DES NOW

[Guest 744nut]

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

[michal 44]

>I decided today to start a early descent (about 150NM from TD)I don't think DES NOW will be working so far from TD. Check the manual.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[Guest 744nut]

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

[michal 44]

Ok, I just found it. The limitation is stated in the original Honeywell FMS documentation for the 744. It says DES NOW will work when within 50 nm of T/D.Michael J.http://www.precisionmanuals.com/images/for...argo_hauler.gifhttp://sales.hifisim.com/pub-download/asv6-banner-beta.jpg

[Guest 744nut]

"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

[Qavion 19]

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>

[Guest Ray51]

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

[Guest 744nut]

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

[Guest D17S]

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.

[jc_150 3]

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

[michal 44]

>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

[Guest D17S]

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?

[michal 44]

>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

[Guest D17S]

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.

[Qavion 19]

"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>