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N1 Limit, settings in FMC

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Ok, question1. In the "N1 Limit" in the FMC, I understand that TO-2 wil get you takeoff from low acceleration and low fuel cost, and TO the oposit if I understand it right.2. But what is the CLB do?3. And what to put in the "Sel/Dat"? I read that if 30C out set it to 36C to lur the engine to get more power? Any one explane pleas thx ;) Sorry about poor english hope you understand my questions ;)

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Hello :1) In case of the standard NGX engine (CFM56-7B26) you have 26k thrust with TO. with TO-1 u will have 24k and then 22k with TO-2. So yes it reduces the performance of the engine, so less fuel consumtion during take off and extended engine lifetime.2)CLB sets a N1 limit so it redces maximum thrust during cimbing. You would damage your engines burn a lot of fuel if you were climbing with the same thrust as Take off.3) Sel (Flex/Assumed temp) simulates a temperature higher than the OAT so it reduces the TO thrust between TO. So it set's a value between 26k and 24k

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he he ok thx Flexman I wil wait ;)------ Safir365, roger thx trying to understand it ;) When setting up the N1 Limit in FMC, should it not be adjusted it would have ben auto setted, not set the TO or CLB just leave it as it is? Same goes for the Sel/Dat but its usely blanked? what happend to just make this blank?

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This is a very complex topic and I can't possibly explain everything about it in a forum post, but here's the basic idea:TO-1 and TO-2 (TO-2 only appears if you have the "Double derate" option selected btw) are called "fixed derates" - these are reduced thrust takeoff settings that Boeing has pre-certified to perform in a certain way. The 26K engines on the NGX will be derated to 24K at TO-1 or 22K with TO-2. There's also a TO-B (bump) setting that will boost the engine up to 27K for short field takeoffs, heavy weights,and other situations like that. The CLB-1 and CLB-2 settings are the same thing as the fixed derates for takeoff, but applied to climb thrust. You can also use an "assumed temperature" with any of the TO options - an assumed temperature tells the engines to perform as if the outside air temperature is actually higher than it really is. Holding the N1 constant, the engines will develop less thrust at higher temperatures because the air is less dense. What an assumed temperature tells the EECs to do is run the engines at an N1 setting that will produce the same amount of thrust that the original N1 setting would produce if the temperature was the higher value. It's kind of a roundabout way of thinking about things, but it makes sense. The main reason airlines use these reduced/derated modes is to save both fuel and wear and tear on the engines. It's worth noting though that several airlines determined that climbing quicker to cruise reduces fuel cost more than derating the climb does. Southwest for instance doesn't use climb derates - they'll do an assumed temperature takeoff, but then it's full climb thrust after that.

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Was gonna go for something a bit more in depth... but you'll be alright with that.

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Thx Gents, I can see that this is an complex answer, I wil summen me with the above info and get my 2 cells to understand it thx again for you replyes ;)

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The main reason airlines use these reduced/derated modes is to save both fuel and wear and tear on the engines
I remember reading somewhere that it's also done to reduce the likelihood of engine failure on takeoff? Martin Wilby

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I remember reading somewhere that it's also done to reduce the likelihood of engine failure on takeoff? Martin Wilby
In the long term yes. Still, full thrust will always have to comply with the engine's limitations and maximum temperatures. What it can prevent is potential compressor stall, but today's airframes are very well designed for the engine's air intake requirements and engines are also well protected aerodinamically. Notice that reduced take off thrust decrease net fuel consumption but increase specific fuel consumption. Something to take into consideration by the operators.

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2 things about Derated TO and ATM TO. 1) TO2 can be useful sometimes when you have a short runway and when the runway is wet (VMCG limitation). Choosing TO2 for take off can increase the maximum payload allowed for a flight by decreasing VMCG until VMCG is lower than V1. 2) If you use a no Derate but only Flex Temp Take Off, in case of any problem, you can manually push the throttle full forward and have the maximum power available to safely continue the take off.If you use TO1 or TO2 with a Flex Temp take off, in case of any problem, if you push the throttle full forward, the thrust available will be limited to the maximum thrust of TO1 or TO2 respectively.

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