Desent Path Question

[jwenham 1]

I have been flying the 737 NGX alot recently and have a question about the desent. Is there a way to have RC alow me to fly my desent as assigned in the FMS? I ask this because RC never matches even close to what the FMC wants and almost always has me taking the bird down before I even get to my TD. If not then what is the point of a desent profile? - Jim

[ronzie 21]

You can ask for a Pilot's Discretion (PD) to delay descending. RC uses the generic 3 in 1 rule to calculate TOD to the hard crossing restriction of 11,000 (or FL110) or 12,000 (or FL120) occurring about 40 nm out from destination depending on whether you will arrive on a near or far side approach. Your FMC calculates your descent as a constant descent approach to the airport MSL. RC steps you down from 11 or 12 thousand feet. You can follow your own descent intially ensuring at forty nm out you will be at the assigned crossing restriction altitude. I use an approximate method of on the ND displaying a range ring around destination of forty nm out. I then monitor descent arcs to insure I'm down to that altitude by then. Failure to meet the crossing restriction rersults in getting delay vectors until you meet it. The crossing restriction is the altitude agreed upon by center and approach where that window is established to transfer control. On the Boeing Smith's FMC system, enter the FIX page. In the scratchpad enter the ICAO code of your destination and LSK the fix ident. In the scratchpad enter /40. LSK the place-bearing-distance button. EXE the change. On the ND you will now see a dotted circle around your destination. This is just a display aid and has no influence on your LEGS navigation. Some will see an existing waypoint near this on the path and put a hard altitude for VNAV on the LEGS page. Some will create a waypoint FIX with a hard altitude but that is a lot of work IMHO. I just adjust speed and V/S as necessary to arrive at the correct altitude trying to maintain idle thrust. The range ring works in case of a change of path. Some also add a 30 nm range ring around destination as a guide to the airport speed restriction area, usually 210 kt or below.

[jwenham 1]

Thanks for the quick reply and for the range ring tip Ron! I will give it a shot on my next flight.

[jwenham 1]

Ron, another question for you... when ATC instructs to descend from FL123 to FLXXX what rate of descent would be good to shoot for? Should I keep in mind the crossing restriction of 11 - 12 thousand feet and base it on that? Jim

[ronzie 21]

If the descent is to a crossing restriction just follow the trend bars by adjusting power and V/S. Idle thrust is ideal but if you reach it the altitude to soon and have to level off, that's fuel inefficient. Allow yourself a little safety area. At any other time descent should be as ordered in an expedient manner. As stated descent steps will occur not too spaced out after RC's TOD calculation to the crossing restriction. You can get an idea from these 737 classic but general pilot notes, a real world source:http://www.b737.org....ulesofthumb.htm http://www.b737.org.uk/rulesofthumb.htm#Speed,_Height,_Distance_conversionplus the section below.

[jwenham 1]

Just did a flight KFLL - KTPA and used your tip on adding a fix in this case TPA and added a /40 which gave me the 40 mile radius visual and it worked like a charm for me. I think this is the first time I never got yelled at for missing my crossing restriction!!!! Thanks for your help and the GREAT support you give all of us in need! Jim