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Guest FPlueckthum

737 Step descent after YXY

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Hello 737 Crews,again I come to this forum with a question, I was not able to find an answer to.Sometimes, during my descent I get a message from my FMC "step descent after YXY (any waypoint)". In this situation, I do not know what the aircraft ( 737 ) "wants me to do", or was was going wrong in the descentperiod before.Maybe someone of you can advise me in this point.Many thanks !

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STEEP DES AFTER XXXXXThe FMC altitude constraint at waypoint XXXXX results in a steep descent path to the next waypoint. Remove altitude constraint at XXXXX if allowed, otherwise the airplane will pitch to increase descent rate to make the altitude constraint and probably exceed the planned FMC descent speed (which is usually OK) to make the path.Here are some other fine points when using VNAV PTH descents:The difference between a VNAV PTH descent and a VNAV SPD descent is in a VNAV PTH descent the airplane pitches to stay on the path and disregards FMC target airspeed. In a VNAV SPD descent the airplane pitches to maintain target speed and ignores the path.When flying a VNAV PTH descent, the speed shown in the FMC is just a target speed for planning purposes. The autopilot will always pitch to maintain the path descent and disregard target speed. The Airspeed may increase up to VMO where then VNAV will disconnect. Airspeed increasing above target speed is generally OK as long as ATC doesn't hit you with a speed reduction. If airspeed is on the low side, then at about 15 knots below target speed the autothottles will add thrust until airspeed is back to target speed. Going over target speed is not a big deal but reducing speed to 240 knots at 10,000 feet is and this is where the speed brake will most likely be needed. Above 10,000 feet during a VNAV PTH descent the FMC will permit descending faster than target speed. It will not permit the airplane to fly faster than the 250 knots FAR rule and this is why VNAV disconnects at about 255 knots in a descent below 10,000 feet. Use the speed brake below 10,000 feet to maintain FMC default target speed of 240 knots. Unforcasted descent winds cause airspeed to go above or below FMC target speed. While still in CRUISE go to the FMC DESCENT page, push FORECAST, and insert the known winds at lower altitudes. This will help the FMC predict the PATH descent and if the forecast winds you put in the FMC are fairly accurate then the airplane will stay pretty close to FMC target speed during the descent.Physics and common sense also come in the play here. An altitude constraint during descent can not be so close to the airplane where by it is physically impossible for the airplane to accomplish it. During preflight check all FMC altitude and airspeed constraints to make sure they are physically practical for the airplane to do. Generally the airplane is capable of a 3 to 1 idle thrust descent in calm winds. A heavier airplane will glide farther than a light airplane.Also, be aware there is a VNAV "bug" in this FMC. If you are flying in a VNAV Pth descent, check the IVSI every now an then and if you see a magenta target bug, this is not right. To get rid of the bug cycle the VNAV button OFF then back ON and the IVSI magenta bug should disappear and VNAV PTH descent will work properly.Good luck.Floyd

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