High VS speeds on Vnav

[MED1473 18]

Hi all!

I've recently experienced some strange behaviour from my NGX, a 700 model ay this time.

Flying from VIDP to VPQR in Vnav and Lnav at FL370. At about 10nm prior to the TOD I select the DESC NOW option on the FMS and the plane starts its 1000fpm shallow descent prior to intercepting the Vnav path. Till here all good and behaviour is normal.

Upon interception of the descent path tho the plane suddenly rises the v speed to something like -3500 fpm for a couple of seconds, presumably as a burst to intercept the path.

Is there a way to avoid/solve this? it seems quite unrealistic and besides my VA doesn't accept reports with v speeded higher than -3000fpm.

I'd like to point out that this was NOT the result of am uncontrolled situation (e.g. weather related) but something connected to the plane logic.

Does anybody know how to avoid this? (eg starting the DESC earlier/later, modifying the parameters such as oat index or other options). I really hope there's another way than overriding vnav and descend in ALT HOLD+VS.

Cheers I'm advance, Matteo Capocefalo, MED1473

[sandpatch 9]

I do not remember the VS of my flights but I don't have the feeling of a "burst". My NGX is careful and is able to intercept the path perfectly. Have you entered the information in the "forecast" page? My only guess is that the wind from behind was not entered so when the NGX tries to get to the path it overshoots it, and the tries to compensate by high VS and having a speed increase.

 

HTH

Manfred

[MED1473 18]

Hi!

Unlikely, I was already on a 1000 fpm descent on about 030 hdg and wind was at 120 at 20 so not really ae factor. Just wondering wether initiating the descent prior to the Tod is relevant I'm this case? It felt more like the plane was pitching down rapidly to quickly "sit" on the DESC path, almost as on the point of intercept it tended to overcompensate.

Puzzled as well as I was obviously below the path so I was expecting that to smooth the situation.

Matteo

[richardnobes14 13]

This is quite normal behaviour, depending on the cost index entered descent rates can vary greatly, this also depends on the path outlined in the FMC, in the descent the PATH is the number one priority over speed so if you have above and below way-point restrictions you'll find that your descent might be quite steep. This is fine and normal as long as you have the space to slow down, as you cant really go down and slow down at the same time .

[rsvit 16]

NGX usually begins intercepting VNAV path at 200' below the path when early descend was initiated. So it looks very smooth. Then V/S depends on many factors.

Similar topic.

[aitrafficgroup 0]

It might depend on the speed you specified in the DES page. Yes it may throw you into a dive but the golden rule is if you dont like what it's doing, either disconnect the autopilot or use another mode. In this situation I'd recommend Level Change on your current speed and you wont get anywhere near over 3000fpm unless you drastically increase the speed in level change mode.

 

Also, as mentioned above remember in VNAV PTH the plane will pitch for the path, and in VNAV SPD it will pitch for speed (same as level change, but protected for altitude restrictions)

 

Henry Lidster

[scandinavian13 5,192]

it seems quite unrealistic and besides my VA doesn't accept reports with v speeded higher than -3000fpm.

 

You might want to have a word with your VA. Unfortunately, VAs are often run by people who aren't real pilots. Because of this, they have no idea how the body feels with certain indications on VS. Most VAs reject high VS reports because they assume this feels like you're plummeting, as a passenger. This is not true. Your body senses acceleration. Speed is not acceleration (and we're talking about Vertical Speed). Sure, you accelerate to get there, and if you went from 0 to -3500 in one second, your passengers would be concerned, but if you did it slowly they'd hardly notice.

 

Don't assume what a simmer tells you is true. Some are very knowledgeable, others haven't a clue.

 

In this case, they haven't a clue what they're talking about.

 

 

 

 

To answer your question, though, be careful with your CI. If it's a high CI, you could run into problems of high VS descents.

[sandpatch 9]

Is it a special type of recorder that only allows 3000 ft/min? Sound really annoying...

[MED1473 18]

Hi all again!

Yeah it's an online (ish) logging system and it hits the fps worst than running Skyrim alongside with fsx so far. They're an awesome VA, problem is sometimes too strict. In this case the mistake is theirs as the don't want you to descend too fast on approach and therefore they just put a hard no -3000fps limit when it might be perfectly acceptable to have suh values in vnav at TOD. Anyway the practical problem stands, does anybody have any idea on how to make sure not to hit that limit when on vnav at tod?

Cheers, Matteo

[sandpatch 9]

I am sorry to tell you there is no way to specifically limit the descent VS via the FMC. Although there are ways past this. The high VS usually occur above (guessing) FL250. What you could do is descend a bit before T/D with level change in VS mode to that altitude and continue on VNAV after that.

I am not sure if this helps but with a lower speed in the descend page should give you a lower VS but I could be wrong. As previously mentioned the CI is quite important.

A third version I can think of is engaging the Anti-Ice at that altitude, it increases the idle N1. But you need to enter that information in the Forecast page or it will heavily overshoot the descent path.

My guess is that you knew most if these solutions but I thought I could post them any way.

 

If all strings break join http://www.scandinavian-va.eu/ which has an awesome fully custom made flight recorder that constantly gets upgraded with new features. :P

 

HTH

Manfred

 

EDIT: Link to screenshots of the software http://www.scandinavian-va.eu/pages/resources/svattng.aspx

[Spin737 102]

Descend early and/or use a lower CI. Even RW, the plane can get steep and fast above FL350.

[Paul_Smith 92]

They're an awesome VA

Doesn't sound like it, but I am sure they have redeeming features.

 

Anyway, stick to a CI below 40 (25~35) and you should be Ok. You do know you can use different CI's for different phases of your flight? Want to get there in a hurry, knock it up to 50, want to come down slow and smooth, drop it back to 20 (but make your change well before TOD as it will pull the TOD much earlier).

[MED1473 18]

Ha thanks Paul no, I actually didn't know about that, will try it next flight! Thanks for the tip. On the flight where I experienced this high vs it was set to 25 (or 28)0for the whole time so I'm a bit puzzled.

Ps I've got a mind o try descend directly on the vnav path rather than using the DESC now option. At -1000fpm its

Matonly a difference of 2000 fpm to drop down to -3000 for the interception while at. 0 fpm it makes a -3000fpm difference and hopefully the logic is limited to a degree of DESC rate or g force. Long shot but with testing

[MED1473 18]

Sorry was replying from my mobile phone, horrible spelling lol! What I meant is that is worth a shot trying to descend without the desc now option hoping that some logic in the fmc prevents it to send you from 0 to -3000fpm!

Mat

[G-CIVA 62]

Just re-itterate what was said by someone else earlier up the thread ....

 

During the cruise & well before the T/D go to the DES Page LSK 6L......

 

<FORECAST

 

Give that lowly little processor (by modern CPU standards) inside the Eqpt Bay as many clues about what lies ahead (& hence below) as you can.

 

It helps ... lots ... thats why the function is there ... Mr Boeing thought of pretty much everything already.

 

Secondly .... fly the airplane ... if it deviates significantly from what has been programmed, annunciated or expected you have the ultimate final vote on what happens next.