Boeing extended centreline

[bucket_and_spade]

Hi guys,

On our B757 fleet, pilots often select, as I'm sure lots of others do, direct to the centre fix on the legs page of the CDU and then type in the inbound course to the runway as an intercept. To those familiar with this Boeing FMC, you'll know that the result is a magenta line extending out from the threshold on the HSI. My question regards the profile deviation info the box now provides on PROGRESS PAGE 2 and the HSI.

The above procedure is often done on the equivalent to a base leg and the profile deviation (vertically) info given on PROG page 2 always seems very accurate. My specific question is - the vertical profile info given always seems pretty spot so what exactly is the box using as it's "track miles to run"? Does it look at current aircraft position (continuously) and then work out the track miles to the threshold if the aircraft is turned directly on a base leg at any given moment?

I can't find anything in the Boeing manual and a couple of training captains weren't sure either.

I use it (along with other indications and my three times table) to make CDAs as it seems to to invariably give good profile deviation info even if it's activated on headings which aren't very base-leg-ish.

Any thoughts appreciated.

B&S

[NW3]

I *think* the miles to go is calculated from the shortest path from aircraft -> active waypoint, and then as per the rest of the active route.

Say you are flying on left downwind, and go abeam the CF for your landing rwy - the CF is 5 miles to rwy, and the CF is 3 miles off to your left hand side.

FMC should say distance to go is 8, but that won't include the turn you will have to make on to finals (which in this case will always be more than 3 miles - you can't make that tight a turn!), as well as any further distance you may fly downwind.

Hope that makes some sort of sense

NW3 (757/767)

[GE90115BL2]

A very wise Check Captain once said it was the direct distance to the next active waypoint and thus the VNav profile assumed you were going directly to it and the progress page distance reflected the same.

[BGQ]

I have flown 737, 767, 747 and 777 among other aircraft and my recollection is that it behaves a little differently on all of them depending on software update and system hardware status.

One thing I am sure of is that the vnav path is correct only when you are intercepting the final approach OUTSIDE of the waypoint that you have selected for the intercept course.

So if you are flying a visual approach or are being vectored and intercept inside that waypoint and use VNAV you will end up high.

Using any waypoint inside of your anticipated intercept point is the way to go. Sometimes that will be the CF waypoint and sometimes something closer.

Later model aircraft make a better job of the VNAV PATH calculations than the earlier versions.

[bucket_and_spade]

Excellent. Thanks guys - there seems to be a consensus on where the profile info comes from with lots of consistent answers.

Strange I can't see anything obvious in the company's technical volume...maybe they are dumbing down pilot training...!

Cheers,

B&S

[GE90115BL2]

Oh yes BGQ's words Using any waypoint inside of your anticipated intercept point is the way to go. Sometimes that will be the CF waypoint and sometimes something closer.

are 100% correct.

If you pick a waypoint outside where you intend to fly, the FMC legs may not sequence correctly and if you then carry out a go-around and select Lnav you'll be in for a rude 180 turn back the way you came.:ooh:

Always check the top right of the ND to see if the waypoint has sequenced to the next:ok:

[Pilot Pete]

As an aside, I fail to see why anyone on a heading in the circumstances described (base leg or downwind vector with an extended centreline in the FMC) would be looking to use VNAV, or follow the vertical guidance.

By then I would ignore VNAV as in my opinion it is 'not an appropriate mode' for the phase of flight. I would be using V/S if low on profile or LVL/CHG if on or high on profile. As for working out the profile, I would be using my 3x table and mark one eyeball/ rough distance to go on my map.

By the way, I thought it did shortest distance to the extended line (i.e. 90 degrees). Will have a look on my next flight to see if it is this or direct to the active waypoint.

Cheers

PP

[GE90115BL2]

I fully agree, one should not be needing/using any Vnav guidance then. However while in IMC being radar vectored the distance to go is another tool in your arsenal of situational awareness and can't hurt.

The distance is calculated directly to the next active waypoint and not to a ficticious point on the centreline abeam the aircraft.:ok: The displayed dist to the destination ( prog page 1 ) is then the least/shortest distance to run and you shouldn't be caught out too high.:ok:

[haughtney1]

Totally agree Pete, VNAV is not really the mode to be concerned with when you are being vectored, particularly from a base leg vector.
Sometimes relying a little too much on the boxes to give you situational awareness can lead you up the garden path..so to speak.
Far better IMHO, is to have some basic DME info available..and a basic understanding of the geometry of the current heading vs distance to the centreline....which will also give you a better situational picture..plus has the added bonus of developing a feel for estimated track miles.
Failing that..a quick peek on the EHSI in map mode should give you a good idea as well!...then there is is the banana bar for the cda etc etc

Ahh how I miss my Boeing banana bar...:sad:

[Spooky 2]

If you wanted to know how far from the runway and say it does not have an ILS/DME associated with it, why would not simply select the runway itself from the available runway selection on the Approach page? I must missing something here as this is a fairly simple procedure.

[TTail]

GE90, do you know the rules for when the FMC will/will not sequence the wpt? I assume the rules are similar to all newer Boeing FMCs...? Can't find that info in the FCOM.
Non-sequencing of wpts is one of the typical Airbus "gottchas" but it has never been highlighted as an area of potential problem on the Boeing with my current airline.
Cheers.

[GE90115BL2]

No, can't say I do know the exact distance inside a waypoint that causes the legs not to sequence. I've had it happen to me in the past and from memory it had to be maybe 2 miles? I'd must be written somewhere in the FCOM 1?

It's mentioned in training and all F/O's I fly with are aware of the trap

Just keep an eye on the top right of the ND for the active waypoint.

Especially since the latest Boeing software update automatically gives you Lnav after pushing TOGA on a go around.

[Pilot Pete]

Sequencing is a gotcha. We are able to use LNAV for non-precision approaches, which reduces workload as opposed to HDG/SEL, which is what we used to have to use! But, I have seen a pilot take radar vectors to the VOR/DME final approach track, only to hit LNAV with the active waypoint (extended final approach course) off behind the left wing....! Confusion reigned and we had a chat about it on the ground after using HDG/SEL. It just goes to show how easily SA can fail with a reliance on the automatics and a 'what's it doing now?' moment.

Another example of the sequencing is the DESIG departure off runway 23 at MAN, which is usually terminated with an easterly heading shortly after departure. This leaves the active waypoint behind you after a few minutes. I have seen (on a line check) unfamiliar pilots staying on this heading and their distance to go increasing, then 'Using Reserve Fuel' displayed in the scratch pad followed by much scratching of heads until someone spots it!

If you wanted to know how far from the runway and say it does not have an ILS/DME associated with it, why would not simply select the runway itself from the available runway selection on the Approach page? I must missing something here as this is a fairly simple procedure. Yes I think you are missing something.

PP

[Spooky 2]

Well, PP why don't you share your point of view and knowledge with me and maybe I'll catch up somehow:confused:

[RYR-738-JOCKEY]

In said example your VNAV path will continously recalculate itself based on the LNAV capture criteria, within 3 miles, or on an intercept course of less than 90 degrees. If you are within 3 miles on a downwind leg it will calculate a path based on the shortest LNAV track which will be an impossible intercept course and less track miles. Therefore you're better off either creating a point or putting CF on top rather than extending from CF. However getting vectors on to finals 15 miles out and more, extending from CF and selecting LNAV will intercept the final approach track (as you know, for sure) and also provides you with an accurate VNAV path.

[BOAC]

Speaking as 'grumpy old retired git' I see the same mental trends here as we saw in the Schipol, Colgan, PGF etc etc accidents - over-reliance on the 'automatics' and computers. I NEVER favoured the use of extended c/line (co-pilots were 'trained' to use it and of course I let them:)) BUT my view has always been that an idea of where you actually are and where you should be is the necessary function for your brain. In the absence of DME. NDB, VOR or whatever on an airfield, what about a mental plot of where you are? Where were you? What headings have you been on for how long? What is the wind? Is this skill now dead? IF you are incapable of keeping such a plot, and I suspect a frightening number of newer pilots have never had this suggested to them - and are - by all means put the a/f in as a fix and work out your energy levels, DTG etc from that. There is, after all (if it is working) a 'map' in front of you with range circles and a built in extended c/line already there!

I recall one 'famous' lady co-pilot (claiming several thousand hours, too) who was COMPLETELY lost without a magenta line, could not plot her position on a map and could not navigate an airway on VORs. ETAs from the PLOG and the clock without the PROG page in front of her were a total impossibility. Probably in command somewhere now - and we wonder why these 'unbelievable' accidents are occurring.

[TTail]

Thanks GE90. I've had it happen to me a few times, e.g. into FRA where we often are let in earlier than the full arrival. I just haven't been able to find the numbers anywhere...

[Pilot Pete]

Well, PP why don't you share your point of view and knowledge with me and maybe I'll catch up somehow

Well, the OP mentioned extending the centreline from the Centre Fix. This means they already have the planned landing runway selected from the available approaches, including the approach itself (say ILS to that runway as an example). Therefore, doing what you suggest is just removing the approach and drawing a line from the runway.....which will be the same unless the approach is offset. All that will be missing will be CF and FF etc. Distance to go will still be the same. So I can't see any advantage in what you suggest, as the OP was talking about an approach that WAS in the FMC already, no mention was made by anyone to making an approach to a runway without an approach in the FMC or an ILS/DME for raw data? Maybe I am missing something?:confused:

PP

[Spooky 2]

Thanks PP. I have flown the 777 for a few years and have been instructing in it for a couple of years now. I failed to get the big picture here so thats my fault.:}