777: VNAV PATH in HDG SEL

[TheBusFlyer]

Hi everyone,

I am having trouble understanding how the 777 is able to follow VNAV PATH in HDG SEL…how can you follow an altitude constraint over a specific WPT if you won’t even overfly it? Does it create ABEAM waypoints on our current track?

If I am not wrong, the 320 would immediately give us a mode reversion and revert to OP CLB (during climb) or VS (during descent).

Any input would be appreciated.

[STBYRUD]

It depends what lateral path you're flying in HDG SEL, but it will command the path equivalent to a point abeam the LNAV path... Needless to say that can be slightly unpredictable, so it's a good habit to change to a more reasonable pitch mode if you're departing the lateral FMC path.

[TheBusFlyer]

Originally Posted by STBYRUD

It depends what lateral path you're flying in HDG SEL, but it will command the path equivalent to a point abeam the LNAV path... Needless to say that can be slightly unpredictable, so it's a good habit to change to a more reasonable pitch mode if you're departing the lateral FMC path.

Thanks for your answer STBYRUD, could you please give me more precision on « it depends what lateral path you’re flying in HDG SEL ».

[rudestuff]

If it's anything like the 757/767 then the FMC should continue to cycle way points and provide vertical guidance, so if you're not too far off the lateral path it won't make much difference. Its generally only something you see during the approach phase, so as others have said if you're way off then either open the window and use Vnav Speed or go for a different mode altogether.

It's also a good idea on vectors to extend the centre line, which gives vnav guidance to the next way point regardless of where you are and where you're pointing. It just means that if you're flying perpendicular to the track you'll essentially be flying level until you turn inbound and if you're flying downwind the magenta line will actually be climbing (Which in approach logic would command a level segment.)

[FullWings]

AFAIK VNAV creates a path, then continues to fly that irrespective of the lateral mode (offpath descent). I find it works pretty well. You can update the path if you want to by executing a direct-to on a waypoint, forcing it to recalculate.

Also, depending on what restrictions it was based on, the path may not be an idle descent one, so the vertical deviation indicator may need some interpretation.

[Commander Taco]

Busflyer,

Whilst in HDG SEL, if your cross track is only 3 or 4 NM, you’ll find you’ll meet your waypoint altitude constraint using VNAV. Much more than that, and start keeping a close eye on it. To do so, enter the name of the waypoint on the “FIX” page. Then select the “ABEAM” prompt. This will shoot a dashed green line perpendicular to the aircraft track. Then you only have to ensure that the green “Predictor Arc” lays over or in front of the “ABEAM” prompt dashed green line.

~Taco

[rudestuff]

Come on, let's use its proper name - it's the green banana...

[Commander Taco]

Originally Posted by rudestuff

Come on, let's use its proper name - it's the green banana...

Aye, the green banana it is! But how best to describe it to an Hairboos pilot who is used to the near unintelligible manuals provided by the mob in Toulouse?!

[ScepticalOptomist]

The dynamic arc was the easy one to name - what about the “donkeys dick”? That’s still the term I use for the thingy-a-majig which is segmented into 30 sec bits…. 🤣🤷‍♂️

[Gweilo747]

To add to the discussion; BusFlyer, the FMC will not sequence the next active waypoint if One deviates more than 21nm. This is important to know as the FMS will not advise you - the only way you’ll know is by looking at the top right corner of your ND and noting what is the waypoint in magenta i.e. the active waypoint.

Also, prudence would dictate that once you are deviating away from the LNAV magenta line (i.e. HDG SEL or TRK SEL), a quick check of the GRID MORA, Minimum Radar Altitude(s), or if within 25nm of the navaid (or ARP) predicated sector MSA, to ensure you will be clearing any obstacles or terrain thus an alternate technique is to use FLCH or VS, update your LEGS page with the amended active waypoint (as per Rudestuff - Post #4), then look at your PROG page and calculate mentally whether you are high, on or below your nominal descent profile. This can also be backed up using the dynamic arc and technique that Commander Taco Post #6 posts.

In essence, there’s various techniques; but certainly have an awarness of what’s adjacent once you’re leaving the LNAV track combined with where the dynamic arc is defined on your ND.

Apologies for the long winded answer.
Safe skies always.