Go Back  PPRuNe Forums > Flight Deck Forums > Tech Log
Reload this Page >

VNAV PTH and VNAV SPD

Tech Log The very best in practical technical discussion on the web

VNAV PTH and VNAV SPD

Old 18th Mar 2018, 00:51
  #1 (permalink)  
Thread Starter
 
Join Date: Jan 2018
Location: Amantido
Posts: 776
VNAV PTH and VNAV SPD

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.
Banana Joe is online now  
Old 18th Mar 2018, 03:02
  #2 (permalink)  
 
Join Date: Aug 2013
Location: PA
Age: 54
Posts: 36
VNAV SPD/PTH Descent?
underfire is offline  
Old 18th Mar 2018, 06:42
  #3 (permalink)  
 
Join Date: Sep 2013
Location: France
Posts: 106
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.
Iznogood is offline  
Old 18th Mar 2018, 09:04
  #4 (permalink)  
 
Join Date: Aug 2007
Location: Brisbane, Australia
Posts: 322
Not in the 737's that I have been flying for 15 years.
ad-astra is offline  
Old 18th Mar 2018, 09:27
  #5 (permalink)  
VJW
Sciolist (look it up) of the first order
 
Join Date: Oct 2010
Location: UK
Posts: 763
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.
VJW is offline  
Old 18th Mar 2018, 09:57
  #6 (permalink)  
Thread Starter
 
Join Date: Jan 2018
Location: Amantido
Posts: 776
Now I have the differences clear, thanks to all for your replies.
Banana Joe is online now  
Old 18th Mar 2018, 12:04
  #7 (permalink)  
 
Join Date: Sep 2013
Location: France
Posts: 106
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.
Iznogood is offline  
Old 18th Mar 2018, 13:15
  #8 (permalink)  
VJW
Sciolist (look it up) of the first order
 
Join Date: Oct 2010
Location: UK
Posts: 763
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.
VJW is offline  
Old 18th Mar 2018, 13:28
  #9 (permalink)  
 
Join Date: Sep 2013
Location: France
Posts: 106
You're absolutely right!
Iznogood is offline  
Old 18th Mar 2018, 14:00
  #10 (permalink)  
 
Join Date: Dec 2016
Location: Oceania
Posts: 7
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).
bravocharliedelta is offline  
Old 18th Mar 2018, 17:48
  #11 (permalink)  
Thread Starter
 
Join Date: Jan 2018
Location: Amantido
Posts: 776
I also fly in Europe and thanks bravocharliedelta for your reply, I've got the differences distinguinshed even more now.
Banana Joe is online now  
Old 19th Mar 2018, 07:26
  #12 (permalink)  
 
Join Date: Feb 2012
Location: Europe
Posts: 9
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...
aviamark is offline  
Old 19th Mar 2018, 07:45
  #13 (permalink)  
 
Join Date: Feb 2001
Location: The Winchester
Posts: 5,261
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 ..

Last edited by wiggy; 19th Mar 2018 at 07:56.
wiggy is offline  
Old 19th Mar 2018, 08:40
  #14 (permalink)  
 
Join Date: Jun 1996
Location: Check with Ops
Posts: 734
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.
Pontius is offline  
Old 19th Mar 2018, 09:52
  #15 (permalink)  
 
Join Date: Sep 2013
Location: France
Posts: 106
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?
Iznogood is offline  
Old 20th Mar 2018, 04:00
  #16 (permalink)  
 
Join Date: Jun 1996
Location: Check with Ops
Posts: 734
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.
Pontius is offline  
Old 20th Mar 2018, 09:32
  #17 (permalink)  
 
Join Date: Jul 2010
Location: EU
Posts: 42
Originally Posted by Iznogood View Post
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.
shaftsburn is offline  
Old 21st Mar 2018, 02:18
  #18 (permalink)  
 
Join Date: Apr 2008
Location: Test
Age: 30
Posts: 184
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?
extricate is offline  
Old 21st Mar 2018, 15:33
  #19 (permalink)  
 
Join Date: Dec 2016
Location: Oceania
Posts: 7
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
bravocharliedelta is offline  
Old 21st Mar 2018, 16:38
  #20 (permalink)  
 
Join Date: Apr 2008
Location: nowhere
Posts: 1
Originally Posted by extricate View Post
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?
JammedStab is offline  

Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service

Copyright © 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.