you get the idea.

True, a few hundread kgs of difference won't be a problem. Maybe 500 - 1000kg if i'm goin to be sitting about for de-icing or something.

 

So if i did want to update my TOW i would re-enter my planned ZFW and that would update my GW on the PERF INIT page and then that would automatically re-calculate my TOW?

The ZFW, thrust limits and V-speeds are loaded into the FMC during the preflight either manually or via the uplink, normally it's uplinked.  At this point it's all based on planned data derived from the load planning system which takes into account how many passengers are booked and known cargo.

 

After we push we receive a load closeout message which among other things contains the actual ZFW, which is also uplinked into the FMC.  The FMC adds the fuel on board to the ZFW to compute the actual takeoff weight.  The actual takeoff weight is then compared to the planned takeoff weight.  If it is equal to or less than the planned weight you are good to go with no changes.  For example if you were planned for a takeoff weight of 135.0 and the actual weight ended up being 133.5 we would still use the thrust rating and V-speeds for 135.0.  If the actual weight ends up being above the planned weight then the V-speeds and possible the thrust rating will need to be changed based on the actual weight.  The same applies to the planned temperature, equal to or colder than planned is OK, hotter than planned requires changes.

 

This is a somewhat simplified explanation but you get the idea.

 

Thanks joe spot on!

 

 

So if i did want to update my TOW i would re-enter my planned ZFW and that would update my GW on the PERF INIT page and then that would automatically re-calculate my TOW?

 

You would update it with the actual ZFW.  Planned weight is just that.  Once you push off the gate you are leaving the planned world and entering the actual one.  In the sim world your planned and actual weight will be the same so you would not need to update anything.  In the real world if your actual ZFW ended up matching the plan you would not have to update anything, it's already in the FMC.  What's important is that the number in the FMC and the actual number are the same.

So how does that work then? When will i update and why? Because as you know this is a simulation world and once i set my load in FS OPTIONS its final, unlike in your world of flying!

 

Thats exactly what i mean't here!

 

Thanks guys. Joe ever considered doing 737NGX flight videos? Tutorials etc

Ok i'm re-entering the ZFW and the TOW doesn't update? i even changed the ZFW and still it doesn't change?

Ok i'm re-entering the ZFW and the TOW doesn't update? i even changed the ZFW and still it doesn't change?

It's been a long time since I've flown without a data link for the speeds, but GW should change when you update the FMC fuel or ZFW. The TOW is simply showing the weight the Vspeeds are based on and that comes to my plane via the data link. I think it was blank back when we didn't use data link.

 

So, if my memory is correct, you should either have a data link simulated TOW or a blank. I'm not sure how your various add ons would change the TOW.

 

My TOW initially shows what weight we are allowed to T/O. If I do a manual data request for new T/O data, the TOW will default to my current GW or whatever I tell it to be in the data request.

 

Say there is windshear and I want to do a no derate, no assumed T/O. I send a manual request and that overwrites the previous TOW and Vspeeds with new uplinked data.

 

Really, though - you're looking at something that is handled differently by different systems and different airlines. The TOW may be blank. The TOW may update with data link. Check that the Vspeeds are acceptable for your current weight and you're covered.

Ahh i see matt. So that TOW can only be changed by an uplink like you mentioned or some sort? So there is no way i can change it?

 

I know its not a big deal and it shouldn't concern me, but i only like to know how it works!