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largo222

VNAV descent disconnects B 737-700

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Been flying captioned aircraft for 2/3 weeks. Getting vnav disconnects which I believe are coming from high speed descents. Flight plans look good and FMC speeds and ALT looks good. Coming down I start with the disconnects.I can usually get the VNav back. Fellow on this forum told me to control speed with S/B. I assumed I could find S/B in manual. No luck! Can anyone explain the use of S/b and where to find it. I usually go to A/P when it disconnects and control things with V/S, alt and manual spd adjustments. Thanks.Frank

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S/B == speed brake == spoilersYou can also try a vnav speed descent rather than a vnav path descent.Jason

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Hi Frank,Before you begin your descent, are you entering a lower altitude in the CRZ page of the FMC? It should (I think, someone please correct me if I'm wrong) be at or below your assigned "descend to" altitude. Type the new level in the scratchpad and hit LSK1, CRZ ALT. Then hit EXEC. You now ought to be able to return to the VNAV descent.You can use the speedbrake to slow your speed in a descent by deploying them (default "/" key). It is located on the pedestal, to the left of the throttles. I also descend with V/S. If the speed is climbing, I'll slow it down by decreasing my vertical speed. If I'm given a restriction that forces me to descend faster than I'd like, I'll use the speedbrake and adjust the V/S to keep the speed I'd like at the descent rate I need.Hope this helps!Chip B.@-@http://www.jdtllc.com/images/rcv4bannerbeta.jpg

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This might help you to avoid the dreaded VNAV DISCONNECT FMC message during a VNAV PTH descent.A VNAV PTH descent is more economical and preferred. Its anidle thrust descent. The airplane will pitch to remain on the VNAV PTH. FMC target speed is just for planning. During the actual descent, the airspeed may go above or below target speed but the AFDS will always pitch to stay on the VNAV PTH. In a prefect world, the airplane would descend at FMC target speed but unfortunately there are some outside factors that prevent this. Wind speed and direction has a greatest effect on the airplane during a descent. Insert known wind speed and direction into the FMC DESCENT FORECAST page to help the FMC predict a good descent path before the TOD point. An unforecasted headwind will cause the airplane to pitch up to stay on the VNAV PTH and therefore airspeed will go below FMC target airspeed. unforecasted tailwinds will cause the airplane to pitch down to stay on the VNAV PTH and therefore increase airspeed above the FMC target airspeed. In the first case the pilot should add thrust manually to at least get the airspeed back to the FMC target speed. If the pilot does nothing, the FMC will eventually add thrust when the indicated airspeed falls at least 10 knots or more below FMC target speed. In the second case, the pilot should use the SPEED BRAKE to reduce the indicated airspeed back to the FMC target speed. If the airplane's airspeed approaches MMO/VMO, VNAV will disconnect. Above 10,000 feet VNAV PTH descents are easy. VNAV PTH descents below 10,000 feet can be more difficult. This is because of the 250 knot FAR speed restriction. In a VNAV PTH descent,the FMC will only allow the airspeed to 10 knots above this before VNAV disconnects.This is the reason the FMC default descent airspeed is 240 knots, to provide a buffer area. If your using VNAV PTH descents below 10,000 feet, keep the airspeed under control by using SPEED BRAKES, FLAPS and landing gear for drag when appropriate.On the PTH DESCENT page, keep an eye on the speed restriction block (SPD REST). It will eventually change from 240/10000 to a FLAPS/airspeed restriction. At this point you defiantly need to be putting flaps out as it suggest.A VNAV SPD descent is more flexible. The airplane may have level off periods between waypoints but it still obeys waypoint altitude and airspeed constraints. It also pitches to maintain the FMC target speed all the time. Its up to the pilot to stay on the VNAV PTH by adding thrust to deduce descent rate or using the speed brake to increase the rate of descent back to the PTH.Good luck,Floyd

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Thank you all for your explanations. Just what I needed. Lots to think about. This will definitely help.Best,Frank

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