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Nixon

Turb penetration N1 setting

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Just flew thru some turbulence and I set the man N1 turb setting to 89.3% as per the FMC. I then started to wonder how the FMC arrives at that figure. I know that you would not want to be too near VMO as you could overspeed so is it to protect against that or does it also protect the engines? If so how can turbulence damage the engines?


Regards

Nixon Thomas

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The Turb N1 is a thrust setting to give you M.76./280 in cruise flight. It's nothing to do with protecting the engines, but like you said, it's to keep you away from VMO (and MMO) on the fast side and stall on the low side.Great question.


Matt Cee

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The Turb N1 is a thrust setting to give you M.76./280 in cruise flight. It's nothing to do with protecting the engines, but like you said, it's to keep you away from VMO (and MMO) on the fast side and stall on the low side.Great question.
Thanks for your reply that makes sense. The setting from my FMC actually gave me M.78 which was my cruise speed. I thought that was odd and am wondering if the value given in the FMC is correct or maybe a little bug has crept in!

Regards

Nixon Thomas

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In the QRH or Performance Handbook there is an Airspeed Unreliable / Turbulent Air Penetration table. It should give you the same numbers as the Turb N1 from the FMC. For giggles, compare the two and see what you come up with. Also, if they differ, you could see which is close to giving you the M.76/280kts.


Matt Cee

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Thanks MattI have just done what you said and the tables are based on M.76/280kts. The info is on FCOMv1 PI 30.42(p748 in the header window) I was at Fl380 and my weight was about 62,000Kg. So by extrapolation the N1% of 89.3 given in the FMC is about right but it gave me a speed of M.78 and I presume I should be at M.76. Out of interest on another flight at the same alt and weight I will select M.76 and see what N1 % I get. Well I guess the problem with being so open with all the info is that such minor things are going to be noticed. In fact it has no bearing on my enjoyment of the NGX it's just that am interested in trying to increase my knowledge of aviation.


Regards

Nixon Thomas

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