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FMC N1 and topcat N1 % difference

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I''ve been having differences between the FMC calculated N1 % and what TOPCAT is providing me. I have everything setup correctly. Bleeds on packs on etc selected on TOPCAT. I'm seeing a difference of .4 maybe .6 at times ie TOPCAT N1% 85.9 - FMC N1% 86.4 but once this part is entered into the N1 page of the FMC it changes over time to a much higher value from 86.4 to maybe 86.6 - 9 ?

 

Is this a normal thing that happens in real life?

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Is this a normal thing that happens in real life?

 

No idea why it's there (FMC vs TOPCAT), but defer to the FMC-calculated values. I'm not sure how/where TOPCAT got its data, but ours is right from Boeing.

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Yeh well those values can't be changed unlike the VSPEEDS. I never pay attention to the N1 % from TOPCAT but rather double check the FMS against whats showing on the N1 reference bugs on the N1 gauges.

 

I do however use TOPCAT Vspeeds!

 

So is this a common thing N1 differences?

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So is this a common thing N1 differences?

 

I mean, you're gonna get differences all over in aviation. The fuel burn table says you are going to burn X amount in an hour. Do you really think you burned exactly that much, or +/- a few pounds on either side? If you've ever tracked fuel burn over a flight, you know that, despite how wonderfully you've planned the flight, you're going to be slightly off on your final number or time.

 

There's a reason we have planned and actual fields. Similar thing here. The planner assumed one thing, the actual (the FMC in this case) shows something slightly different. As long as it's close, you should be fine.

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Yeh exactly! Well PFPX and the FMC do predict fuel burn/remaining quite good, well..untill ATC vectors, Of course!

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I've asked the same question and got the same answer from Kyle lol.

 

I know there's a efb manual usually. I guess the efb manual usually has the tolerances published in there, along with everything else.

 

E.g

- crew need to recalc performance for wind velocity change of x knots.

- efb calc reference speeds must be within x knots of fmc generated ref speeds, if not, re-check data for correctness, then .... Blah blah blah.

 

I don't know what the actual experience has been in real aircraft using efb and how closely efb calc N1 matches fmc generated N1.

 

Brian Nellis

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Funny how simmers seem to be more "anal" about this than actual pilots. When we did a dual engine run on an A320 earlier this week, the difference in idle N1 between both engines was more than two percent (18,6% and 20,9%, left and right respectively). However, neither the mechanics, nor the pilots flying the aircraft, cared about it. Those differences are within limits. The difference between a new engine and one that has a couple thousand hours on it can be quite dramatic, both in performance and especially fuel burn. Since it would be cost prohibitive to change both engines when one is due (for whatever reason), you'd be flying around with new and old engines on the same aircraft quite frequently.

 

All of that to say... 0,4-0,6 of a difference honestly isn't that much of a difference, if at all.

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Hi,

yes 0,4-0,6 difference is nothing...you should try a MPA check (maximum performance check is an excel spreadsheet for testing CFM56 performance in which you have to set your engine at a given N1% (normally 65% or 75%) and input into the spreadsheet your altitude, outside temperature, CFM56 type (i.e. CFM56 7B26 or 7B27) your EEC setting (it seems to remember that CFM56 can have seven different settings i.e. variable bleed vanes schedules giving seven slight different corrected N1% vs corrected N2% curves) etc. and that spreassheet returns as output the max values than the others values (N2%, EGT) to be into normal limits.

You should find that spreadsheet here: www.sjap.nl

 

Best

 

Andrea

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Hi,

I remember that during a fixed base trainer session throttles weren't aligned after selecting a/t takeoff power but the EEC syncro system equalized the thrust of the engines in spite of throttles position...

 

Ciao

 

Andrea 

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Also worth bearing in mind that there will be slight differences in the data that each is using: the FMC will be using a combination of data that you have entered and sensed values based on the actual ambient conditions (I have no idea what the resolution of the TAT probes is, for instance, but I reckon there's a good chance that the FMC itself could easily be using fractions of degrees to calculate with (even if you only have integers displayed). TOPCAT (or any other performance calculator) on the other hand may be using more granular data (especially in terms of temperature) and exclusively predicted/loadsheet values.

 

As mentioned above, I really wouldn't worry about fractions of a percent N1: there are far too many other variables that will affect the takeoff to a much greater extent. Any time you calculate something like this with a computer you're relying on a perfect world, where amongst other things the wind is exactly steady, the temperature is static, the pressure is rounded to the nearest millibar and everybody weighs 80kg: clearly real life deviates from these parameters and thus some variation is to be expected!

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The next natural question would be from my perspective - how much of a split is too much?

Obviously there are tolerances for maintenance purposes - but if pilots experience mismatch, at what point do they consider a mismatch reportable?

 

This question would probably/normally get dealt with by Fleet Captains/Technical Pilots/Boeing and/or GE/Snecma, but I was wondering if any real world NG pilots or aircraft engineers loitering here might be able to shed some light on that.

 

And if you're going to tackle that question - in preflight, what is the typical acceptable tolerance between OPT calculated N1 and reference speeds and FMC generated N1 and reference speeds. Do they typically match, or are they usually within x% N1 and y Knots respectively.

 

This world ain't perfect... That doesn't mean the tough questions ought not be asked, as they are valid questions, even though the answer is usually of no real consequence (more of a - nice to know, but now I'm going to forget it - kind of nature). And to be honest, these questions are probably a bit too technical for a sim forum.

 

Brian Nellis.

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This world ain't perfect... That doesn't mean the tough questions ought not be asked, as they are valid questions, even though the answer is usually of no real consequence (more of a - nice to know, but now I'm going to forget it - kind of nature)

Of course!

 

I can't speak for N1 values -- however, the guidance on the 747 from the operator I'm familiar with is:

 

Takeoff performance calculation
State FLAP/ACCEL HT, EO ACCEL HT and THR REDUCTION.
State V1.
State VR and V2. Both pilots must check these values are within 3 knots of the FMC speeds prior to entering data into CDU (When performance calculator data not available use FMC VR and V2).
State MACTOW to nearest integer.
 
WARNING: If VR or V2 differ from the FMC figures by more than 3kt, check that the data entered into the FMC and performance calculator or manual performance data is correct. If the input data is correct and the difference remains unresolved, use the higher of the FMC or performance calculator VR and V2.

 

 

 

 
I'd imagine the NG tolerances would be similar.

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Of course!

 

I can't speak for N1 values -- however, the guidance on the 747 from the operator I'm familiar with is:

 

 

I'd imagine the NG tolerances would be similar.

As I suspected... Thanks for sharing.

 

Brian Nellis

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Id have to look at the boeing manuals to check as i only work exclusively on engines and not airframe, but the N1 and N2 are always going to be different for every engine, as stated the difference is efficiency and EGT margins on individual engines is calculated into the EEC, its called a trim setting. So lets say engine 1 has 3500 cycles on it, and to create 24k of thrust the N1 speed (actual engine) is 99% with no trim, but engine two is brand new, and at 99% N1 will create over 24k, we have to down trim the engine in the EEC so the cockpit will still display 99% but real fan speed would be now 97%, thus creating asymmetrical thrust. at takeoff thrust they should both produce the same thrust setting. The variance in RW is the trim setting may be slightly out and does have a tolerance.

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I'm just happy when the power levers line up together.

Not a fan of the 1" lever split takeoff? ;)

 

(Nor am I)

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A couple of tenths difference is not a big deal.  As part of our load closeout process we compare the FMC calculated N1 with the one given in the takeoff performance.  As long as the actual N1 is in the ballpark it's all good.

 

If it's off by a substantial amount then something isn't right and you should figure out the cause.  There is a known anomaly in the real plane that can cause the wrong temperature to be uplinked into the FMC.  In one case the actual temperature was 18C and the uplinked temp was -18C.  The crew didn't catch it and they took off with substantially less power than planned.

 

Another thing I look for when checking the values is that the FMC calculated N1 is the same for both engines.  If they are different then you probably have either a pack or an engine bleed switch off.  There are times where a split is normal, if you had a pack MEL for example.

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A couple of tenths difference is not a big deal.  As part of our load closeout process we compare the FMC calculated N1 with the one given in the takeoff performance.  As long as the actual N1 is in the ballpark it's all good.

 

If it's off by a substantial amount then something isn't right and you should figure out the cause.  There is a known anomaly in the real plane that can cause the wrong temperature to be uplinked into the FMC.  In one case the actual temperature was 18C and the uplinked temp was -18C.  The crew didn't catch it and they took off with substantially less power than planned.

 

Another thing I look for when checking the values is that the FMC calculated N1 is the same for both engines.  If they are different then you probably have either a pack or an engine bleed switch off.  There are times where a split is normal, if you had a pack MEL for example.

 

Brilliant

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A couple of tenths difference is not a big deal. As part of our load closeout process we compare the FMC calculated N1 with the one given in the takeoff performance. As long as the actual N1 is in the ballpark it's all good.

 

If it's off by a substantial amount then something isn't right and you should figure out the cause. There is a known anomaly in the real plane that can cause the wrong temperature to be uplinked into the FMC. In one case the actual temperature was 18C and the uplinked temp was -18C. The crew didn't catch it and they took off with substantially less power than planned.

 

Another thing I look for when checking the values is that the FMC calculated N1 is the same for both engines. If they are different then you probably have either a pack or an engine bleed switch off. There are times where a split is normal, if you had a pack MEL for example.

Thanks Joe!

 

Brian Nellis

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