Question about VNAV on Boeings

[Midland63]

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.

[Intruder]

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.

[wiggy]

Originally Posted by Midland63

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...

[Midland63]

Originally Posted by wiggy

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?

[rudestuff]

Originally Posted by wiggy

Correct...

Is it? If you're on vectors and extend the centerline, you'll still get calculated track miles and a vnav profile.

[wiggy]

Originally Posted by rudestuff

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.....

[Flap40]

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.

[misd-agin]

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.

[TryingToAvoidCBs]

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.

[condor17]

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 .

[wiggy]

Originally Posted by Midland63

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.

[Midland63]

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.

[Spooky 2]

The simplest answer without getting in the weeds is the FMC is a planning tool and VNAV is simply one of those functions the pilots have to work with. There are several ways to make it work for the crew and since ATC is seldom delivered in a prefect world, the pilot has to be prepared to intervene in the FMS logic. VNAV has greatly improved over the years and whether it be a Honeywell or Smiths application, the newest "boxes" do a pretty good job. The pilot needs to be prepared to intervene when they see the airplane going high on the descent, i.e. UNABLE crossing restrictions and they can do that with Vertical Speed or Flight Level Change, both of which have added limitations and operator hazards awaiting the user. Boeing aircarft have a descent ARC which give a excellet graphic didplay on the ND that shows how your descent profle is working. Airbus has something simailar but I don't think it's as intuitive. It's proably a matter of what youre use to so no big deal. I would dare say that less than 10% of the descent profiles in todays world work without some sort of pilot intervention. Take all this with a grain of salt as its been a few years since I have actually done one of these in a real airplane.

[rudestuff]

Agreed there's no substitute for the 3x table, we should still do it when using VNAV and we HAVE to do it with FLCH. The standby glide slope is also a great tool: "It's showing a little high but that's ok because we're still on base" etc..

VNAV is great when we're flying a procedure: keep it on the path and it'll take you there: leave the LNAV path under vectors and it's suddenly useless. Or is it? We see it it all the time: "Fly heading xxx" - BAM! - FLCH, Speed brake out, extend the centerline. Stick with what you know.

Few people know or understand what the Vnav is doing, and fewer care to find out during the most critical phase of flight. If you've extended the centerline, Vnav is drawing a line upwards from a known point in space and a known speed/altitude constraint. The problem for us is that we're not flying directly to it, we're flying at an angle to intercept the LNAV/LOC, which makes our track miles unreliable. Here's the thing though: you ARE STILL going to that waypoint, which means that Vnav path WILL STILL funnel you to that waypoint at that altitude.

So if you get turned in early you can open the window, pop the brakes and follow the diamond down whilst doing your 3 times table as well. Opening the speed window and coming off the VNAV path into VNAV SPEED has exactly the same effect on the flight modes as FLCH. There is no difference between THR HOLD/SPD and THR HOLD/VNAV SPD: both hold the speed shown in the window and both control rate of descent with throttles and speedbrakes. It'll descend just as fast as FLCH, and once you get to flap 1 it'll even recapture the path and fly a nice neat CDA for you...