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737-700 Over Speed Disconnect

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Guest mrants

Can someone PLEASE! Help!I

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You need to give some more info please.With what speed are you descending ?Is the speed controlled via the FMC by enabling "LNAV" or isthe speed controlled via de MCP via LvChg or speed hold ??Regards,Niek

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Mr Nameless,Please take a moment to read the forum rules. You need to sign your posts with your name. It's always nice to know who you are talking to. ;-)You may do a search on VNAV PATH descents and you'll get a good bunch of answers to your question. Below is the short one:When descending in VNAV PATH mode the speed is not managed by the automatic flight system. You are responsiblle for that (you're responsible for the flight whatever mode on the A/P as a matter of fact ;-)). The only thing the A/P is trying to achieve when in VNAV is keeping the calculated path.Hope it helps,


Mats Johansson
PMDG Flight Test Dept
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Niek,LNAV does not control speed. VNAV does and A/T has to be armed as well.Hope it helps,


Mats Johansson
PMDG Flight Test Dept
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| Asus Z270-A | Intel i5-7600K @ 4.8 GHz OC/H2O | nVidia Geforce GTX 1070 8GB OC/O2|

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Guest mrants

Thanks all for replying.The descent mode is (path) descent.Yes Autothrottle is on. The descent speed is around (284) KIAS.I did search this forum thoroughly but with no avail.At TD the descent begins ok, its only when the bird descends into lower altitude i.e. 10000ft

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Tony,While in a VNAV PTH descent, below 10,000 feet , the airspeed must not exceed 250 knots or you will get a "over speed disconnect" every time. This is why the FMC descent target speed below 10,000 feet is defaulted at 240 knots which provides for a 10 knot buffer. The FMC will not allow the airplane to exceed this FAR speed and VNAV will disconnect, giving speed control back to the pilot. The pilot can exceed FAR speed if he wants to. In a VNAV PTH descent, the FMC controls the PATH and the pilot controls airspeed. The FMC target speed is just for descent planning to build a PATH. The autopilot will disregard airspeed on the high side in order to remain on the VNAV PTH. A couple of things to remember:-A VNAV PTH descent is a idle thrust descent for planning.-In a VNAV PTH descent , the FMC will pitch as necessary to stay on the path and the pilot will have control airspeed with thrust or speed brake if necessary. To help fix the problem, insert descent winds into the descent forecast page of the FMC. This will help the FMC plan a VNAV PTH descent at the FMC target airspeeds. One problem that often arises is when descent winds produce excessive tailwinds that force the FMC to pitch the airplane down to stay on the VNAV PTH, disregarding FMC target speed. The problem is not so much above 10,000 feet as it is blow 10,000 feet because of the FAR speed restriction. Above 10,000 feet airspeed may increase to VMO/MMO before VNAV disconnect will happen. Check the Legs page altitude and airspeed constraints for reasonableness. Make use of the speed brake. When the airspeed is starting to creep above 240 knots , apply speed brakes then, not later to maintain airspeed below 250 knots. Good luck.Floyd


John Floyd

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Guest mrants

Thank you Floyd, that sounds a lot like what

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A VNAV SPD descent would be easier but where's the challenge? Might as well use LVL CHG.I don't know if you use a weather add on or the FS9 weather but you should be able to estimate the descent winds from how you have the winds set in FS9's Weather. Descending with a tailwind will be more difficult to manage than descending into a headwind. Use the speed brake and/or configure the airplane with Flaps 1 or 5 if necessary during an approach.Good luck.Floyd


John Floyd

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Hi Floyd, I've had the same problem as Tony and found your answers really helpful. But would you please spell out what you mean by "where's the challenge". First, how do you get into an FMC SPD descent? I'm very familiar with the FMC, but I must have missed that point. And then I've seen in another forum post a recommendation to use an FMC SPD descent to get around the overspeed problem. Why do you say "might as well use LVL CH"? I hope the answer isn't too obvious. :-)ThanksGeoff


Geoffrey Kent

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Guest mrants

Thanks a lot for all your help Floyd. . . greatly appreciated.Regards,Tony.

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The challenge in the NG is to learn to use LNAV and VNAV to their fullest extent. This includes PATH descents and VNAV approaches which are probably the most difficult maneuver


John Floyd

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