VNAV PTH and VNAV SPD

[Banana Joe]

Going through my first type rating and I am a bit confused on the difference between VNAV PTH and VNAV SPD. Am I correct in saying that:

in VNAV PTH the FMC will command pitch to mantain pitch and if required it will command the autothrottle to mantain FMC SPEED
in VNAV SPD the FMC will retard the throttle levers to IDLE and mantain speed with pitch and will not attempt to stay on path similar to LVL CH

I do understand that the ideal descent path, provided forecast winds are correct, will result in a idle descent.

[underfire]

https://www.pprune.org/questions/380827-vnav-spd-pth-descent.html

[Iznogood]

You are correct! May I just add that VNAV SPD descent is available only if your aircraft is equiped with SPD INT on the MCP, but the functionality is exactly like LVL CHG.

[ad-astra]

Not in the 737's that I have been flying for 15 years.

[VJW]

What you say is pretty much correct Joe - when in VNAV Path the pitch mode on the FMA is maintaining a desired Path not Pitch. Iznogood isn't technically correct regarding how VNav Speed exactly like LVL CHG.

The difference between LVL CHG and VNAV SPD is that imagine you were descending from A to C via B, and B had an AT or ABOVE altitude set in the FMC. VNAV SPD would ensure you crossed at or above, and would level you off to ensure that restriction at B was met if necessary, regardless of what altitude was set in the MCP window. (And unless I'm mistaken, it'd actually flick over to VNAV path if it needed to level off to comply with the restriction at B, even with speed window open. It'd flick back to VNav SPD once the restriction at B had been complied with).

LVL CHG wouldn't. It would simply be an idle descent from A to C at the Speed set in the MCP window, ignoring any restrictions set at B.

[Banana Joe]

Now I have the differences clear, thanks to all for your replies.

[Iznogood]

Yes VJW is right about this detail. Off course, VNAV SPD will maintain its VNAV features and will fly exactly all the altitudes restrictions that you have set in the FMC (that's why it is VNAV).

What I meant is in case there is no altitude restriction (most of the time in my part of the world), both of them will fly you with idle power until you reach the MCP altitude...but it goes without saying abouth the VNAV features.

[VJW]

Someone wanting to know what the functions actually do because it’s their first type rating - probably wanted to know exactly that and not what happens in ‘your part of the world.’ ;)

In Europe (a sizeable chunk of the planet) at or above/below or hard altitudes are extremely common fwiw.

[Iznogood]

You're absolutely right!

[bravocharliedelta]

Went to check the Boeing CBT,

On the FMA [Thrust Mode, Roll Mode, Pitch Mode]; whichever has SPD, means airspeed is being controlled by that specific mode.

E.g. [THR REF, LNAV, VNAV SPD]; Pitch controls airspeed

E.g. [SPD, LNAV, VNAV PATH]; Thrust constrols airspeed

According to the CBT, you only get SPD in the Thrust Mode, when you are leveling off or in level on the MCP/FMC altitude.

To the question of (assuming we are only looking at descents),

in VNAV PTH the FMC will command pitch to mantain pitch and if required it will command the autothrottle to mantain FMC SPEED

VNAV PTH would most likely give you a Thrust Mode of IDLE/HOLD, as the FMC computed descent profile is based on IDLE thrust.

in VNAV SPD the FMC will retard the throttle levers to IDLE and mantain speed with pitch and will not attempt to stay on path similar to LVL CH

VNAV SPD may go into THR then IDLE/HOLD. VNAV SPD will not maintain the original calculated descent profile which starts from the calculated ToD. It may parallel the profile.
To get VNAV PATH back, you will have to try to increase/decrease your RoD to fall back into the calculated profile, depending whether you are high or low (VNAV profile indicator may assist you).

[Banana Joe]

I also fly in Europe and thanks bravocharliedelta for your reply, I've got the differences distinguinshed even more now.

[aviamark]

Just one thing to add, be aware of what the approach logic is and VNAV SPD automatically changing to VNAV PATH, has caught me out the one or other time when i started off back then... hehe ;) Can be quite an unpleasant surprise and you have to quickly change to another mode. Easiest thing to avoid this is just using LVL Change when you are a bit higher than usual...

[wiggy]

At the danger of making heads spin yet another way of looking at it:

As has been pointed out PFD FMA “101” is that the right hand FMA simply tells you what the elevators are trying to control and how.

VNAV PTH means the elevators are trying to fly the VNAV programmed path (a really important need to monitor/need to know when you get to using VNAV for approaches).

VNAV SPD means the elevators are controlling speed. TBH most commonly seen when the desired vertical path has (perhaps only temporarily) gone out of the window... sometimes seen if you have to reprogram the FMC during a descent, e.g. for a direct, or change the VNAV descent speed since it takes time for VNAV to recalculate the path, also often seen if you’ve gone high on the profile - appropriate use of speedbrake (horror of horrors to some) should get you back on path (at which point you should get automatic transition back to VNAV PTH again). It’s not unusual to see the Pitch FMA shuttle between PTH and SPD in a descent....which leads into the final point...

Above all and regardless of the fancy pitch modes mentioned above the big health warning in all this is to make sure somebody is monitoring the FMAs and really importantly the speed ..

[Pontius]

Some jolly good advice (especially Wiggy's FMA monitoring). Going back to one of the original questions:

in VNAV PTH the FMC will command pitch to mantain pitch and if required it will command the autothrottle to mantain FMC SPEED

I'd have to dig into my old 737/747 books to make sure I'm not giving duff info but I'm 95% certain they're they same as a 767. At the top of descent, in VNAV PTH, the auto throttle mode will change from SPD to IDLE and then THR HOLD. So, in answer to the question, no, the auto throttle will not maintain the FMC speed. The aircraft will pitch to maintain the path but if you get an unexpected headwind and the aircraft pitches up to maintain the path then the speed will reduce as the thrust levers are in HOLD (at idle, unless previously moved). When you reach a restriction it will revert to SPD/VNAV PTH and will kick the auto throttle back into life to maintain the speed.

Treat this, however, with caution as things have moved on from the 767 and the likes of the 737NG may well be more intelligent in working out what needs to be done i.e. wake the auto throttle up if a headwind is encountered in VNAV PTH. That's why those FMAs are so important.

[Iznogood]

Pontius, on the 737 NG, in normal cruise you will have FMC SPD.....VNAV PTH
Then at top of descent, after setting a lower MCP altitude it will switch to RETARD....VNAV PTH then ARM (white)....VNAV PTH when thrust levers reach the fully closed.
In this mode the path only is maintained regardless of the speed and if the speed exceeds more than 10 kt the target FMC you will have a "DRAG REQUIRED" message. If the speed drops by 15kt or less the Autothrottle will be "back into life" as you said and you will have on your FMA : FMC SPD ....VNAV PTH till the aircraft reaches the target FMC then RETARD & ARM again.
That is for VNAV PTH
If your aircraft is equipped with SPEED INTERVENTION (Optional) and you push the button SPD INT under VNAV PTH your FMA pitch mode will change into VNAV SPD, and the airplane will descent on IDLE power at the current or selected speed (speed window will be opened on the MCP) Here similar to LVL CHG until you reach the altitude selected on the MCP or the altitude programmed in the VNAV (in case you have an altitude restriction) whichever is higher. If you have altitude restriction in the FMC which is higher than the one set in the MCP the FMA pitch mode witch automatically to VNAV PTH again. If you want to continue descent you can delete the altitude restriction from the FMC legs page or just press the button ALT INTERVENTION (Optional also)
I guess the 767 is slighlty different but the basics are the same?

[Pontius]

I must apologise to my maligned steed; she does indeed wake the auto-throttle up if it gets too slow in VNAV PTH. Depending on whether the first speed constraint has been passed it will occur at either -15kts or -10kts but it does not remain indefinitely in THR HOLD and will engage SPD or tell me that thrust is required if the A/T is not engaged.

I've never actually seen this, only "drag required" but that was no excuse for me being lazy and not looking in the book before posting.

I will take myself out behind the bike sheds and give myself a thorough talking to.

[shaftsburn]

You are correct! May I just add that VNAV SPD descent is available only if your aircraft is equiped with SPD INT on the MCP, but the functionality is exactly like LVL CHG.

SPD INT is not always required.

There is the "SPEED" option on 6R of the Descent page, which takes you to the SPD DES page.
The required speed is entered via the FMC, because of the lack of SPD INT on the MCP.

[extricate]

What are some of the situations on descent, the FMA will read THR|VNAV SPD?

I only know of one situation as documented in FCTM, that is in an early descent. What else can you guys think of?

[bravocharliedelta]

Quite a few.

During a descent you may have for the pitch mode:
V/S
FPA
FLCH SPD
VNAV SPD
VNAV PATH

Thrust mode will probably be,
SPD
THR
IDLE/HOLD

[JammedStab]

What are some of the situations on descent, the FMA will read THR|VNAV SPD?

I only know of one situation as documented in FCTM, that is in an early descent. What else can you guys think of?

It was earlier stated that there will be IDLE with VNAV SPD but that is not always the case. When doing an enroute descent during cruise flight, you get THR/VNAV SPD and the thrust levers have a partial reduction in power.

Not sure if there is some sort of variation in the amount of thrust reduction based on the amount of descent required which there apparently is with FLCH. Anybody know?

[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]

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.

https://image.ibb.co/dZff7x/des_page.jpg

[Iznogood]

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.

https://image.ibb.co/dZff7x/des_page.jpg

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]

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]

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]

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]

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.