Intercept Course To: INTC CRS TO VNAV switches to HDG/TRK ?

[sloppysmusic]

Just checking this is normal. In legs page when adding legs or using direct to by selecting a later leg and 'pasting' over the active leg normally the ND will show you your planned changes until you press EXEC and then will activate your changes or if you press ERASE it will return to original route (without changing current roll mode).

All this is working as expected except if I change the default INTC CRS TO at R6 to any other radial/course (that is NOT on current intercept heading) then the moment I place it in R6 VNAV cancels (message NOT ON INTERCEPT HEADING msg appears) and immediately I am in TRK or HDG HOLD mode depending on what is set on MCP. This happens BEFORE I press EXEC. Even if I press ERASE VNAV is still canceled and I have to manually select again on MCP. I am just checking this is normal as I EXPECTED nothing to change at all until I press EXEC or ERASE? It works normally on ABEAM PTS or even if I select the R6 with the default course. ONLY if I put in a course of my choice that is NOT on intercept heading the AP will immediately drop out of VNAV. 

Note: If track IS on intercept heading it works as I expected, no changes until EXEC (or press the L1 key again).

I understand if I want to CONFIRM I wish to use intercept NOT on current heading I need to use TRK or HDG mode to align but why does VNAV immediately cancel in this one circumstance when for every other situation I HAVE to press EXEC or L1 (the new WP) to confirm?

Thanks for your help in advance. Loving all the intricacies!

[downscc]

I followed about 20% of the problem statement.. using specifics would have helped but let me try with an example of how I use this feature on most of my departures from my home base.  Here is the setup:  Departing on RNW 31 flying runway heading 310 and there is an airway I want to join that is parallel to the extended runway center line to the right (Northeast) and it is a 310 course.  ATC gives me a 340 heading to intercept the airway and the next fix on the airway is THX.  On the FMS, THX is doubled LSB to give it a direct to and the INTC CRS TO value is changed to 310.  I've been on a HDG hold of 340 and I change to VNAV and the aircraft continues on 340 heading until intercepting the 310 course to THX, which puts me on the airway.

You don't change the INTC CRS to the value of your current heading, you change it to the course that you want to intercept and fly to the next waypoint.

[sloppysmusic]

1 hour ago, downscc said:

you don't change the INTC CRS to the value of your current heading, you change it to the course that you want to intercept and fly to the next waypoint.

OK first things first my humble apologies but it is LNAV that drops out not VNAV. I was typing while flying last night and trying to do 2 things at once. So swap VNAV for LNAV in every instance(this should make a lot more sense as we are talking roll modes here not pitch modes!).

Yes I understand that 100%. The question I am asking is it it normal for the AP to drop out of LNAV IMMEDIATELY you put the required course into the legs page R6 whereas every other scenario shows the new UNCOMFIRMED course on the ND but lets you EXEC or ERASE it, THIS scenario instantly takes you out of LNAV and even if you erase it you are already in TRK/HDG HOLD. I noticed the first time when my plane flew of course as I had not expected it to drop LNAV when I had ERASED the proposed change.

I understand exactly 100% what the manual is explaining I am just completely surprised by the AP reaction to this particular situation.

You can try it yourself. Legs page copy leg 2 over leg 1. new leg 1 is now highlighted ready to confirm with EXEC. R6 shows current intercept heading. (Theres a storm ahead so you want to come round the other side of the cloud for example). The moment you put in any course that is NOT on a current intercept to the new fix into RK6 LNAV drops. If you put a course that IS on an intercept heading LNAV does NOT drop.

by LNAV drop I mean roll mode instantly changes from LNAV to HDG/TRK HOLD or even ATT if plane is on a current turn! WIll post a video here in a short while which should explain this easier.

[Ephedrin]

I‘m totally confused by your VNAV being dropped for HDG HOLD.. first I thought you meant LNAV and accidentally wrote the vertical nav as you were too concentrated on all the other stuff... but now you talk about vnav again... 

LNAV would be disconnected into HDG HOLD/SEL when you activate INTC HDG. But VNAV??? 

[downscc]

Any course that is not an intercept must drop VNAV because it can no longer calculate a vertical path... the lateral path is indeterminate.

[sloppysmusic]

9 minutes ago, Ephedrin said:

I‘m totally confused by your VNAV being dropped for HDG HOLD.. first I thought you meant LNAV and accidentally wrote the vertical nav as you were too concentrated on all the other stuff... but now you talk about vnav again... 

LNAV would be disconnected into HDG HOLD/SEL when you activate INTC HDG. But VNAV??? 

Sorry forget every single mention of VNAV in every single instance in my above posts except when I say i made the mistake!. It is entirely not an issue here. ONLY LNAV. 

11 minutes ago, Ephedrin said:

LNAV would be disconnected into HDG HOLD/SEL when you activate INTC HDG. But VNAV???

EXACTLY!!! It disconnects BEFORE you activate! Video will make it very clear soon.

[Ephedrin]

Isn‘t VNAV also responsible for the thrust setting on the 777? So will it swap into SPD | HDG HOLD |   VS/ ALT HOLD? 

[sloppysmusic]

Here is video showing EXACTLY what is going one. Apologize for the title of this thread and the first post it should read LNAV in every instance of VNAV. Please allow YouTube a few minutes to render HD versions as it's probably only viewable in low res now. Will be 4k when YouTube have rendered all formats.

Please note that LNAV drops out when ABEAM PTS is selected as well. 

 

[downscc]

Russel, why are you entering a value that is not on an intercept course?  You are telling it to do something it cannot do. 

Are you trying to do direct to TEKNO?  If this is what you want to do then don't click on the INTC CRS LSB.  If not then explain what you are trying to achieve.

[Ephedrin]

Dan,

I would call this an entry by mistake, but LNAV shouldn't deactivate into ATT without the entry being executed, should it? The warning comes up in the Scratch field that it's not possible, so far so good. A pilot would be warned and wouldn't execute.. This far the assumption 😄

 

FCOMv2 at Page 4.10.7 says:

- LNAV maintains current heading when: 

         - activating the inactive route or activating an airway intercept and not withing LNAV engagement criteria

- LNAV is deactivated:

         - (I summarize) by selecting HDG (TRACK) HOLD or SEL, when localizer captures or with a dual FMC failure. 

 

neither in the LNAV section of the FCOMv2 nor in the route section regarding heading intercept is anything mentioned about a deactivation of LNAV prior to executing any insertions.

 

 

[downscc]

Good question and one that someone on the tech team would have to respond to.

My problem is that Russel is in LNAV mode when he tries to set up a heading to intercept a course to a waypoint.  Normally, one is in heading mode when selecting INTC CRS TO and this arms LNAV, which then activates when the interception of the course occurs.  So I think it is possible that LNAV is being deactivated by the action that normally arms LNAV because it cannot be armed when already active?  Circular logic, I'm not sure what the correct action would be when trying to do this but the best thing might be to change to heading, set the INTC CRS  and let the machine do what it knows how to do.

[sloppysmusic]

26 minutes ago, Ephedrin said:

neither in the LNAV section of the FCOMv2 nor in the route section regarding heading intercept is anything mentioned about a deactivation of LNAV prior to executing any insertions.

This is my point exactly. The real life reason would be say bad weather avoidance but FCOM states VALID course for interception is 1-360 degrees so therefore ANY course is valid, and the deactivation should NOT occur before confirmation of route change.

[sloppysmusic]

15 minutes ago, downscc said:

Normally, one is in heading mode when selecting INTC CRS TO and this arms LNAV, which then activates when the interception of the course occurs. 

Good point but I am in the section of the FCOM where it explicit talks about course alteration from the legs page IN FLIGHT. So you logically have to already be in LNAV mode right? Will edit this to give FCOM page number when I locate it.

Flight Management, Navigation FMC CRUISE

11.42.9

[downscc]

15 minutes ago, sloppysmusic said:

VALID course for interception is 1-360 degrees so therefore ANY course is valid

I don't understand how you can have a valid intercept course if you heading/track will not cross or intercept that course.

Please explain what you are trying to achieve in the video.  What flight path are you trying to set up with the FMS?

[sloppysmusic]

6 minutes ago, downscc said:

I don't understand how you can have a valid intercept course if you heading/track will not cross or intercept that course.

Please explain what you are trying to achieve in the video.  What flight path are you trying to set up with the FMS?

It doesnt matter, you can fly IN from any angle you want, but if NOT on intercept course you have to do it using TRK/HDG of course. Thing is plane drops you out of LNAV immediately before you press EXEC or ERASE. That is the entire problem, the highly unusual logic there. It does not matter WHY I want to do this, the FCOM clearly states you CAN do it and does NOT state "entering a heading not on intercept will immediately cancel VNAV and put you on a TRK/HDG hold before you even press EXEC or ERASE"

I go through the FCOMS and test out every scenario in flight so I learn it and understand rather than just forget it. Each time I follow the example shown in the FCOM, using similar scenarios. This situation results in entirely unexpected behavior inconsistent with any other related FMC procedure in any plane.