I wonder where im thinking about that 1000 fpm down rate? Am I getting that from vnav descend?

 

VNAV DES NOW> function...

Level Change plus speedbrakes will bring you down real fast while maintaining speed. Leave it like that for a couple of minutes and you will be 1000-1500 feet below profile. Go back to VNAV and the a/c will slow rate of descent to recapture the profile and the speed will not increase thru the roof like it does on some STARS even with speedbrakes deployed. But you must always consider altitude and speed constraints as depicted on the STAR.

This works best on a STAR with no constraints such as CYPRESS SIX into KMIA. It would be very difficult if not impossible to use on a KOOLY4 in KPHX.

Pcubine if i get an early descend can i stay in vnav or do i have to switch out to level change...

Pcubine if i get an early descend can i stay in vnav or do i have to switch out to level change...

You can stay in VNAV. You can:

 

1. Use the DESC NOW prompt as Kyle noted
2. Use ALT INT from the MCP (same effect)
3. "Recruise" at the lower altitude (Change the CRZ ALT in the FMC)

All while staying in VNAV, if that's what you want.

well i'm using PFE atc great little bit of kit but staying in vnav and descending at 1000 fpm made me very high on app....

well i'm using PFE atc great little bit of kit but staying in vnav and descending at 1000 fpm made me very high on app....

 

You're either not properly utilizing the autopilot modes properly (again: verify your throttles are AT idle if you're using the setting that allows your hardware to override the sim in ARM/HOLD MODE ONLY - if they're off the idle stops, you're going to get slow descents), or PFE isn't as great of a little bit of kit (I personally think all ATC programs are pretty rubbish, but that's just me).

 

My bet is a little bit of both, which is compounding the issues of both.

Hi kyle i got to des now stay in vnav and it descends me at 1000 fpm? I don't see why you should stay in vnav as you are no longer required to stay on your path untill i reach my star which i then fly my STAR myself.

 

I've set it up to ARM/HOLD MODE ONLY in the FMC !

Hey Vernon

 

The DES NOW function in VNAV is only available before you reach the TOD point. If you select it, it will descend you in VNAV at 1000 fpm until you intercept the original FMC calculated VNAV PATH. At that point VNAV PATH takes over and you will be back on the normal descent profile.

 

If you are getting ATC descent instructions it really does not matter whether you stay in VNAV or use LVL CHG, as long as you monitor your descent to make sure you have a sensible profile and dont bust any altitude constraints. For example.... you are told by ATC to descend from cruise to 20,000 feet. Your next waypoint is 40 nm away and has a requirement to be AT FL200 at that point. VNAV DES NOW will be the best option here as the original path will have planned to be at FL200 at the next waypoint. There is no point changing to LVL CHG and flying a high speed descent which will get you to FL200 quickly then have to cruise at FL200 for a considerable distance.

 

Hope this helps.

 

 

got to des now stay in vnav and it descends me at 1000 fpm?

 

This is the correct behavior.  Peter explains it well in the post above.  Basically, since you're descending early, you descend on a shallower path because you want to eventually re-merge with your calculated (ECON) path.  DES NOW is a way to meet both the ATC instruction, while still maintaining some of your ECON descent path (once you recapture it).  In order to do this, you descend early,  but at a shallower rate.  Again, it's behaving as expected.

 

 

 

I don't see why you should stay in vnav as you are no longer required to stay on your path untill i reach my star which i then fly my STAR myself.

 

The path requirement isn't an issue of ATC, it's an issue of fuel and money.  ATC giving you an early descent doesn't mean you immediately toss out the FMC-calculated path.  Why would you do so?  Remember that you want to operate the aircraft to your own goals (as an airline), while still meeting ATC instructions.  The FMC calculated path isn't something that the controller knows (and when people use ATC programs, those definitely don't know - again, one of the reasons I think they're heaping piles of trash).  That path is a dynamic path that the FMC is constantly re-evaluating to make it more economical.  That data doesn't show up on controller consoles, so the controller doesn't know what's best for your flight.  This is the precise reason that most control facilities now simply say things like "cross FALKO at 250 knots and 10,000 feet."  This leaves the top of descent up to the pilot, which is usually the most economic choice.

 

Again, this idea that "you're no longer required to stay on your path," is rather misguided.  Your goal is to stay higher, longer, for better fuel economy.  Just because ATC clears you to descend earlier doesn't mean you're going to want to drop like a brick as soon as possible.  First, they shouldn't be issuing "descent and maintain" from the cruise levels like that unless it's absolutely necessary.  Second, even if they do, you'll want to initiate a shallow descent until you can use an idle descent to save fuel.  Airplanes are most efficient up at the cruise levels airlines use (that's the reason they cruise up there :wink: ).

 

 

 

I've set it up to ARM/HOLD MODE ONLY in the FMC !

 

Good!  That's a realistic setting.  Now - again - where are your hardware throttles during all of this?  If you do not reduce them to IDLE before your descent, you're going to get a shallow descent.  If you don't want to have to mind your throttles at all while A/T is on, change that setting to NEVER.

 

Again, your hardware position can affect your descent path and cause shallow descents in either VNAV or FL CH.

On descends from cruise i never reach the new alt before ATC gives me a new FL so i my throttles are always at idle.

 

Well i find it impossible to stay in vnav in a shallow descend and reach my FAF at the correct alt so i have to into level change! Maybe in the real world but PFE its impossible unless someone can prove me wrong?

Hi pshlute well PFE isn't making any alt changes that corresponds to waypoint alt restrictions because you fly your own star! Thats where i grab my profile is when i reach my star, but like my problem i'm having which you know about ATC doesn't let me fly my star.

 

Thanks!

>Well i find it impossible to stay in vnav in a shallow descend and reach my FAF at the correct alt...

 

Hmmm...what does your vertical path profile show on the right side of the ND? Are you on your vert path or high or low?

 

If you're on your vert path then you should be able to meet your FAF altitude. Have a look again at my first and second ILS video flight demos...I set my descent altitude down to my FAF altitude and VNAV gets me there ok.

 

Now, IF you are high or low on your vert path profile, you need to correct to get back on profile, do you know how to correct to get back on profile?

Yeh got it set to show! Its always high as PFE descends me earlier so i change mode to level change because if i dont i'll arrive at my faf to high which has happened a few times now.

 

Kinda but not really....

So, I have more questions:

 

(1) What happens if you fly this without using PFE? (2) Does the vertical descent problem go away? If so, then PFE is causing the problem.

 

(3) I don't understand how PFE can cause you to descend early?

 

(4) Is it telling you to descend before your T/D point?

 

I'm trying to figure out, without seeing a video of your flight, if PFE is messing with your aircraft (which of course it should not) or if you are just following their instructions and by doing that on the MCP, you're causing yourself to get off your vertical profile?

 

You should not have to exit VNAV on your descent.

 

Without knowing how PFE works, it sounds like it has no clue as to what you're route is and that it is giving you ATC instructions that have no relevance to your route of flight...(5) would that be a correct or incorrect statement?

 

I think without a video to see exactly what is happening, its going to be difficult to spot exactly what is causing the descent problem.

 

Screen Cast-O-Matic is pretty easy to make a video with and its free. It will even upload directly to YouTube.

Vernon

 

There was a post on the NGX Forum right after the plane was released. The poster was having problems descending without over speeds. One of the replies was to press the F1 key once below 30000 feet. This insured that the plane was at flight idle when the FMA indicated ARM. Due to a calibration problem with the physical throttles it looked like they were at idle when in fact they weren't. Try this only in the ARM mode not the FMC SPEED mode. This well reduce N1 by several percent points and bring the plane's throttles back to the stop.

Hi ralph! If i don't use PFE i can keep on my profile if i do everything right and take action to any FMC messages but most of the time its all good.

 

About 40nm roughly from my TOD

 

If i descend early and follow PFE instructions and switch to lvl ch i find myself arriving at good altitude!

 

But if i stay in vnav and descend early at 1000 fpm i come in way too high as i'm not descending steep enough to follow PFE altitude there assigning me.

 

PFE does a good job once you set it up but only problem now is it can't read your FMC profile and doesn't know your path as it can't read it. All works well but not in vnav. Only time vnav does work when you fly your star and catch your profile again.

Ok cubine i will retard the throttles to idle once in arm mode thanks!