Drift with LNAV and HDG SEL

[Driver170 42]

Ok i quickly set the 737 NGX for a quick demo.

 

Winds are from the right 236/33, Heading 159 to HON

 

Case 1, Picture 1 shows LNAV engaged

https://www.dropbox.com/s/n0vfihp7hlipb40/2015-2-11_20-4-10-102.BMP?dl=0

 

I'm using the FPV and the purple vertical strip representing the MCP HDG 159 lays in the middle of it and the airplanes nose is pointing into the wind to hold the FMC route 159.

 

Case 2, picture 2 shows HDG SEL engaged

https://www.dropbox.com/s/50bnenn7abllzgf/2015-2-11_20-5-12-383.BMP?dl=0

 

I'm using the FPV and this time the purple vertical strip representing the MCP HDG 159 lays directly under the airplanes nose.

 

I just can't get my head around the drift and why this changes?

[Spin737 102]

Why are you using the FPV to track an airway?

 

Anyway, it's a Track vs Heading issue. Get some basic IFR and airmanship books and it'll clear this right up.

[Driver170 42]

I didn't say i was. I was just curious why the vertical purple strip on the PFD (HDG) changes between LNAV and HDG SEL?

 

Bit by bit matt i'm on ground studies PPL navigation book the now. DED RECKONING before i hit the ATPL stuff

[scandinavian13 5,192]

 

I just can't get my head around the drift and why this changes?

 

As Matt mentioned, this is very basic pilot nav stuff.

 

When you're in LNAV, you're flying between points with a fixed location in a GPS coordinate system. This means you are flying tracks between the waypoints. In other words, in order to fly a track line between the two coordinate points, the aircraft must correct for wind drift.

 

When you're on HDG SEL, you're flying a simple compass heading. You may or may not be compensating for wind when on a compass heading. ATC provides vectors with the wind already factored into the assigned heading (mentally), or, if you're flying by ded reckoning, you've already factored the wind into the heading. Regardless, you're still pointing the aircraft nose at that heading.

 

When you're in LNAV, look for the white triangle on the outside of the compass rose of the ND (just under the word MAG in your picture). This is the actual heading the aircraft is pointed in (and, while we're talking about it, where your heading bug should be - not straight up like in your picture). If you were to keep your HDG SEL cue right on this triangle and change from LNAV to HDG SEL, the aircraft wouldn't turn.

 

If you'd been placing the heading cue properly, it would probably have made a little more sense already. LNAV compensates for wind automatically by adjusting the heading properly. HDG SEL requires you to adjust for the wind on your own. In both cases, though, given the same wind, the nose (and therefore, heading) are pointed in the same exact direction.

 

The white bar that runs from the top of the ND to the center, however, is the track line. The line is the direction you're actually tracking across the ground. That's probably what you're getting confused with.

 

Heading - nose.

Track - line across the ground.

 

 

 

Bit by bit matt i'm on ground studies PPL navigation book the now. DED RECKONING before i hit the ATPL stuff

 

haha - that's quite the jump.

 

Might want to jump into the instrument rating and commercial rating stuff in between.

[ark 1,290]

If I understand correctly, when in LNAV the a/c is following a specific track "over the ground" and will have to fly a heading that compensates for the wind. When in heading mode, the a/c will fly (point its nose) according to the selected heading and the track you get will depend on the wind. A common example of track vs heading would be on flying an ILS. To stay right on the localizer (track) the a/c will have to crab appropriately to compensate for any cross wind.

Al

 

Edit: Never mind, Kyle posted while I was still typing, and his answer is better!

[skelsey 765]

Because the PFD compass rose displays heading up, whilst the ND displays track up.

 

In the first (HDG SEL) picture, you are holding heading 159 and track 155, with heading 159 selected.

 

In the second picture, you still have heading 159 selected, but in LNAV the autopilot has turned the aircraft to track 159, on a heading of ~163.

 

EDIT: as above, Kyle has posted a much more comprehensive answer!

[Driver170 42]

When you're in LNAV, look for the white triangle on the outside of the compass rose of the ND (just under the word MAG in your picture). This is the actual heading the aircraft is pointed in (and, while we're talking about it, where your heading bug should be - not straight up like in your picture). If you were to keep your HDG SEL cue right on this triangle and change from LNAV to HDG SEL, the aircraft wouldn't turn.

 

But don't you always set the HDG cue to the course thats displayed in the FMC legs page for each waypoint?

Thanks for the rest of the help, much appreciated to everyone

t's quite the jump.

 

Might want to jump into the instrument rating and commercial rating stuff in between.

 

Well i have a few more books to get through, before the big stuff

[skelsey 765]

 

 

But don't you always set the HDG cue to the course thats displayed in the FMC legs page for each waypoint?

 

It may depend on the airline, but in my experience the general practice is to keep the heading bug lined up with the aircraft's current heading (i.e. over the white triangle). That way you're always keeping the appropriate drift correction in and if you need to deviate around weather/ATC asks you to turn ten degrees right or left for spacing etc you can drop straight in to HDG SEL and make the adjustment as necessary.

 

On a personal note, your pictures just reminded me why I'm not a fan of the track-up presentation on the ND, especially when the PFD displays heading up. I had to look about three or four times at each to compare the ND and PFD heading rose and work out what was different between the two! Some airlines have the ND display heading up, which I personally think leaves far less room for confusion.

[ark 1,290]

I was going to say that setting the heading to initially match the FMC value is a good starting point until you see how the a/c compensates for the wind, and then set the heading as discussed above, but I know Kyle will have already answered this in a much more informative way by the time I'm done typing, so I won't say anything!

Al

 

Edit: I mean Simon!

[Driver170 42]

note, your pictures just reminded me why I'm not a fan of the track-up presentation on the ND, especially when the PFD displays heading up. I had to look about three or four times at each to compare the ND and PFD heading rose and work out what was different between the two! Some airlines have the ND display heading up, which I personally think leaves far less room for confusion.

 

Track up and heading up? This is new to me.

[skelsey 765]

 

 

Track up and heading up? This is new to me.

 

Track up = aircraft track at the top of the display

Heading up = aircraft heading at the top of the display.

 

So if you look at the ND, you'll notice that it says TRK 158 M at the top. TRK indicates that the ND is displaying "track up" -- so the white track line (showing your track across the ground) will always point directly up, whilst the little white triangle (which indicates your heading) will move around depending on the drift.

 

Compare this to the "heading up" presentation on the PFD compass: where the current heading is always at the top and the white track line moves to display your heading across the ground.

 

Personally I think it's more intuitive to have both displays in "heading up" -- I think that makes it much easier to see the wind correction angle being applied by the autopilot in LNAV, without squinting at the little heading triangle (not to mention making crosschecking between the two displays much easier!)

[Driver170 42]

Found this

 

http://www.pprune.org/questions/431821-heading-bug-cruise.html

Ahh i see what you are on about. I think its because on the PFD you have the dot representing the nose and the FD bars and they sit directly under the white triangle? It looks like its giving a birds eye view maybe? Improving situational awareness.

[scandinavian13 5,192]

 

 

But don't you always set the HDG cue to the course thats displayed in the FMC legs page for each waypoint?

 

Nope - put the heading cue on the white triangle (which is the aircraft's actual heading) on the ND. Otherwise, you're kinda defeating the point. The cue should always be on the nose. The idea is that, if you need to put yourself on HDG SEL for whatever reason, the aircraft should not turn when you select the new mode. Using your picture as an example, the aircraft would turn a few degrees to the left, which would take the aircraft off course.

 

Additionally, if ATC provided a vector off of your route, it's easier to identify turning the knob left or right if it's on the nose.

 

http://www.flightlearnings.com/2012/07/16/effect-of-wind-on-navigation-part-two/

[Driver170 42]

Thanks kyle it all makes more sense  

[scandinavian13 5,192]

Thanks kyle it all makes more sense  

 

Welcome - it was a good question. Hopefully the answers here will help in the PPL ground school. Honestly, with all my time messing around in Flight Sim (back to 5.1, but FS2002 by the time I started taking ground school), it was pretty much all old hat to me. I just went there to get the signature in my log book, though I did learn a thing or two in the process.