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T/D always too late.

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I know there has been discussion on this before but I am perplexed on a couple of things.The 737 reaches T/D and starts to descend normally. However after approx 2 mins I get the message Drag Required. I apply the Speed Brake. Why does N1 stay at 40%. Why does the aircraft not idle in order for it to slow down? When I reach 10,500 the aircraft levels off to reduce speed for the restriction. And from there on the I struggle to meet further speed restrictions and inevitibly VNav will disconnect and I have to go around and perform a manual landing.Can anyone explain why this is? Surely speed brakes should not be applied so early. Any tips most welcome.Thanks.


Keith Sandford.

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Hi Keith,Couple things:1. Do you have weather loaded at all, and if so do you have the winds entered into the FMC? It needs that info or else the VNAV predictions will be off.2. The N1 value for idle thrust isn't always the same - it depends on altitude and you'll see a higher N1 at higher altitudes when you start your descent.


Ryan Maziarz
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I am currently flying with zero wind. Do I have to manually input that figure into the FMC?Also, what do we have at our disposal to slow the 737/900 series down? By this I mean control surfaces that create drag. I do not mean climb or level out.Are speed brakes and spoilers the same thing?FLAPSSPOILERSGEARSPEED BRAKE


Keith Sandford.

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Keith,You should also know that the NG is a slippery bird. You can't expect her to slow down and descend at the same time. Also below 10k feet and with a lot of hard altitude constraints loaded in the terminal procedure, the aircraft might have a hard time doing what you want. Check your profile well before T/D and make adjustments as necessary. You're in command, not the FMC. You make the calls! ;-)Best way of slowing down is level flight, second would be the spoilers/speed brakes, and third would be gear when below max speed for gear operations.Hope it helps,


Mats Johansson
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Guest tomahawk_pa38

Keith,VNAV Path is calculated by the FMC based on the atmospheric info you put in. If you don't insert wind speed and direction from the Descent / Forecast paage, the FMC will assume there is now wind aloft. If you pick up a tail wind you will indeed get various messages (Drag required or VNAV Disconnect). Since I started estimating the wind speed and direction, I rarely get these FMC messages now. It doesn't appear to matter how acurate your forecasts are as long as there's something in the boxes. it salways interesting to check the wind speed and direction arrow on the master display - this will give you a clue for future flights in the same areas as to what the prevailing winds generally are. I think there's a website that will give you winds aloft I think but don't use it myself.

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800 ft above the path and the trend isn't towards improvement then the brakes go up.Trending is important to smooth control.


Dan Downs KCRP

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Guest frazer

A great tip from a real-life 737NG-pilot is the following, always used by me, especially when commanding the -600:Aprox. 10 miles before the calculated T/D, set lower MCP-altitude and press the ALT INTV.-button, placed just aside the ALT-knob.This will engage a descendpath with 1000 fpm until intercept of the FMS-calculated DES PATH, which will then be intercepted and the descend will continue as usual. This will make the aircraft slow down to the set descend-speed in advance, and will in a normal descend speed reduce the need of speed-brakes to a minimum. Very good tip!

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