VNAV PTH and VNAV SPD

[Iznogood]

shaftsburn, yes as I said SPD INT and ALT INT are just options and many aircrafts are not equipped with. The DES NOW you are talking about, if pressed before TOD under VNAV PTH pitch mode, will start the descent at a rate of 1000ft/mn and at FMC SPD speed until aircraft catches the descent path, then it will follow the descent path at IDLE power in normal VNAV PTH. you will always be under VNAV PTH during that time.
I would say that the SPD INT is like a shortcut to change your speed without manipulating the fmc but still under VNAV.

exteicate, as far as I know, and with the 737 NG, there are only 2 situations where you will get VNAV SPD during descent:
1- By pressing SPD INT on the MCP (if equipped) while under VNAV PTH (speed window will be opened)
2- If for any reason you cannot start descent at TOD: FMA will change from VNAV PTH to VNAV ALT first, then after setting a lower MCP altitude, it will start descent in VNAV SPD (normal because aircraft cannot maintain path anymore) and the aircraft will start descent at idle power at the descent speed set in the FMC

[shaftsburn]

Originally Posted by Iznogood

The DES NOW you are talking about

Think you misunderstood what I was talking about. I wasn't referring to "DES NOW" on the CRZ page. That's something completely different.

I was referring to "SPEED", which is on the PATH DES pages. (On planes without common VNAV)

So in addition to your 2 methods in post #21, you can also enter VNAV SPD via this SPEED button.

My mistake previously, it's actually button 5R.

[Iznogood]

Originally Posted by shaftsburn

Think you misunderstood what I was talking about. I wasn't referring to "DES NOW" on the CRZ page. That's something completely different.

I was referring to "SPEED", which is on the PATH DES pages. (On planes without common VNAV)

So in addition to your 2 methods in post #21, you can also enter VNAV SPD via this SPEED button.

My mistake previously, it's actually button 5R.

Got it, quite interesting function...I don't have it on my a/c. I have only FORECAST L6 and RTA R6

[Enigma01]

Hello i do still have a question about the vnav speed/vnav path:

Lets say i am flying from waypoint A to waypoint B and waypoint B has a restriction 210/fl150B.

Due to circumstances i am well above (lets say FL300) my vnav path, and the aircraft is in vnav speed maintaining the fmc descent speed of eg. 260kt. So pitching the aircraft to maintain the speed.

Will my aircraft still be flying at my programmed speed of 260kt and ignoring the restriction of 210/FL150B overhead waypoint B, or will it be reducing speed to 210kt but being well above FL150? (Assuming i dont intervene).

Thank you.

[Banana Joe]

It will go in VNAV speed and try to recover the path as path has priority and eventually it would overspeed if you don't take action (speed intervention or LVL CH). At least this is what happened to me a few times. And I don't trust the built-in overspeed protection.

[Enigma01]

Mmmm i dont get it. I always thought it would just reduce speed to 210K by pitching up. How can it try to regain the path by ignoring my fmc vnav descend speed and pitching down (so increasing speed above 260K by piching more down) if it is in the vnav speed on the FMA???

Pegasus FMC.

Sorry still all unclear to me.

[Banana Joe]

If you don't open the speed window, the FMC will gradually increase the speed to attempt to recover the path. If you open the speed window with speed intervention or use LVL CH it will keep that speed regardless of path. VNAV SPD will always comply with altitude constraints. Again, this is what I have experienced. Sometimes what the FCOM says doesn't seem to be incorrect i.e. when I had to discontinue an ILS approach and I deselected the frequency, the outcome was not what was the FCOM says it would be.


In your scenario, I would just report unable to comply with either restriction and ask ATC what if they prefer me on speed or on profile. And open the speed window if I have to descend faster.

[rudestuff]

Originally Posted by Banana Joe

If you don't open the speed window, the FMC will gradually increase the speed to attempt to recover the path. If you open the speed window with speed intervention or use LVL CH it will keep that speed regardless of path. VNAV SPD will always comply with altitude constraints. Again, this is what I have experienced. Sometimes what the FCOM says doesn't seem to be incorrect i.e. when I had to discontinue an ILS approach and I deselected the frequency, the outcome was not what was the FCOM says it would be.


In your scenario, I would just report unable to comply with either restriction and ask ATC what if they prefer me on speed or on profile. And open the speed window if I have to descend faster.

What plane is that? The thing to remember is all VNAV systems are similar yet different. A VNAV path is projected backwards from the last constraint at an angle that assumes idle thrust at the FMC decent speed. The only way to ensure meeting a speed/alt constraint is to be on the path doing that speed (and even then with a tail wind you might need drag).

Put simply, in descant with VNAV engaged you're either on the path (PATH) or you're not (VNAV SPD) - ignoring VNAV ALT for the time being (!)

Descend early? Leave the path > VNAV SPD
Descend late? Same thing > VNAV SPD
Open the SPD window in descant? > VNAV SPD
Modify the path? > VNAV SPD

VNAV SPD gives speed on pitch, just like FLCH - either the MCP window speed or the FMC speed.

High on path? You can (a) pull speed brake and 'drop' onto the path, (b) you can open the speed window, increase the speed and 'dive' onto the path (just remember to close it again or you'll go straight through the path!) or (c) you can increase the FMC speed to give a later, steeper descent and effectively bring the path to you.

Keeping the window closed has the advantage of automatically recapturing the path.
VNAV speed will start an automatic deceleration to 240 kts by 10,000' and any other speed restriction you program (clean/6000 is a good one)

[Banana Joe]

A 737 with FMC U10.8A and in one year on the line I have risked overspeeding already a couple of times with VNAV SPD during descent.

As for VNAV ALT, that is not installed on our fleet.

[Skyjob]

Originally Posted by Banana Joe

It will go in VNAV speed and try to recover the path as path has priority and eventually it would overspeed if you don't take action (speed intervention or LVL CH). At least this is what happened to me a few times. And I don't trust the built-in overspeed protection.

I'm not too sure, as the 250?FL100 is equally observed by reducing speed even when above PATH...
Maybe logic for intermediate speed constraints is handled differently by VNAV, but it may also depend on FCC version

[Banana Joe]

I have experienced that above FL100 with VNAV trying to recapture path following stronger than forecasted tailwinds.

[rudestuff]

Originally Posted by Banana Joe

A 737 with FMC U10.8A and in one year on the line I have risked overspeeding already a couple of times with VNAV SPD during descent.

As for VNAV ALT, that is not installed on our fleet.

Do you have the cruise descent function? On ours if you descend more than 50NM early it'll reset the cruise alt and descent at cruise Mach instead of descent schedule. It can be a gotcha as the IAS creeps up with nothing to stop it.

[rudestuff]

Originally Posted by Enigma01

Mmmm i dont get it. I always thought it would just reduce speed to 210K by pitching up. How can it try to regain the path by ignoring my fmc vnav descend speed and pitching down (so increasing speed above 260K by piching more down) if it is in the vnav speed on the FMA???

Pegasus FMC.

Sorry still all unclear to me.

I can only speak for the 757, but in that aircraft in VNAV SPD the priority is speed. In PATH, when the auto throttle is unable to maintain speed within parameters, it reverts to speed. If you open the speed window and slow down, it prioritises that speed, the nose goes up and it departs the path in VNAV SPD. The only time path is prioritised is in descent logic with flap out: you get the option to descend in SPD: VNAV PATH.
​​​​​​
On the LEGS page, the FMC decent speed is shown against all legs in small characters, unless a hard speed has been inserted (in large characters) - so it's reasonable to assume that VNAV SPD would follow that speed, leaving you the problem of getting back down to the path with speed brake, gear or just saying you can't do it (which is reasonable if its their fault you're high).

Of course the 737 might be quite different.