TOPCAT N1 Performance question

[tango4]

Hi everyone !
I have some issues with performance calculations on TOPCAT regarding the N1 values for takeoff when compared to what I get in PSX.
My basic question is: are the values I get normal, and is the difference between TOPCAT and PSX normal ? If not, is it because of the N1 model in both software is different ?
So now, the details  
Situation is:
LFPG RWY 26R
QNH 1013 temp 20°c no wind, no cloud or precipitation. (Weather is stable and not in smooth transit or something like this).
I'm using F-GITH, B747-400 Pax with GE engines. ZFW 200t fuel 20t.
In topcat I chose the same profile, just edited the values according to Brian's wonderful tutorial (DOW 179500 MZFW 270000 Fuel Capacity 173032), though I don't think it has an impact on N1 calculations.
In TOPCAT, I have TOW at 220000, Flaps Config Optimum, Thrust Config TO, Air conditioning On and Anti Ice Off. I made sure I have the same in PSX.
Here is what I get:
In TOPCAT: *No derate: N1 106.7%
                  *Max Derate 74°c: N1 98.1% (Althoug PSX won't accept anything more than 63°c)
                  *63°c: N1 99.7%

In PSX, I get: *No derate: 105.4%, 20°c displayed as OAT
                     *63°C: N1 95.9%

Another strange thing is that if I use engine antiice, TOPCAT changes the values to 106.2% without derate and 99.2% at 63°c!.
In PSX there is no change if I switch engine antiice on. (But there is a change if I switch off air conditioning).

Hope I did not give you a headache !!

If someone could reproduce the situation or explain me if this is normal, that would be great !

Charles

[Vnav]

Bonjour Charles,

Looking at the FPPM, I can confirm PSX full rated N1 is absolutely correct, not TOPCAT. I can't say for the reduced N1, not having the Performance Dispatch section available. I can also confirm that the N1 corrections applied by TOPCAT for anti ice are in agreement with the FPPM. The lesser N1 value is due to the anti ice system using bleed air from the engine thereby decreasing the maximum thrust available. Conversely, switching off the packs results in increasing the available thrust (N1).

I suspect TOPCAT might have an small issue there, but I don't know where. Engine model (B1/B5) is not the cause as TOPCAT only has the B747-400 available,  not the -400ER.

Bons vols,


Olivier

[tango4]

Thanks a lot for taking the time to check all that, Olivier.
So that would probably answer the main part of my question : Topcat model is probably less accurate than PSX.
Regarding the anti ice, I understand the lesser value in TOPCAT. What i don't get is why it does not decrease takeoff N1 in PSX when I actually switch antiice on. (Wheras when I switch packs off, I can see the increase in takeoff N1 directly)

Charles

[Pierre Theillere]

Hi Olivier!

I had also noticed the disrepancy between TOPCAT and N1 settings. Assuming that performance given by TOPCAT depending on N1, and PSx performance were both matching, I was tweaking with assumed temperature (using either TO, D-TO1 or D-TO2, depending on TOPCAT's computation) so that PSx takeoff N1 was same as TOPCAT's results.
Proceeding in such a way, I seemed to get quite accurate takeoff predictions.
I remember I have some real (paper) Air-France docs, where pilots were told to fiddle with the MCDU, until their N1 was matching the one given by their own "OCTAVE" software. So, somehow, my way of doing things seems... operational!

[mk777]

Is the n1 discrepancy due to the packs and / or bleeds not being configured owing to the fact that the engines aren't started?
Bear in mind, with an assumed temperature, you'll always get slightly more thrust than the performance figures at say 60* owing to the temperature being considerably less than 60*. That will be a lower N1 too I think. 0.5-08% is quite common on the B777 between performance and CDU.
Cheers
Matt

[GodAtum]

I have also noticed this discrepancy too. I just input the Topcat temp (leaving the FMC d-rate as is) and vspeeds. Although sometimes Topcat gives lower V1 then the FMC so I use the FMC value.

[tango4]

Thanks guys for all your contributions !
The only question remaining would be the anti ice one.
When I have engines running, ready for take off, no derate, here is what surprises me:
If I switch 2 packs off, I see an N1 increase. (I mean a take off N1 in green, my engines are idling). That's normal. If I switch off the remaining pack, another increase. Still normal. But engine antiice seems to have no effect. Is it normal ?

[Vnav]

Hi Charles,

The FPPM says that no adjustment to the N1 is required below 7500' pressure altitude. Indeed, if you try and takeoff from SKBO at 8400' you'll notice that FMC does adjust the N1 depending on the engine anti ice setting.

Cross checking the N1 value in the FMC against the N1 obtained from the Boeing OPT is part of our performance data crosscheck. They are usually within 0.2% on my aircraft (737), knowing that 5 deg of assumed temperature roughly translate into a 0.5% N1 difference or that bleeds off make a +1% N1 difference.

Happy landings,


Olivier

[Vnav]

Hi Olivier!

I had also noticed the disrepancy between TOPCAT and N1 settings. Assuming that performance given by TOPCAT depending on N1, and PSx performance were both matching, I was tweaking with assumed temperature (using either TO, D-TO1 or D-TO2, depending on TOPCAT's computation) so that PSx takeoff N1 was same as TOPCAT's results.
Proceeding in such a way, I seemed to get quite accurate takeoff predictions.
I remember I have some real (paper) Air-France docs, where pilots were told to fiddle with the MCDU, until their N1 was matching the one given by their own "OCTAVE" software. So, somehow, my way of doing things seems... operational!

Hi Pierre,

Quite smart! At least you're sure you won't lack thrust.

Olivier

[tango4]

Thanks a lot Olivier,
I guess you might have saved me hours of searching to find that answer !!
I don't know who you are working for, but if you come at LFPG sometimes, we might talk to each other on the freq  

Charles

[Vnav]

Hi Charles,

My pleasure!

I used to fly in and out Paris before, but now is a bit different! Better chance of meeting each other at the next (first?) PSX event in France!

Olivier

[Ray_CYYZ]

Just curious about those temperature numbers, where is that 74°c degrees coming from?

The hottest place on Earth is considered Furnace Creek Ranch in Death Valley which holds the record at 56.7°c!  :shock:

[Hardy Heinlin]

Althoug PSX won't accept anything more than 63°c

Hi Charles,

the FMC validates pilot entries up tp 99°C, but internally limits them to 63°C, and 63 is then indicated. This is not documented in certain manuals, but has been observed on the real 744.


Hi Ray,

luckily, these high temps are just "assumed" :-) -- (too fool the FMC).


Cheers,

|-|ardy

[tango4]

Thanks for this precision, Hardy.
I keep being astonished by this forum. I asked a somewhat technical question. In less than 12 hours I have a complete answer, coming from pilots and the developer.
I really don't regret my 350€ !!

Charles