GW in FMC and Topcat question

[andreadebiase 5]

I am trying to better understand how is it possible that the FMC, after setting winds, fuel, payload, runway departure, GW and more.....is telling me that i am good to go and Topcat instead disagrees, telling me for example that i am 20K pounds too heavy. Of course i do go with what Topcat tells me and drop some cargo (and btw it works very well) but i was curious to know what is the big difference between the two, what main variable makes topcat really necessary and why can't the FMC do the whole job. 

thanks

 

[scandinavian13 5,192]

 

 

telling me for example that i am 20K pounds too heavy

 

Could be a runway-specific example, or a terrain issue, but 20K pounds sounds more like you have a KG-Lbs discrepancy between the aircraft in TOPCAT (TOPCAT is expecting KG, but you're giving it pounds).

 

TOPCAT is also calculating other parts of the departure profile that the FMC does not, which also provides limiting factors for the departure.

[andreadebiase 5]

Could be a runway-specific example, or a terrain issue, but 20K pounds sounds more like you have a KG-Lbs discrepancy between the aircraft in TOPCAT (TOPCAT is expecting KG, but you're giving it pounds).

 

TOPCAT is also calculating other parts of the departure profile that the FMC does not, which also provides limiting factors for the departure.

 

I am pretty sure that my weight units in Topcat is in pounds but now that i think about this i was departing from MMMX and......does Mexico use  Kgs in aviation? Any chance 737 pmdg FMC was talking to me in Kg because i was in Mexico......but this makes no sense because if my weight is expressed in Kgs and "ok-ed" by FMC then the same number in Topcat (pounds) should also be "ok"  as it is lighter so i should have got a go ahead also from Topcat right?

.....most likely is the "other parts" you mention that made the difference 

[scandinavian13 5,192]

 

 

I am pretty sure that my weight units in Topcat is in pounds but now that i think about this i was departing from MMMX and......does Mexico use  Kgs in aviation. Any chance 737 pmdg FMC was talking to me in Kg because i was in Mexico......but this makes no sense because if my weight is expressed in Kgs and "ok-ed" by FMC then the same number in Topcat (pounds) should also be "ok"  as it is lighter so i should have got a go ahead also from Topcat right?

 

That's not how it works...

 

KG or LBS is a company option, so that would only change if you were flying an aircraft for an operator that uses KG. From there, you'd need to choose the profile in TOPCAT that matches the aircraft you were flying in the sim (this is why they have the aircraft broken down by tail number in TOPCAT - each aircraft could have different KG/LBS setting).

 

Additionally, the FMC will accept a high weight. If the plane is looking for KG and you give it LBS, it'll just assume that it's just heavy (it's assuming the LBS you handed it are KGs).

 

 

 

.....most likely is the "other parts" you mention that made the difference 

 

20K of difference is not likely to be anything other than a KG/LBS issue unless you've made an incredibly bad error with the program.

 

Either way, from all I can see on this end, it seems that it would serve you well to learn how to tie TOPCAT into your workflow, and also pay very close attention to ensuring all of the values match up, and are in the correct unit.

[downscc 1,327]

I've experienced TOPCAT and TOPER giving different solutions, and will use TOPER over TOPCAT when the latter fails to provide a solution or when planning for B739 or B77W flights.  Probably just as you say Kyle, TOPCAT does have some obstacle data incorporated into the algorithm and neither FMC nor TOPER does.

 

 

I am trying to better understand how is it possible that the FMC, after setting winds, fuel, payload, runway departure, GW and more.....is telling me that i am good to go and Topcat instead disagrees,

 

Best thing to do is to use a flight planner such as PFPX and before you release the flight, check the takeoff solution. You may have to reduce payload and fuel or change departure runway. Once you have a takeoff solution you can then release the flight and start preflight on the FMC.

[BlueStar 323]

 

 

i was departing from MMMX

 

MMMX in the summer will have DAs above 9000 feet which is a real performance killer especially for the NG especially the 800 and 900.

 

If you will give me your routing I will calculate the flight for you and let you know what I get. :smile:

 

blaustern

[andreadebiase 5]

MMMX in the summer will have DAs above 9000 feet which is a real performance killer especially for the NG especially the 800 and 900.

 

If you will give me your routing I will calculate the flight for you and let you know what I get. :smile:

 

blaustern

 

I am good thank you.  I followed Topcat advice, dropped some cargo at departure to get in to the green zone and was able to take off smoothly and arrive with 6 left in the tanks to KMIA. CRZ altitude of 36000 feet step to 37800 as soon as FMC said was ok to do so. Landing was smooth. By the way I was flying COPA.

 

So let me ask this then. When landing do you guys approach minimums with the FMC recommended speed (+5) or the Topcat one? sometimes they differ by few digits... 

[skelsey 765]

i was curious to know what is the big difference between the two, what main variable makes topcat really necessary and why can't the FMC do the whole job.

 

The FMC only provides very basic balanced field calculations. It knows the runway length and the conditions you give it, but nothing else. Also, I don't know about the NGX -- does it calculate an assumed temperature reduction or adjust the speeds based on the assumed temperature you enter? If not, the speeds will all be based on full thrust/full derated thrust (which is safe, but not necessarily optimal).

 

TOPCAT has an obstacle database and therefore will take in to account the performance required for the climbout which at MEX could be quite limiting, especially with the high density altitude. It will also calculate optimum speeds, the optimum assumed temperature and/or derate for the runway and the maximum possible takeoff weight, as well as taking in to account any defects you select.

 

Finally, performing an external performance calculation is a good failsafe -- the FMC only knows the weight you told it. If your weight entry in the FMC is wrong, the speeds it gives you will be wrong. You could plug the wrong weights in to TOPCAT as well, but you'd be doing well to independently enter the same wrong weight in both TOPCAT and the FMC and so if there was a very large discrepancy in thrust or VR/V2 speed (the latter of which are based on weight and shouldn't generally vary too much assuming the takeoff performance calculator isn't optimising with artificially high V2s for improved climb) that would/should set some alarm bells ringing.

[downscc 1,327]

 

 

When landing do you guys approach minimums with the FMC recommended speed (+5) or the Topcat one? sometimes they differ by few digits...

 

I always use the FMC Vref for landing... however, TOPCAT is still handy when determining stop and remaining distances and best Autobrake setting. No necessary when you have 11,000 feet but certainly important at KMDW or KSNA where you are working with 5100 ft.

[andreadebiase 5]

I always use the FMC Vref for landing... however, TOPCAT is still handy when determining stop and remaining distances and best Autobrake setting. No necessary when you have 11,000 feet but certainly important at KMDW or KSNA where you are working with 5100 ft.

la guardia and kdca are also no joke...

[BlueStar 323]

la guardia and kdca are also no joke...

 

American use to fly DC-10s into KLGA.  I can name half a dozen airports in Central and South America that are harder than

KLGA and KDCA.   :smile:

 

blaustern

[andreadebiase 5]

American use to fly DC-10s into KLGA.  I can name half a dozen airports in Central and South America that are harder than

KLGA and KDCA.   :smile:

 

blaustern

 

didnt know KDCA had the arresting gear

[pcubine 143]

 

 

CRZ altitude of 36000 feet step to 37800 as soon as FMC said was ok to do so.

You are heading east. I believe your cruise altitude should be odd flight levels and step climbs are not 1800 ft.

[andreadebiase 5]

You are heading east. I believe your cruise altitude should be odd flight levels and step climbs are not 1800 ft.

 

correct, that was not the right thing to do