I've a question concerning the "INTC CRS" feature :
When we are under radar vector for approach, I usually ask to set the FAF of the runway on top of the leg page and select the localizer course on the LSK 6R (INTC CRS). What does the VNAV deviation scale indicates when we perform this?Is it the deviation between the current altitude and the FAF altitude?Or the deviation from the altitude we should have when we intercept the LNAV route with the actual heading?
There was a thread on this a couple years back. I *think* it was "decided" that the FMC transposes you laterally onto the FAC and does not include any offset for a base or downwind. So, if you were on a base 5 miles from the threshold, 3 miles from the centerline, it would say you're at 4 miles. (3-4-5 triangle.)
Now I need to look that up again. That doesn't sound right, now that I think about it.
Last edited by ImbracableCrunk; 1st Jul 2012 at 19:09.
Never tried this as i don't really need a VNAV Path, but if you're on an intercept course and you extend the centerline. I guess you could just click LNAV, then it will draw a line to the extended centerline and probably give you an accurate path then go back to HDG.
I'd always thought the PROG 1 distance was the nm's from present position on your intercept HDG to capture the magenta line and fly the planned route thereafter. Your altitude relative to the required path to achieve the next alt constraint generates the VNAV deviation.
I think it calculates from aircraft position to that FAF and then the rest of the flight plan regardless of your aircraft track or heading.you can check it out by setting intercept course to at FAF and you will see the same distance at progress page in different intercept track even your track is from the runway(outbound ).
Another method to check,by selecting the same track to intercept but change your intercept point (may be FAF IF or the others),the distance is changed (on aircraft or simulator ).
Last edited by Noknoipobin; 2nd Jul 2012 at 16:14.
Never tried this as i don't really need a VNAV Path, but if you're on an intercept course and you extend the centerline. I guess you could just click LNAV, then it will draw a line to the extended centerline and probably give you an accurate path then go back to HDG.
Again i've never actually done this for real.
That works, I've done it before. VNAV gives a real VTRK with LNAV and an intercept. It's not correct in HDG SEL if you're not on an LNAV direct course to the FAF or some fix along the way. In HDG SEL, it shows higher than LNAV until you get on the FAC.
Okay, so that clears it up a bit... The difficulty here is that the ac, using this method, you will intercept the ILS beam, perhaps 'at or above', then the ac will drop, to re-acquire the beam, especially when using baro-vnav to ILS intercept...
This is all very possible, but is certainly 'outside the box'...
Okay, so that clears it up a bit... The difficulty here is that the ac, using this method, you will intercept the ILS beam, perhaps 'at or above', then the ac will drop, to re-acquire the beam, especially when using baro-vnav to ILS intercept...
I was just wondering how the FMC calculate the VNAV path when you put the extended centerline (FAP with the intercept course equal to the localizer course) when you are under radar vector for an ILS approach.
What does the VNAV deviation scale indicates when we perform this?Is it the deviation between the current altitude and the FAF altitude?Or the deviation from the altitude we should have when we intercept the LNAV route with the actual heading?
As to your specific question (assuming you're speaking NG here)...
The correct answer for FMS stuff always seems to come out, 'it depends'.
In HDG SEL & VNAV on vectors with the LOC extended, if the assigned vector altitude in the MCP is put in as the VNAV cruise altitude, the FMA mode will be VNAV PATH and the PDI will show no deviation from path as long as you maintain that altitude, regardless of position, until you get back around to the on track T/D point created for that altitude.
For discussion purposes, if a new altitude is assigned on vectors, for example a descent of 1000 feet, if LVL CHG is used for the descent and then VNAV is reselected after level-off without updating the active cruise altitude, the FMA mode will be VNAV ALT and the PDI will display a deviation from the vertical path that resulted from extending the LOC course out to infinity.
So, for example, if you continue level on a downwind, in VNAV ALT, the vertical path deviation will continue to increase as you get further out. The PDI deviation in this case is relative to being below the geometric plane that would be the glideslope along the LOC, not the active cruise altitude.
In any case, the distance to the active waypoint on the ND, whether in LNAV or not, is straight line distance from present position to the waypoint, regardless of offset from the LOC or heading. Conceptually, no different than distance to the next waypoint along an RF turn segment, it’s not the actual track distance to be flown but the instantaneous straight line distance.
I need a nap.
(naturally current NG guys are invited to hack away here)
So complicated, so confusing, and in effect absolutely **** useless (since ATC will probably never send you that way) which is why I never used the 'extend the centreline' (luddite!) but always knew where I was and what I height I needed to be. Much simpler. Is it that difficult?
Unfortunately it became the 'darling' 'clever trick to show' for those TC's who presumably did not really know where they were and every F/O came out of the sausage machine programmed to push the buttons and then not really know what to do with it.
The very best in FMC vertical guidance was with an un-executed (dotted white on 737) leg to a point, but that sadly fell out of favour with de management and became a no-no..
Long live range to waypoint and the three times table. Why complicate it?
As to your specific question (assuming you're speaking NG here)...
I'm speaking about the classic but I suppose it's the same as in the NG?
And how is the profile computed if we don't put the VNAV cruise altitude on the FMC? Imagine we "just" use LVL CHG and not VNAV...
Let's say we are on a 8Nm downwind (HDG SEL and ALT HOLD and the FAP is on top of the leg page with the correct intercept course) and the VNAV deviation indicates 2000ft high ... How is this value calculated?
So complicated, so confusing, and in effect absolutely **** useless (since ATC will probably never send you that way) which is why I never used the 'extend the centreline' (luddite!) but always knew where I was and what I height I needed to be. Much simpler. Is it that difficult?
Unfortunately it became the 'darling' 'clever trick to show' for those TC's who presumably did not really know where they were and every F/O came out of the sausage machine programmed to push the buttons and then not really know what to do with it.
The very best in FMC vertical guidance was with an un-executed (dotted white on 737) leg to a point, but that sadly fell out of favour with de management and became a no-no..
Long live range to waypoint and the three times table. Why complicate it?
I was just wondering how the VNAV profile was computed when do the "extended centerline"on the FMC... just out of curiosity
It does not need a 'VNAV' cruise entry - it uses current altitude. I was told it computed the 'direct to' vertical profile with no allowance for heading or turn radius, but that could be wrong - as I said, I never used it. If that is the case, the only time it would be meaningful would be when you were on that track, so I would wager that '2000ft high' would be just about perfect..That is why the 'un-executed' route was always more accurate as it 'drew' the turn.
Let's say we are on a 8Nm downwind (HDG SEL and ALT HOLD and the FAP is on top of the leg page with the correct intercept course) and the VNAV deviation indicates 2000ft high ... How is this value calculated?
adri737:
I only know the current box in the NG, classic for me was years ago and I've slept many times since then. So you may want to check it in a sim session. That said, in the NG...
You won't have a PDI to show VNAV deviation in HDG SEL, without VNAV being selected. If ALT HOLD is selected/annunciated for the ATC vector assigned and presumably MCP selected altitude during vectoring, you're not in VNAV.
(I believe also that you won't have a PDI if being vectored from the opposite direction even in VNAV until you pass the waypoint on the LOC you've made active with the INTC CRS function, in others words in a position to actually intercept the extended LOC outside the active waypoint, but I'm not positive when it pops up.)
In this situation, if VNAV is then selected (accidentally, whimsically, or for whatever reason), you will see VNAV ALT annunciated, a T/D for your altitude will be created at the proper on-track point, but the PDI will reflect the deviation from the vertical profile for the extended on-track LOC & GS as loaded into the box, I don't think it's a 'direct to' profile simply because you're not doing a 'direct to'.
As you proceed outbound in VNAV ALT on the downwind at that assigned altitude, the VNAV deviation will indicate that you are getting lower, not higher relative to the on track vertical profile, just like the GS indication when proceeding outbound on downwind. You're on a vector to the ILS (you will eventually select APP) at presumably a reasonable assigned altitude, not a 'stratospheric' downwind to a visual.
BOAC, he's just asking how it works, not whether or not it's a 'clever' idea to use it.
Pilots get locked in to constantly using only what they're familiar with, and very often don't want to learn other options because they're too 'complicated'.
You don't have to use it, but you sure should know how to.
(No, it does not need a cruise altitude entry, but the PDI functions differently if you don't. Not better, not worse, and hopefully not obscure to anyone who operates the system. And it's not really an 'extended centerline', it's an FMC extended LOC course from an active fix on the LOC course which might even be offset from the runway centerline and either way can be visually useful during close parallel ops.)
Last edited by OK465; 3rd Jul 2012 at 23:25.
Reason: grammar
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