Go Back  PPRuNe Forums > Misc. Forums > Spectators Balcony (Spotters Corner)
Reload this Page >

Question about VNAV on Boeings

Spectators Balcony (Spotters Corner) If you're not a professional pilot but want to discuss issues about the job, this is the best place to loiter. You won't be moved on by 'security' and there'll be plenty of experts to answer any questions.

Question about VNAV on Boeings

Old 10th Nov 2019, 15:09
  #1 (permalink)  
Thread Starter
 
Join Date: Jan 2006
Location: LPFL
Age: 56
Posts: 74
Question about VNAV on Boeings

Hopefully there will be a kind pro out there willing to take time to answer this question.

As I understand it, in the crudest and most general terms, VNAV plans a descent from cruise by drawing a line up from the runway to a point (T/D) on the active route where, taking account of forecast winds and other known constraints, if you close the throttles, the aircraft ought to be able to glide down in idle thrust all the way to the runway (or to 5-10 miles out on the glideslope, anyway). I understand there are Speed and Alt Intervention modes to take account of temporary unforeseen ATC speed or alt constraints. Roughly right so far?

if so, my question is, how does the computer (FMC?) which calculates the position of T/D on this basis know in advance how many track miles Approach is going to vector you round? Does it, for example, use some sort of assumption about this and, if the track miles turn out to be materially different, the crew just has to abandon VNAV and use some other more "traditional" A/P modes?

Thanks in advance.
Midland63 is offline  
Old 10th Nov 2019, 15:24
  #2 (permalink)  
 
Join Date: May 2000
Location: Seattle
Posts: 3,123
The FMC doesn't know anything about vectors. If there is a discontinuity between the route, arrival, and/or approach, it assumes a straight line between the fixes that are the ends of the discontinuity.
Intruder is offline  
Old 10th Nov 2019, 17:07
  #3 (permalink)  
 
Join Date: Feb 2001
Location: The Winchester
Posts: 5,391
Originally Posted by Midland63 View Post
if the track miles turn out to be materially different, the crew just has to abandon VNAV and use some other more "traditional" A/P
Correct...
wiggy is online now  
Old 10th Nov 2019, 18:43
  #4 (permalink)  
Thread Starter
 
Join Date: Jan 2006
Location: LPFL
Age: 56
Posts: 74
Originally Posted by wiggy View Post
Correct...
Thanks for a commendably succinct reply wiggy.

So what assumptions as regards the portion of the flight over which it will be vectored does the FMC assume when calculating T/D? For example, do published approaches to airports say something like "anticipate vectors for X-Y track miles from waypoint ABCDE ..." or something like that?
Midland63 is offline  
Old 10th Nov 2019, 19:02
  #5 (permalink)  
 
Join Date: Dec 2005
Location: UK
Age: 41
Posts: 131
Originally Posted by wiggy View Post
Correct...
Is it? If you're on vectors and extend the centerline, you'll still get calculated track miles and a vnav profile.
rudestuff is offline  
Old 10th Nov 2019, 19:24
  #6 (permalink)  
 
Join Date: Feb 2001
Location: The Winchester
Posts: 5,391
Originally Posted by rudestuff View Post
Is it? If you're on vectors and extend the centerline, you'll still get calculated track miles and a vnav profile.
You'll get a profile, sure, but is it worth having? I don't doubt it will well work very well at some airports but I've certainly seen enough of a miss match between ATC track miles vs. calculated track miles when on vectors going into the likes of LHR to prefer to "use some other more "traditional" A/P modes" and the three times table.....



wiggy is online now  
Old 10th Nov 2019, 20:17
  #7 (permalink)  
 
Join Date: Aug 1999
Location: Too Far North
Posts: 994
The FMC plans the T/D based on the route entered by the flight crew. Most crews, if operating into an airport that they know, will enter a route that is close to what they expect ATC to do rather than what the flight plan says while retaining the option (using the FIX page etc) to modify the route back to the flight plan as required.
Flap40 is offline  
Old 10th Nov 2019, 21:52
  #8 (permalink)  
 
Join Date: Jan 2006
Location: US
Posts: 2,070
Doesn’t the FMC add a fixed distance if the final fix has a heading after it and the next fix is behind you? Like a downwind from the east for landing to the east? Isnt it 30 miles??? So the FMC inputs 60(??) miles from the radar fix to the next fix on the approach?

That’s why understanding the FMC DTG (distance to go), planned/expected track miles, and straight line distance is good to know/estimate. Figure that out, add a dash of old school 3:1 math, add put the aircraft if the appropriate energy state (altitude/airspeed).

When in doubt remember ‘Tiger’ (*) errors will always allow you to get a bomb to a target but being a wimp just puts yourself at risk.

“We might be high but at least we’re fast.” :-/

* - tiger errors - fast, steep, high.
slow, shallow, low can put you in the frag envelope.
misd-agin is offline  
Old 10th Nov 2019, 22:30
  #9 (permalink)  
 
Join Date: Apr 2017
Location: Event Horizon
Posts: 58
737 specific (no idea how other Boeings operate).
The T/D point and rest of the entire vertical profile is only accurate provding you are flying the entire approach following the magenta line, be that manually, LNAV, HDG SEL, VNAV or V/S. As you have said, any height or speed restrictions entered into the FMC should update the profile and move the T/D point accordingly. If you get vectors away from the predicted path laterally or vertically, you can either use a different mode (LVL CHG or V/S and use the 3 x table), or update the FMC with a new customised waypoint (radial and distance from an existing waypoint) to update the profile for your new estimated track.
Using LVL CHG or V/S will open the MCP window and allow higher/lower speeds to be flown than those in the decent page. This will only give you an accurate vertical profile if you also seperatly update the decent page with the new updated speed. This will also take into account any further restrictions e.g 240kts/FL100 and move the deceleration points accordingly to ensure you meet your future restrictions in the LEGS page. Flying a largely different speed to that in the desent page will give you a false vertical profile.
As long as you can fly a track that approximates the magenta line, you shouldn't have too many problems.
Note: Using SPD INTV will open the MCP window, but keep VNAV engaged. You will still be required to update the FMC and the FMA will now read MCP SPD not FMC SPD.
TryingToAvoidCBs is offline  
Old 11th Nov 2019, 08:00
  #10 (permalink)  
 
Join Date: Aug 2012
Location: N . Daarset
Age: 66
Posts: 167
Wiggy , like it . DME and a the 3 X table works like magic . Stil ask kids on trial lessons if they know the 3 x table ... Most just give 'wot one mother [ a Teacher ] called a list ... 3, 6, 9, 12,.... One 7 yr old spouted out 1 x 3 = 3 , 2 x 3 = 6 etc. very quickly and easily ... Upped the bar ...
'Wot's 17 x 3 ? .mutter , mutter ..''51'' .
'' That's Brilliant lad , you can fly Jets '' !
If I had a pound for every time time we'd turned final at LHR at 17 nm out .. = 5100 '

rgds condor .
condor17 is offline  
Old 11th Nov 2019, 08:49
  #11 (permalink)  
 
Join Date: Feb 2001
Location: The Winchester
Posts: 5,391
Originally Posted by Midland63 View Post
So what assumptions as regards the portion of the flight over which it will be vectored does the FMC assume when calculating T/D? For example, do published approaches to airports say something like "anticipate vectors for X-Y track miles from waypoint ABCDE ..." or something like that?
Midland..they might do, they might also publish a full procedure to cater for a radio failure but as an example of the real world at places like Heathrow shortly after you've left one of the holds on a vector ATC will tell you your landing runway and tell you your track miles to run....that varies from one approach to another at LHR depending on how they are fitting you in with preceding or following traffic..

Some days you'll get a "snatch" other days you flog downwind for miles. LHR are also very keen on you flying a continuous descent pretty much from leaving the hold to touchdown for noise abatement reasons.....most folks I fly with stop using VNAV at or before entering the hold and switch to something like Vertical speed or Flight Level Change.

At other airports VNAV might well be appropriate until closer in.
wiggy is online now  
Old 11th Nov 2019, 15:18
  #12 (permalink)  
Thread Starter
 
Join Date: Jan 2006
Location: LPFL
Age: 56
Posts: 74
Thanks again wiggy.

So would I be roughly right in thinking that VNAV's fine for the first bit of your descent and approach down from cruise until you contact approach and they say "Vectors to runway XY, Z track miles to run" whereupon 9 times out of 10, you'd be as well switching VNAV off and applying the 3 x table and requesting more track miles if necessary? If that's approximately correct, then it's the answer to my question.
Midland63 is offline  

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


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.