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DES NOW

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I decided today to start a early descent (about 150NM from TD) by pushing the DES NOW Button. As far as I know this should happen:- the activate button lights up and after activating the AC starts descending (1200 FPM)- The VNAV deviation scale appears and the calculated T/D disappears.In the sim this nothing happens so and you have to avtivate the descent by pushing the alt knob. The descent speed however is 1200 FPM. The DES NOW button only funtions when you are about 10 nm from the T/D sign.Did anyone have this experience?Ton

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Hi Michael,The FMC manual nor the RW manual give any limitations.Ton

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"The limitation is stated in the original Honeywell FMS documentation"Found it! figure 3.5.3 ;-) Thanks that is solved than! Funny it is not mentioned in the RW AOM or the Bulfer manual Ton

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Our AOM is slightly ambiguous...In one section it reads..."During cruise, setting an altitude below the current cruise altitude in the MCP altitude window and pushing the altitude selector activates the DES NOW function when the aircraft is within 50nm of the T/D or if the MCP altitude is set below the highest descent altitude constraint in the VNAV descent profile"In another section it reads..."DES NOW-Displays when T/D created and descent phase is not active"These sound like two different processes.However, I seem to recall reading, also, that this function should not be used too far from TOD as predictions can become inaccurate. Just can't remember where I read it, though (thought it was in the Bulfer Manual, but couldn't find it... Perhaps in a Notam).Cheers.Q>

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on page 204 of "Big Boeing FMC Users Guide" on the DES NOW> it says:"If pressed during the climb will delete climb and cruise constraints"This would imply that DES NOW is not restricted by distance from T/D.However it is only important how PMDG modeled the behavior.Ray

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Ambiguous information indeed -The honeywell "B747-400 FMS Pilot guide" does mention it-Bill Bulfer mentions the use of DES NOW with 2 different situations (with or without altitude intervention)where the 50 nm limit is only mentioned in case of the latter situation-Several AOM don

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The DEC NOW function is available immediately on transition from Climb to Cruise mode (in the model). Doin' some test flights now. Will a DEC NOW, 1200 fpm ROD ever intercept a projected VNAV PATH, idle descent path, say 150 miles out? Lets see.

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HiThis is not related to the 747, but to DES NOW. I was online friends with a retired pilot who was a controller with an online flying system, and of course on quiet times i would bombard him with questions, and on one occasion he told me that on the 737's that he flew he would operate the DES NOW at 10nm from TOD to achieve a 1000ft/min decent rate till it intercepted the projected path, and thats what i do now. regardsJohn CallejaBAW352

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>that on the 737's that he flew he would operate the DES NOW at>10nm from TOD to achieve a 1000ft/min decent rate till it>intercepted the projected path, and thats what i do now.10 nm seems like an awful short distance - it is barely a minute so I wonder what practical difference it would have on your descenet profile - very little. I would rather see practical benefit from engaging DES NOW at least 20-30 nm out.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

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I found that the DES NOW feature can very helpful, but it will allow some pretty 'creative' virtual piloting. For instance, if I am too far from TOD, the DEC NOW will simply put me into a 1200 fpm descent and never intercept PATH. The path deviation scale comes into view and I just keep going further and further below PATH. Keep that MCP altitude setting above the dirt, because otherwise, that's where you're going. Sim pilots beware.I would think the DES NOW feature would be inhibited if that 1200 fpm descent would not intercept PATH by some altitude. Anyone have any real world experience with this?

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>path deviation scale comes into view and I just keep going>further and further below PATH. Hmm.. that sounds strange. 1200 fpm should always be less than a typical rate of descent while in PATH - resulting in slow convergence of both paths. Yes, if there is not enough time left they will never converge but the trend should be to converge rather than diverge.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

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Yea, the old path is always still there. I can tell when I'm under it because the deviation decreases, then reverses. But if I'm still too far away, the airplane never closes for the intercept before I have to pull up to miss a north Atlantic white cap. Seems odd the FMC would even allow me to initiate DES NOW when there was no chance of intercepting PATH.

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"Seems odd the FMC would even allow me to initiate DES NOW when there was no chance of intercepting PATH."The Descend Now function allows the pilot to manually add thrust to slow the descent. The FMC has no way of knowing how much thrust the pilot is going to add during Descend Now, so it doesn't know if the plane will intercept the path at some point in the future.It's kinda nice to know that the 744 still allows the pilot to be the master.... and assumes that he/she knows what what he/she is doing ;)Cheers.Q>

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Wrong impression. This is about machines, not people. Breath.

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>Pilots should know how to crunch a few numbers in their heads>after flying aircraft for decades/thousands of hours.I would hope so. 150 nm is equal roughly to 20 minutes of flight time - 20 minutes times 1200 fpm and you get almost 25000 ft. Clearly even with doing less accurate arithmetic than this one would expect that initiating DES NOW 150 nm before TOD will have rather significant impact on the profile.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

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From a practical viewpoint you would only use the "descend now" feature that far out if you were given a lower level by ATC say due to converging traffic. This would intiate a cruise descent to the new MCP altitude and when there recalculate a new TOD point. Inside 50 mls it doesnt and assumes that you are going to eventually try to pick up the original descent point at some place further down track. It is a useful feature if ATC tell you to decend now and not at pilot discretion (Planned TOD). I notice that the ATC in FS9 always seems to clear you down very early and this is where that feature comes in handy.Q is quite right that pilots do have ball park numbers in their heads the best one being the 3 times table. There are many techniques to descend this beast and "descend now" is just one of them.If ever you get confused with what the aircraft is doing or the VNAV seems to be screwed up just hit FLCH. A brilliant oh s..t mode. As long as you have the altitude you want set in the MCP and the AT engaged it will either climb to or descend to that altitude and all at the press of one button. To check on descent progress multiply your height to lose by 3 and add 10 and this will cover you for about 90% of all eventualities. From then on you can look for a trend and make adjustments as necessary. Use the green arc. Getting a little high? Crank the speed up first (assuming smooth and no procedural speed limitations). This has a radical affect on descent path. Still too high or getting to fast? Speed brake. Too low? either dial the speed back slightly on the MCP (not my personal favourite as jets should be flown fast) or apply a little bit of power. The AT will be in HOLD so this is easily done by hand. Once you have a descent looking VNAV profile again intercept it and select VNAV. Just be careful and remember that if you select VNAV and your current speed is well above the VNAV programmed speed that it may shoot out the top. CheersSteve

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