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Vnav

Started by Toga, Wed, 6 Mar 2019 09:50

Toga

Is it just me or is vnav slightly confusing, particularly on approach? It's quite uncertain and can catch you out fairly easily. Begs the question as to why it wasn't designed to sync the fms speed values with the speed window if opened and changed. Nothing worse than closing the window after an atc speed restriction only to find the speed runs off back up to 250 knots! I'm also not certain what's going on with vnav speed in the descent either. Think I'll stick to FL CH!

On further practise, is Vnav speed a little like open climb/descent on the airbus?

localiser

#1
This is akin to forgetting to activate the approach phase in Airbus aircraft. You'd be on vectors and when cleared for approach push for managed speed and whooops! 250 knots is bugged with the engines going for it...

The 744 is different in that although the FMC phases to on approach logic, the speed is still controlled by the pilot. On a VNAV approach you're meant to select speed intervention prior to the FAF or once you have gear down flaps 20. It's not really SOP more of a technique, but in addition to the speed constraints in the LEGS page we update the speed in the VNAV DES page to match what we are currently flying - whether that's a LEGS constraint or a selected speed (for want of a non-Airbus phrase  :D ). This way, if at some stage we close the speed window the speed bug won't go for 250 or 240 knots as you described but stay within the area of what we were already flying.

What are you experiencing with VNAV speed in descent? This should be ECON following cost index or what you have selected as your descent Mach/speed on transition in VNAV DES.


(edit to add video link)
https://www.youtube.com/watch?v=ewJklYJujeg

Jon aka emerydc8 explains it rather nicely in this video in the tutorials thread

Hardy Heinlin

If you want to keep the FMC command speed as low as the MCP SPD, you should set the respective SEL SPD before closing the MCP SPD window -- at least in the legacy FMC.

In the NG FMC this is no longer necessary.

localiser

QuoteOn further practise, is Vnav speed a little like open climb/descent on the airbus?


Mmnnnnnngggggg yes and no. VNAV respects the speed and altitude constraints that are programmed in the FMC.

In a climb VNAV SPD will select a thrust and will pitch for selected, or ECON climb speed. In this way you could compare the notion to FLCH, LVL CHG or OPEN CLB.

In addition to this VNAV will level off, climb, and maintain correct speeds, as on a SID or a STAR. If you use FLCH, you will climb direct at the selected MCP speed to the selected MCP altitude leaving a trail of destruction!

VNAV PTH only exists for descent and prioritises the ECON or modified path. If you're from an Airbus background you could think of it as managed descent, but it's really not good to translate Boeing to Airbus as although they are different methods of achieving very similar goals they are quite intricate and can differ in very nuanced ways.

Toga

#4
Quotethey are quite intricate and can differ in very nuanced ways.

They most certainly are! After having a play around with it I think I've gotten the hang of it... sometimes i find myself engaging Vnav and thinking I wonder what will happen next.
Regarding go-arounds, which is the best technique for acceleration and leveling off? I find with a reduced thrust g/a after I select Vnav the thrust sky rockets once more and it's quite a task to get the flaps in on schedule.

-- Thanks the video was very helpful and also helped with speed control on the G/A.

With a reduced G/A thrust at lighter weights to prevent the thrust going to full G/A thrust on selection of Vnav, is it good practise to press THR at AA to continue at climb thrust? Not wanting to compare apples to oranges but on the bus we would move the throttles from TOGA to CLB at this point and accelerate at CLB thrust.

Cheers!

localiser

The speed issue on your go around could be down to the weight of the aircraft, speed constraints or lack of them or a low go around altitude. One press of the toga button will give you a reduced thrust with a climb rate of 2000 fpm while a second press will give you full GA thrust.

Generally we stay in VNAV. So like in Jon's video at 1000' aal press VNAV and that will look after commanded speed, pitch, thrust and constraints.

There is a second method, used less often but there could be a good reason for it. Again at 1000' aal but this time press FLCH, and then select the maneuvering speed for the planned flap setting. Once at the flap setting and speed you require, 180kts flap 10 perhaps for downwind and another approach, or clean and 250kts for diversion, press the THR button. This changes the reference thrust limit to CLB to match the flight phase.


Quotepress THR at AA to continue at climb thrust

I've not seen that done while in VNAV, VNAV really should cope with it. For take off though it is possible to select CLB, CLB1 or CLB2 thrust on the CDU thrust limit page preflight.

Toga

Thanks for your help.

I don't know if its a PSX issue or it's something I'm doing incorrectly but when I select VNAV at AA with reduced G/A thrust, the thrust then increases to max as it would with two presses of TOGA. The only way i'm able to climb out with CLB thrust and not full G/A thrust on selection of a vertical mode is to press THR first to reduce G/A to CLB on the upper eicas. -- or go through the FMC as you say and select a CLB mode.

Hardy Heinlin

It's not a bug. In climb, VNAV SPD is always coupled with THR REF. And THR REF always goes to the current ref limit (with VNAV SPD).

There is no "reduced" G/A thrust ref limit. Only CLB and T/O ref limits can be reduced.

Toga

Ok cheers, Hardy. I didn't think it was a bug, most definitely a problem my end!

Hardy Heinlin

It's not a problem at your end either. It's a normal feature :-)

Even if you keep the TO/GA pitch mode with THR for 2000 fpm -- at MCP ALT capture it will go to THR REF | | ALT and accelerate. But in that case (level flight) THR REF will reduce the thrust before the airspeed reaches the red bricks.

Toga

Thanks for the help!

Toga

Regarding Vnav in the descent, im finding the pitch changes abruptly when switching between vnav path and vnav speed. Is this just a nuance of the system?

Hardy Heinlin

No, it's smooth.

What's your definition of "abrupt" and what exactly are your situational parameters? And what PSX version?

Toga

I'll have a play around with it this evening. Latest update. I think it could be an issue with an addon i'm running - (xview, psx.net or bacars) as i'm having  trouble with wind updates and random freezes of the sim. I don't have such issues running psx natively, i don't think.

Toga

#14
After testing this eve it seems to an addon thats the issue. During a wind uplink request the fmc threw out all the route info and kept saying resyncing fmc or something. Then When I went DCT the FMC put me in descent mode with no path. Once i restarted psx without the addons these problems stopped. Vnav descent was perfect.

Hardy Heinlin