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Virgin_A340

Overspeed Disconnect

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I keep getting very dodgy approaches with the 737NG when flying STARs with LNAV and VNAV. It seems to hit all the alt restrictions ok during the first part of the descent but when it gets into the STAR it's another story. First off slowing to 250kts at FL100 is almost impossible. When I finally get it to 250kts it stays there but then when I descend as per the STAR it always shoots back up again even with speed brakes out. I also always get overspeed disconnects when following STARs. Yesterday I was flying into ESSA on the ELTOK 2V STAR seen here. http://www.lfv.se/upload/ANS/AIP/AD/AD%202...SSA_4_73_en.pdf Everything was set in the FMC with the alt/speed restrictions. I got down to FL100 by SA911 but then the descent to 8000 by SA491 caused an overspeed disconnect. This has to be some kind of error as it's only a descent of 2000ft in 8nm. The 737 MUST be able to do this at 250kts right? As I say I have set everything correctly I think in the LEGS pages by doing a SA491 250/8000 input. Can anyone tell me what's going wrong here? Is there some more info I have to put in? Cheers,Andy S

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Are you putting the winds in the FMS? Important if there is a tailwind component. Actually, 2000 ft in 8 nm is a standard slope of 250 ft/nm; which is not difficult unless you have tailwind that increases your groundspeed. The key is slowing down your IAS, thus allowing a lower descent rate. Many terminal areas will slow the traffic to 220. Also, many use VNAV for decents but I always use MCP SPD & V/S and let the VNAV computer provide cues rather than letting it control. The MD11 is the only sim I have found that does a really good job descending in VNAV mode.


Dan Downs KCRP

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I have not really tried putting in the winds. I will give it a go if I can get them off Active Sky. I'm not sure it gives the wind for all the points on the STAR though. Last time I looked it only gave winds to about 30nm out then didn't list any more until the winds at the destination airport.Andy S

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Current winds are calculated and displayed on your ND. The FMS navigation data is calculated from the change in location with reference to time by GPS and/or raw data such as DME, etc. Ground speed and path is derived from this and the IAS is known and with OAT true airspeed can be calculated. Take in the wind correction required offset heading (where the nose is pointed) and the wind speed and angle can be calculated. I think I've got this correct. It is kind of a reverse dead-reckoning navigation procedure referenced to known ground position.I also use VNAV as advisory and prefer to use MCP control with IAS and V/S keeping an eye on N1 to maintain idle. Anticipate where you will encounter a steep descent and reduce speed ahead of time. If you need additional drag you can drop the landing gear once you are slow enough within gear drop speeds to assist getting down to reaching flap speed limits. Make good use of the descent trend arcs on the ND. It also helps to put range rings (using the FIX procedure) around your destination to anticipate crossing restrictions of altitude and speed and see how the descent trend arcs relate to them.

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I have not really tried putting in the winds. I will give it a go if I can get them off Active Sky. I'm not sure it gives the wind for all the points on the STAR though. Last time I looked it only gave winds to about 30nm out then didn't list any more until the winds at the destination airport.Andy S
You don't need to get winds for each fix, you just need to enter the forecast at different levels on the descent forecast page.

Dan Downs KCRP

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You don't need to get winds for each fix, you just need to enter the forecast at different levels on the descent forecast page.
Ah ok. I said that because in the 767 I enter the wind for each fix on the RTE DATA page. Andy S

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Ok I tried it with the winds on the descent forcast page and it was better but still go overspeed disconnects, just not as many. The FMS only allows I think 4 different altitudes for winds on the descent forcast page. What winds do I put in as Active Sky gives them from cruize alt right down to 3000'?Andy S

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Andy,

Ok I tried it with the winds on the descent forcast page and it was better but still go overspeed disconnects, just not as many.
Do not forget that you, the pilot, has to monitor and manage the speeds during VNAV descents. This will not be done by the FMS. If speed tends to increase deploy the air brakes. The NG is known to be a slippery bird.
The FMS only allows I think 4 different altitudes for winds on the descent forcast page. What winds do I put in as Active Sky gives them from cruize alt right down to 3000'?
You need to evaluate the winds given by Active Sky. If they are changing evenly from level to level you can just pick 4 level evenly spaced and enter. If you can see that some abrupt changes are happening between two altitudes you should make sure that is entered into the FORECAST page.Hope it helps,

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