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Guest tomahawk_pa38

Steep descent After XXXX

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Guest tomahawk_pa38

Just a quick fundamental question - I sometimes get this message when descending - what are you supposed to do about this message or is it just for information.

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I recommend using MCP SPD and V/S to control the descent. The VNAV is telling you that it's forecasting a descent (usually the next segment) that it can't achieve but you usually can if you intervene manually. Sometimes it's due to a speed limitation, where VNAV holds a certain IAS until it's time to deaccelerate. I've found that it's smoother to descend at a set V/S and allow the IAS to decay to the speed constraint, as you slow down you can increase the ft/nm without increasing V/S. At other times is because the VNAV wants to cross a constraint like "AT OR BELOW 19000" at 19000 where a lower crossing altitude would make a smoother descent to the next constraint.Bottom line, I rarely use VNAV for descent and never below FL180.


Dan Downs KCRP

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Dave,Dan is absolutely correct (IMHO). I also never use VNAV below approx 18-20000ft in the descent. At that stage I switch to another descent mode & therefore remove some of the automation from the equation, using the mcp & directly influencing where the a/c needs to be at certain points. If you fly online terminal ATC will most likely be giving you specific instructions which may negate the use of LNAV/VNAV by that stage anyway.I came across a very interesting view on VNAV while reading Bill Bulfers Big Boeing FMC Guide.He asks you to consider VNAV as a strategic tool, it navigates you in 4 dimensions; the 3 we know & of course the last one is the capability to accurately predict optimum altitudes & step climb points. It gets you to your destination providing you have fed the correct data into the FMC...just like any computer programme would, in the most ecconomical or quickest method posible.But in a descent in a crowded & busy terminal area, if given specific instructions by ATC to 'cross XXXXX at 7000 feet, speed 220 knots' - consider these options....Go head down to the FMC & start 1 fingered typing...there is always the chance of making an error....what he/she is asking you to comply with may be beyond the capabilities of VNAV at that stage anyway...I dont know about you but by the time I have started typing with my 1 finger I'll be trying to remember what he/she said...orgo straight to the MCP & dial in the new altitude & select FLCH or V/S & directly control the a/c by perhaps placing the green descent arc over the fix in question, should you need to decelerate to meet a speed restriction over that fix you can wind in a little more V/S & trade height for for speed, then reduce your V/S slow down...more often than not you'll be at the right height & speed...just where ATC want you to be. If it looks like you won't make the gate...say a late clearance to a lower altitude or instructions to expedite your descent then you can always use the spoilers to increase your RoD without increasing your speed drastically - I personally try & keep away from the spoilers unless I really need them...it upsets the G&Ts in first class...LOL!!!!You'll already be looking at the PFD, watching the ND for TCAS conflicts etc & using the MK1 eyeball to check the environment outside, you can also cross check with some raw data to confirm you are on the right page.Just food for thought.Kind RegardsSteve B


Steve Bell

 

"Wise men talk because they have something to say.  Fools talk because they have to say something." - Plato (latterly attributed to Saul Bellow)

 

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"should you need to decelerate to meet a speed restriction over that fix you can wind in a little more V/S & trade height for speed, then reduce your V/S slow down...more often than not you'll be at the right height & speed..."This really doesn't make sense to me, Steve.... aircraft inertia is the same, whatever mode you're in. As you say, you have to trade something for something else.... which then has to be re-traded to lose both height and speed.To me, adding drag (through spoilers) would seem the most logical (and surefire) thing to do (unless part of your programmed VNAV descent has, at some point, a relatively flat section, where you can catch up (i.e. there was originally a section where thrust would be used, but now you can fly that section with the engines at idle and reduce speed that way).... but how often, other than the flat section just before G/S capture, does this occur in real life?... Almost every descent/approach I've selected in FS gives me a constant battle to slow down (engines are almost always at idle from TOD, down to G/S capture).Thanks.Cheers.Q>

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Steve,What do you think about the logic, of going head down and programming the 1 fingered data into the FMC first so that you get some backup data via the deviation indication as you decend. That way after you switch to FLCH or VS, you can still monitor how you're doing, compared to the 'required' path?Or does that waste too much time, and is it better to just go FLCH or VS straight away without any additional indication?I always wondered how much pilots have the mentally required path 'numbers' in their head regardless of indications showing in the cockpit (in this example, the VNAV path deviation, even if you are in FLCH or VS).Thanks,Subin

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Q,Its probably the way I have worded it...What I was trying to point out was that this particular situation may require a compromise...height first then speed or vice versa, I agree that spoiler deployment is the surefire way of getting down quickly, or slowing down quickly.I some situations as you point out you will have no option other than to deploy the spoilers.If its a speed restriction before a height I might use FLCH & wind the speed back, in my limited experience once shes at the commanded speed she sinks like a brick, if the gree arc then indicates that I am going to reach the next altitude before the required point I might go back into V/S as the a/c decends within say 1000ft or so of the new altitude, reduce the RoD, & watch the power come back on gradually to maintain the speed, trying to keep her balanced between the speed commanded &/or the vertical speed required.Kind RegardsSteve B


Steve Bell

 

"Wise men talk because they have something to say.  Fools talk because they have to say something." - Plato (latterly attributed to Saul Bellow)

 

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Subin,I guess if you have or are given that '1 fingered information' early enough, say prior to or shortly after the ToD...(it may already be known as a part of a published approach proceedure), then yes, it's just my personal preference during the mid to latter stages of the descent to take a mode other than VNAV, retaining a positive pilot input (via the MCP).Its a fascinating subject!!Kind regardsSteve Bell


Steve Bell

 

"Wise men talk because they have something to say.  Fools talk because they have to say something." - Plato (latterly attributed to Saul Bellow)

 

The most useful tool on the AVSIM Fora ... 'Mark forum as read'

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Guest tomahawk_pa38

Steve,I suppose my original question really revolves around what you call 'positive pilot input'. I guess this is what I'm having trouble with especially controlling speed whilst descending. I always end up using spoilers much earlier than what I have witnesed as a passenger or end up too fast or too high. Is there any tutorials around that help with descent good practice. (By the way I'm flying the 737 models).

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Dave:The 737 is notorious for difficulties in bleeding off speed and height. Careful descent planning is the best approach. I find that slowing before starting descent in that a/c (and most others) is actually the best way to avoid needing the boards. I have even often created a PBD waypoint before TOD, simply to reduce speed just a bit, to allow for a more comfortable descent profile (first in VNAV and then usually no lower than FL180 in VS). With a little experience in a particular descent (destination) often just a few knots reduction early on, allows for a much more comfortable ride.If under positive ATC control, it is acceptable to ask for earlier descent, or to ask for descent if ATC does not offer it at the optimum time. If flying into rigid descent profiles (STARs at busy airports) again, getting slower BEFORE the descent is the surest way not to to trade too much airspeed for altitude reduction.This is one reason I absolutely agree with Steve- the MCP is the place for anything tactical (read as anything not carefully pre-planned for optimum flight profiles). That SPD INTV knob press is invaluable. When flying, I am very aware of passenger comfort, and attempt to keep all g-loads and deck angles at non Mimosa spilling limits so I'd rather slow a little early and avoid the boards. Keep in mind though, there will be times where you are kept high and fast and they are unavoidable.Best-Carl F. Avari-Cooper BAW0225http://online.vatsimindicators.net/980091/523.png


Best-

Carl Avari-Cooper

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Guest tomahawk_pa38

Thanks Carl,I feel a string of questions coming on so apologies for that but I don't recall finding much written which explains the Speed Intervention button. How does it work. Do I enter a speed into the MCP and then press the button and what actual action does pressing the button do when in Level Change.Also if I select Level Change what speed / rate does it descend at as when I have tried that I usually end up having to using VS mode and then start getting problems with speed and / or height again. I've tried various tutorials over the years but don't recall reading much good tips for descent planning. Also are you guys suggesting slowing to what sort of speed at top of descent ?Thanks

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I just want to reiterate what Dan and Steve said - in a busy terminal environment real pilots are not going to program the FMC to do what ATC tells them, they're just gonna do it using the MCP and the green arc on the ND.


Ryan Maziarz
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Ryan,It sounds like you'd be surprised at what your PMDG products can actually do.:DAll,In the real world I've heard both ways...your dads way, and then FMC-FMS/(M)CDU software versions which help a bit better depending on Airbus/Boeing....mostly Airbus for software issues in the MCDU.It also depends on the published STAR (in this case). Some STARs seem to be a bit easier to fly in the VNav then others. You also need to double check those STARs that have speed restrictions with ALT restrictions. That makes a huge difference, especially if you're flying outside the US (I have learned). Some arrivals have multiple parallel paths which require slower speeds just to make the turns. Some STARs here in the US just plain have close in waypoints with different ALT restrictions that cause issues.I think what it boils down to is individual technique and lots of experience in your particular airport or airspace to be more comfortable in the aircraft. The more you fly, the more you might be able to keep your aircraft in the VNav during through tough arrivals and even through ATC adjustments and vectors.In other words..personal preference where there might be alot of right answers...and...sometimes it works and sometimes it doesn't.I used to be mainly an MCP-V/S flyer until I started really diving into learning VNav. Now I can generally maintain VNav all the way down till the IAF. Sometimes I even cheat and use a GPS APPR as an RNP APPR...LOL The nice thing about sim is it doesn't know any better...hehe.

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Q: This works because of the significant decrease in groundspeed at a given IAS with altitude loss. The best strategy is to decend at high V/S rates while high (and high groundspeed) and then allow the IAS to bleed off during the intermediate descent (FL180-FL140) to arrive at the speed constraint without needing to pull the nose-up for deacceleration. 'Course, ATC might want you to do something else for spacing.


Dan Downs KCRP

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Ryan,Unless there's an actual maint. issue with the CDU/FMS software, I'd be surprised if it was an actual SOP.More plausible is pilot preference based on the way ATC is giving them their instructions. Sometimes you get instructions that might warrant the crew getting a clarification...and in busy terminal areas it might be easier just to revert to V/S mode so they don't tie up the radio freq constantly....or...they get different verbage from ATC for the same instruction at the same airports or actual different instructions each time they fly through for traffic flow purposes at the same airport, and it's just easier to revert back to the V/S mode. Could be a thousand different reasons.It's hard saying without talking to them directly.Anyways for me in the sim, alot of times VNav doesn't always work for certain situations just like, I presume, in the real world.It's one of those interesting water cooler topics I supose, just like everyone's got a different technique for landing. :-)I am however, looking forward to your 'new' or improved VNav logic in future version....as I think you guys eluded to a while back.Should be fun!

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