Posted same question over at Aerosoft but figured I could post in here as well since the question is related to the NGX which is the only a/c I fly.

I'm using TOPCAT to calculate takeoff performance data and have a question about the table you get by pressing the 'Compute' button after entering all other data such as weights, fuel on board etc.

More specifically I'm wondering how to use the flex (assumed) temp in a realistic way? This since there are a number of temps to choose from where the higher the temp, the shorter the runway margin. Is it simply up to the captain to decide and if so, based on what exactly?

Reading the TOPCAT manual it says it's airline specific but I guess most big airlines use a similar approach to this.

Anyone who knows more about this and I'm particularly interested in how SAS does it.

6 minutes ago, WebMaximus said:

Posted same question over at Aerosoft but figured I could post in here as well since the question is related to the NGX which is the only a/c I fly.

I'm using TOPCAT to calculate takeoff performance data and have a question about the table you get by pressing the 'Compute' button after entering all other data such as weights, fuel on board etc.

More specifically I'm wondering how to use the flex (assumed) temp in a realistic way? This since there are a number of temps to choose from where the higher the temp, the shorter the runway margin. Is it simply up to the captain to decide and if so, based on what exactly?

Reading the TOPCAT manual it says it's airline specific but I guess most big airlines use a similar approach to this.

Anyone who knows more about this and I'm particularly interested in how SAS does it.

Richard,

Could you share a screenshot of the tables for those who aren't familiar with TOPCAT?

I flew this route SWA3936 (PVD-DCA) tonight with the parameters below.  I used PFPX with TOPCAT enabled, so my result within PFPX looks like the text screen display on the second screen shot.  I don't typically open TOPCAT. 

My config was also Flaps 1 and TO1, and I chose the higher temp of 34 degrees.  I had little room to spare; I ate up a lot of runway   I would say the 400 and some feet remaining was accurate.  I almost always pick the higher or highest temp.  Just nicer and easier to hand fly with derated engines.  Although, some airports have obstacle clearance minimums and factors that prevent much derate if any.  TOPCAT will let you know if you can or not.  It seems to be pretty accurate overall in my estimation.  Obviously real world is gonna vary a lot, depending on SOP for each airline and environmental factors, such as runway shortenings and such.  

Hopefully someone can answer as to what SAS does.  Might not be set in stone though.  Being a sim I just go with my mood.  That's the joy of it, most realism with a spice of personal flare.

 

 

 

 

Hi Richard,

I usually choose the flex temp which gives me at least 100m / 300ft of margin. Though I have to say I stopped using it for the NGX as I found strange results especially on short runways. So I would not recommend it for the 737. It works well with the 744 GE engines.

No idea about SAS S.O.P...

Cheers

40 minutes ago, alex98 said:

Hi Richard,

I usually choose the flex temp which gives me at least 100m / 300ft of margin. Though I have to say I stopped using it for the NGX as I found strange results especially on short runways. So I would not recommend it for the 737. It works well with the 744 GE engines.

No idea about SAS S.O.P...

Cheers

This is a strange observation.  I fly out of BUR and SNA often and can't say I've ever noticed a problem. 

Edited to rewrite the info about PFPX since I noticed I wasn't using the same numbers for the calculations although I thought I was.

Thanks for your input guys although I'm still hoping to find a more real/official answer.

I just did an experiment where I first manually used TOPCAT to calculate the performance data for a given flight. Then I did the same thing using PFPX which is connected to TOPCAT.

When I manually used TOPCAT I was presented with a t/o performance table ranging from +30C and N1 87,8% to +48C and N1 85.3%. Looking at the runway margin, going with flex temp +30C it's 573m where using flex temp +48C would only give a margin of 2m.

Doing the same thing in PFPX using the same data the t/o calculation picks the highest flex temp which is +48C in this example. So I guess at least in PFPX the goal seems to be to pick the highest possible flex temp as long as it's authorized.

I can't speak for SAS but from what I have seen of BA's A380 performance system (which returns a similar format to Topcat - the other fleets use a different system which doesn't) it's perfectly common to use the highest possible assumed temperature.

Remember that the "margin" is a theoretical measure of the amount of ASD you will have in front of the aeroplane after an RTO at V1. This is already a very conservative calculation because there are lots of safety margins already built in to the calculation and the actual performance should/will be significantly better. There is no need to keep adding safety margins on top of safety margins. The amount of runway left on rotation is not necessarily an issue: once you are past V1 essentially all that is required is that you make the screen height (35ft dry/15ft wet) at the end of the TODA (and could do so with an engine inoperative).

Of course, that assumes that the TOPCAT data matches the actual NGX performance.

As for your second question - are you positive that absolutely everything was identical between the two calculations? Weights, intersection, wind, QNH, temperature, configuration etc?

What you say Simon makes a lot of sense and this also mirrors the PFPX behavior picking the highest flex temp available.

Regarding the numbers for the calculations, as mentioned in my edited post above they were off. After correcting that PFPX picked the highest flex temp available just like we discussed.

RW, we just use the solution with the highest temp that works. That can change if you have windshear (no assumed, no derate, T-O Bump if available) or a tailwind (no assumed) or contamination (no assumed).

Okay, actually I just push a button now and it loads itself, but I have looked at the charts for fun every once in a while.  ;)

Oh, and sometimes I delete CLB-1 or CLB-2 if it looks like we'll be close on a crossing restriction. Our software doesn't account for those.

Thanks a lot for the input.

On 28/06/2017 at 11:46 AM, Orlaam said:

This is a strange observation.  I fly out of BUR and SNA often and can't say I've ever noticed a problem. 

I have seen situations where Topcat was very conservative and computed lower than expected max TO weights, even at airports like LIME (Bergamo).

I find it to be very pessimistic as soon as the airport is 1000ft amsl and/or there is obstacles and/or the runway is short. Sometimes you would not be able to get out of places like Skiathos or TNCM... And this was checked against the FPPMs I have.

On 6/28/2017 at 4:04 AM, alex98 said:

Hi Richard,

I usually choose the flex temp which gives me at least 100m / 300ft of margin. Though I have to say I stopped using it for the NGX as I found strange results especially on short runways. So I would not recommend it for the 737. It works well with the 744 GE engines.

No idea about SAS S.O.P...

Cheers

I've never had any problems with PFPX.  Most likely what you are considering as strange with the "short runways" is most likely the  "balanced field" length.   

blaustern

Look at the weights for each assumed temp.

If your weight is higher than the max TOW listed for a certain assumed temp setting, then decrease the temp until your TOW is below the MAX TOW for a given assumed temp setting.

1 hour ago, ahsmatt7 said:

Look at the weights for each assumed temp.

If your weight is higher than the max TOW listed for a certain assumed temp setting, then decrease the temp until your TOW is below the MAX TOW for a given assumed temp setting.

As far as I understand TOPCAT will take this into account for you after you put in all the figures meaning it will only show you flex temps that will be OK with your current TOW.

5 hours ago, alex98 said:

I have seen situations where Topcat was very conservative and computed lower than expected max TO weights, even at airports like LIME (Bergamo).

Sure, you can use TOPER to give you much higher weights for same condition, the difference being engine out obstruction clearance not included in TOPER.  Sure, you can get off the runway and as long as you don't lose and engine the higher weights are .... hate to say okay.