Recommened TO/GA values for 737-900 Using Simbrief

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Hey! I’ve started calculating takeoff values using Simbrief. My first experiment is the 737-900 (not surprising if you see my pfp). Here are my results!

Conditions

Locations: KJFK 31L/13R & KSFO 01R/19L
Elevation: 13 feet MSL
Runway Conditions: Dry
Temperature: 30°C
Altimeter: 1013 hPa
Winds: Calm
Flaps setting: 5° (Considered optimal for these tests)

% Payload % N1 V1/VR/V2
10 80.5 106/106/123
20 83.3 111/112/127
30 86.1 117/118/132
40 89.9 123/124/137
50 92.0 128/130/142
60 94.6 134/136/147
70 97.1 139/141/151
80 99.6 144/146/156
90 101.8 149/151/160
100 104.0 154/156/164

To me, these numbers seem pretty predictable and the acceleration in game follows the same pattern. With pitch, you should experience around 17.5 degrees, consistently at that. It’s a bit of a climber, but what do you expect of a 737-900 that produces 39,000 LBF per engine (Exactly How Much Power do Infinite Flight Aircraft Really Have?). As much as I could look for an FCOM online and maybe find information, I’d rather not use that. IF aircraft are unique, and the modeling may cause issues for that. I’d rather reverse engineer and generate values based off of that instead. I mean, isn’t that kind of what flight testers do in reality when a new aircraft is built? Anyways, I’ve rambled on enough. Have a go at it. Hopefully, you find this useful. Probably won’t get to de-rates and assumed temperatures until a later time.
Disclaimer: Please note these values are variable based on environmental factors as pointed out by @TheGlobalAviator . Also, you may find the values like a derate, so less power is needed for TO/GA at lower weights compared to higher weights (@ToasterStroodie). Please don’t try to substitute this for an actual derate. You’ll use even less power and maintain a lower pitch, not 17.5 degrees.

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I’m not sure I quite get the end goal of this post. Is it to say we are in a very overpowered plane and need to constantly derate our takeoffs, or is there something else?

Thrust output values depend on temperature and altitude, not weight. I could be at 50K kg or 80K kg taking off from the same airport at the same temp. using a non-derated takeoff power setting, and it will always be the same number for N1.

It would make a bit more sense if you re-title your thread to say “recommended takeoff thrust values for 737-900” and you put disclaimers that the lighter you are, the more you de-rate the thrust. It makes no sense to manipulate T/O N1 using weight alone - I can’t use, for example, 86% N1 at DEN for a 66K kg 737-900 when it’s 30C out while I absolutely can at BOS when it’s 15C out.

As far as your V-Speeds go, it seems okay for TO/GA thrust, but then again, what you’re really doing with your table is applying derates. You need V-speeds designated for the derated takeoff mode for that.

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This reasoning completely contradicts your methodology; as SimBrief figures are based on real-life FCOM data as well (from V-speeds, to thrust settings, etc.)

In terms of the thrust on the IF 737NGs, they are modelled with a sufficient degree of accuracy to the real thing, that using real-world FCOM data thrust settings would not cause any major issues contrary to what you have described.

If you were to insist that real-world FCOM data is unsuitable for IF use, it would be best to not use SimBrief data altogether - which I do not recommend - but it would be most suitable for your chosen method of reverse engineering the IF model and generating values off that instead.

V1 is defined as the takeoff decision speed; where afterwards there will be insufficient runway available to stop if a rejected takeoff is initiated.

As a result, this figure is highly dependent on the actual conditions at the airport and of course, the runway itself - especially at higher weights or on shorter runways; and it definitely isn’t a specific “one-size-fits-all” speed as you have suggested here.

For the CFM56-7B26 that you have most likely used in your Simbrief calculations, the maximum allowable N1% at your described conditions (30 degrees C, basically sea level) would be 100.4%.

It may very well be the case that the CFM56-7B27/E offers even greater N1% limits to generate the 27K thrust rating; but that will come with different V-speeds that are not accounted for here.

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Thanks for your replies. I would say IF are overpowered and you would need to derate.

@TheGlobalAviator , the thing is, I’m not derating to the extent of a 26K-27K rated engine. It’s a 39K rated engine calculated for the IF aircraft based on it’s acceleration. So far, I haven’t found a single source that says a CFM56 engine can produce 39K LBF. Maybe 34K-35K, but not 39K. The 737-900 doesn’t even use that much thrust per engine. I think that’s for an a340. Simbrief uses the 26K by standard (pretty reasonable). The v-speeds could be the same from the FCOM given the plane’s weight, which requires certain airspeed to gain sufficient lift and maintain tail clearance. However, the distance to reach them can be different because of the increased acceleration. I’ve seen this happen in SimBrief’s calculations when I increased thrust numbers. At that point, I wondered if the Simbrief devs actually went out of their way to calculate this manually. I dont think an FCOM would cover such large thrust values like that for a 737 and the distances. That’s why I mentioned I strayed away from the FCOMs (in that sense, which I failed to address).

@ToasterStroodie , are you positive thrust output values don’t include weight as well? I’m pretty sure you need to include this into takeoff calculations as well. If you factored out weight, then the engines use excessive thrust, leading to uncomfortable climbs and high pitches that a flight tester would be doing. Certainly not economcial or practical at all. I think weight is thrown in to act as a limiting factor in a sense and to enhance overall efficiency (fuel, engine life, etc.). Altitude and temperature do affect thrust output, but I’m not tricking the plane into anything with my tests. It’s at its flat-rate temp and I’ve taken altitude into my calculations for my locations. My takeoff values are just matching the v1 distance in SimBrief, so I’m accelerating correctly. I could test at other temps and get different results for sure, but I’m testing under one set of conditions. I can specify that in my topic. For altitude, I would have to test at a different airport, but they have sloped runways, something IF doesn’t acount for if the airports are all on flat surfaces. More work to calculate, but still doable I guess. Despite that, for your example of a 66K 737-900 at KDEN at 30°C, 86% n1 for sure isn’t going to help you get off the ground well with that acceleration. V1 distance from Simbrief averaged as 4,090 feet and IF went 6,605 feet. In my table, you are close to 70% load, so you would be somewhere about 97.1% n1. I ran tests at 97.1% n1 and I got 4,335 feet, so maybe just slightly more power needed (couple tenths of a %), especially for the high altitude. KBOS should be more feasible to get away with less power, though I doubt it would be less than KJFK and KSFO based on elevation. @TheGlobalAviator , I believe you questioned me on something similar here and you’re right. The values will vary based on conditions. Though, I did specify the conditions I was using for my test. I just never cautioned that they are variable based on actual conditons.

These are just my calculations though.I know my TO/GA calculations appear to just be derated values, but I’m just matching Simbrief. I purposely turned off the derate option so I know what I’m dealing with. If anything, I expect actual derates to pitch from like 10-15 degrees. I’ll take what you guys said and make changes to my topic.

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At 15C, 1000ft MSL pressure altitude, D-TO (26K) will always be the same number: 98.6% N1. That will never change, regardless of how heavy you are. You only change the N1 used by selecting another takeoff setting, that being a standard 10 or 20% derate (TO-1/TO-2) or by adding an assumed temp. on top of any TO setting. When you see people having lower N1s for lower weights, that’s because they choose a derated takeoff setting because they don’t need all 26K.

Again, we’re not factoring out weight here. I’m simply stating that while you are correct in that lower weight typically results in a lower takeoff N1, that’s a product of choosing a derated takeoff setting, not the N1 just magically decreasing because you told the plane “I’m only 45000kg on takeoff.” Like I mentioned above, at any given temperature and pressure altitude, the N1 will always be the same for those conditions. It’s up to you to then determine if you want to limit the thrust output by choosing TO-1/2 or adding assumed temp.

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Perhaps consider the 39K figure to be greatly inflated following some tests I have done after reading your methodology used when coming to the conclusion that the 737NG in IF has that much power.

The method I used is as follows:

B737-900 at MTOW - 79,001kg.
ISA + 15 (30 degrees C) - standard atmospheric pressure - OMDB 30R (solo)
26K TO/GA N1 = 100.1% (according to FCOM.)

Dependent variable being the time taken to accelerate from 0 to 100kts - using F = ma we can work out the net resultant force in the direction of the thrust fairly easily.

The results:

Mean time taken = 18.005s, +/- 0.025s for all non-anomalous results taken from several repeats.

Now we know that 100kts is approximately 51.444m/s, and the mean acceleration will therefore be about 2.857m/s^2.

Using F = ma, with m being 79,001kg as above, the calculated resultant force F will therefore be approximately 225706 N.

Conclusion:

The CFM56-7B26 has a rated thrust of 26,300lbf, which is equal to 116988 N (per engine.)

116988 * 2 = 233796 N
Resultant force calculated = 225706 N which is less and to be expected considering that air resistance increases with speed and is modeled in IF.

The thrust produced by the B739 in IF, therefore, is nowhere close to the 39K you’re describing.
An issue in the methodology of the topic you linked, is that you are measuring the distance left after coming to a stop at V1 - it may very well be that the braking power of the 737 in IF is much more than that in real life (a variable not considered in your investigation); and that the accelerate-stop distance computed by Simbrief automatically has margins added to it as follows:

Screenshot 2025-06-01 231853
Screenshot 2025-06-01 2318531138×447 37.7 KB

(Source: SKYbrary)
…and not that the 737 is really that overpowered compared to real life.

Hope this helps.

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