What are N1 limit, Acceleration, reduction and cutback in the fmc?

[heroturkey 0]

Hi,

In the fmc of b737 ngx, there is a section called n1 limit. There are few selections like TO, TO-1, TO-2 and CLB, CLB-1 and CLD-2. They are the power of engine while takeoff and climb but i don't know how to calculate and select the power. How do you select it? According to what? And in the takeoff ref page, there are few AGL settings such as ACCEL HT, REDUCT HT, EO ACCEL HT and Cutback switch. And when i enable cutback,RESTORE HT or something like this appears.What do they mean?

Sorry for asking terms meanings but i'm trying to learn advanced fmc

[FLEX1978 373]

This is all covered in the documentation.  I would strongly suggest at least following the tutorial.

 

Here is a very basic explanation-

 

TO, TO-1, TO-2 and CLB, CLB-1 and CLB-2

 

As you say they are different power settings for take of & climb.   (They should not be confused with the assumed temperature method)

 

Real world you choose the correct setting based on company policy/aircraft take off weight/ambient conditions/runway length and a host of other factors.

 

They are commonly calculated using an onboard laptop loaded with  take off performance software - Best on the market for sim use http://secure.simmarket.com/topcat-take-off-and-landing-performance-calculation-tool.phtml

 

Or you can calculate manually using runway specific charts and performance data from the QRH (PMDG provides you with this data)

 

Noise Abatement Procedures

 

Each airport has it's own procedures, the company you fly for will also have it's own policy.  NADP1 or NADP 2 

http://www.b737mrg.net/downloads/b737mrg_noise.pdf

 

You adjust the thrust reduction & acceleration height accordingly in the FMC.

 

EO ACCEL HT - ​This is the safe  altitude that you can lower the nose and start accelerating the aircraft in the event of an engine failure.

 

Once again, my explanation is very basic and just scratching the surface.  You really should study the documentation that comes with the aircraft

[zubart 5]

N1 Limits are selected automatically by the FMC. If you go to the N1 page, you'll see AUTO is the default mode. You can manually select N1 though. If you don't want the default TO (max N1 limit) you can select a de-rate or put in an assumed temperature in the SEL TEMP on the Takeoff Ref page. For climb, the FMC defaults to CLB (max N1 climb limit). CLB-1 reduces climb thrust by 10% and CLB-2 by 20%. Normally these last two are not selected as it uses more fuel in the climb phase.

ACCEL HT is the altitude that flap retraction begins and the airplane accelerates towards flaps up climb speed. Cutback is an altitude the N1 rolls back and restore is the altitude that CLB N1 is restored. This is use at airports like KSNA that require a quiet climb out after takeoff (Noise Abetment procedures). At cutback altitude, (800 feet AGL at KSNA), N1 TO thrust goes from TO to about a 25% roll back (about 78% N1) The pitch attitude must be lowered to maintain V2 + 15 until the restore altitude.

[BeechPapa 32]

TO, TO-1, or TO-2 are usually calculated with a tool like TOPCAT or onboard the real plane through an EFB or iPad. The calculations that effect required N1 for takeoff include: weight and flaps setting of the plane, length of runway, wind, temperature, barometric pressure, and dry vs. wet runway. The calculations required for this are complex, and where as the flight engineer used to make these calcs, that's become the territory of computers. CLB, CLB-1, CLB-2 are more or less fuel saving settings for climb and I haven't seen a way to calculate these with computers. I believe it may come down to different airliners' procedures for climb, and how much fuel they want to burn at climb. CLB makes the plane climb fastest at max n1, CLB-2 is the slowest climb with least n1 percent.

 

I am familiar with engine cutback because I often practice the noise abatement procedure leaving KSNA. Reduction height is the AGL height at which the engines will cut themselves back to the lower N1 you see on the cutback page. Restore is the altitude at which normal climb N1 (CLB, CLB-1, CLB-2) are resumed. At KSNA for instance, after a very steep climb at max n1 from a short field, engines must be quieted at 800ft and can only resume once the plane is high enough or far enough away from neighborhoods around the airport. By setting Reduction at 800ft, and restore at 3000ft you can effectively simulate the noise abatement procedure. EO (Engine Out) Acceleration height is the height at which the plane, if it senses an engine failure, will lower its pitch to exchange climb power for acceleration power. The reason for this is that if you had an engine failure you can't maintain the same climb rates and you need to start thinking about maintaining speed (and lift) instead of gaining altitude. For EO Accel HT you are effectively telling the FMC: if you have an engine failure at this height, all bets are off for the cutback, go to max thrust and pitch down for acceleration instead of climb.

 

Hope that helps. I really dig the FMC cutback options, they make it possible to mimic real world procedures to an exact T... or an exact N1 to be more precise.       

 

Edit: Whoops, looks like previous posters covered the same info already while I was typing. You've got plenty of info now.