Question about using "descend now"

[Guest jettjokk]

After studying the FMC manual and reading many posts here about how to prevent the dreaded FMC ding-dong when it's unable to follow a descent profile, I decided to see if I could silence the ding-dong.First I modded waypoint restrictions so as to not allow descent and decel on the same leg. In some cases a leg was split into 2 legs by adding in a waypoint fix about halfway between the original two so that the first half would be descent and the second would be decel. The FMC will not warn you when the waypoint restriction is unachievable until you get there. Ding dong and VNAV disconnect time.But I now see that if you put the MFD into map mode, zoom in, and use the step carefully you will see on the flight path where the decels are too close to the next waypoint. Splitting the leg can effectively move that decel point farther back up the leg which will prevent help VNAV disco.I also read some posts and read in the manual about the "descend now" feature. So I decided to activate descend now as soon as possible (right at 50 miles from t/d) to further help prevent VNAV disco. It had the exact opposite effect. The manual says that upon descend now activation a 1000 fpm descent is begun that will continue until the original VNAV path is intersected. I think what I did was initiate a new descent path that never did intersect the original VNAV path. Must have been parallel below the original. The FMC error was "unable next altitude" over and over and I sat there and watched it for some time not being able to figure what was wrong.I knew I was way below the original VNAV path but I didn't know how to fix it. Finally figured out the plane was already below the restrictions for the next 3 waypoints. Had to pause the flight ad reprogram the restrictions at the waypoints adjusting all of the altitudes down to reflect how low the plane was. This worked but I wonder now how to prevent in the future. Usually I pick FL190 for cruise because everytime I allow the super high altitude arising out of the cost index, I can never get the plane down anywhere near in time. Even an FS IFR flight plan to FL300 will present you with an impossible descent. So it seems at FL190, activating descend now 50 miles from t/d is much too soon. Maybe it's not needed at that lower cruise alt. Anyway to figure out more than what is in the manual regarding that descend now option?

[crvm 0]

That's why you don't use VNAV PATH with many restrictions, try VNAV SPD insted. There is VNAV criteria which means that the angle between two points is not realistic so modify or remove the restrictions. VNAV is complicated to be sure and one has to be one step ahead of it at all times to be effective. [h4]Randy J. Smith[/h4]

[Guest jettjokk]

I knew this would happen...Everywhere in this forum, PMDG guys preach about sticking to the real airplane and procedures. Reality rules. This is good but the FMC manual clearly states that airlines prefer VNAV PATH not VNAV SPEED. Saves fuel with high speed,idle thrust, rapid last minute descents. So that makes me want to make VNAV PATH work to preserve the committment to reality, right?BTW is the descent angle listed in the manuals? What is the actual angle in degrees? Pretty simple calc to see if any leg violates. I still think close examination of each leg in the MAP mode paying close attention to the locations of t/d and decel on the leg will also show up future VNAV disconnects.Also today I flew the same exact flight omitting using "descend now" and had no problem like before. I still got a disconnect on the last leg but it was like down 7500 and slow down from 250 to 190. I am positive a little more tweaking of this route will allow full use of VNAV PATH. It was on this flight where I programmed a 25 nm. leg to descenf from 19000 to 11000 at 315 kias. The speed was the same at the two waypoints. As I sat there and watched it descend, the ring was on the money and it made the leg ok. But during the descent I heard a power reduction and watched IAS drop from 315 to 295. It accelerated back up to 315 once it reached 11000, still before the next waypoint.So it seems the 737-700 can descend and decelerate (within limits) at the same time. Actually did not the FMC realize the a 20 kt. speed reduction was required to make the next restriction? Didn't use the boards, flaps, or gear. FMC just did it. Apparently that leg with the automatic speed reduction still fit the angle criteria.

[crvm 0]

No the NG cannot decel and descend at the same time WITHOUT help. VNAV does not keep speed it only keeps path. The reson for chosing SPD over PATH with restrictions is because SPD will keep SPD and wpts easier since it is altiude prohibited regardless of the point on the PATH and will actually level off prior to the wpt fix restrictions (if it happens to reach the altitude prior to the res) and then resume descent at SPD. And I will state it again, VNAV PATH should not used with some many restrictions regardless of what the manual says (PMDG's).I have discussed with more than one NG pilot on the beta team concerning this very issue. You can look at Bill Bulfer's 737 Guide that talks about this very thing. VNAV and descents are the most challenging for a 737 pilot. As for the manual, I don't know I have not read it. I believe that the angle is 7 degress max above 10000' and 6 degrees below.[h4]Randy J. Smith[/h4]

[Guest Hawkeyeted]

JettJokk,The "descend now" function is used if you need to descend before your calculated TOD point. Say you're 35nm from TOD, but ATC clears you to descend from cruise to FL210. Dial FL210 in the MCP, and hit the "DES NOW" on the FMC. The aircraft will descend at approx 1000 feet/min until it intercepts the originally calculated descent PATH.

[Guest mangold]

Guten Tag Jettjokk,thanks for expressing MY problems in comprehensable English;-)Regards G

[Guest jettjokk]

I tried the VNAV SPD instead and as usual you are right. It's not often in the FS world to get information from real pilots so this is pretty much gold. Thanks and I need to get that book.