B737, FMC Question.

[mangatete]

U10.6. On departures/arrivals page I select a runway and a runway extension (say 6nm). Select the extension to the top of legs page followed by runway underneath and track direct to the extension waypoint. The runway has a speed and altitude limit which is providing the EOD point and providing valid information for the vertical deviation indicator (VDI) on the EHSI. and also providing valid information for VNAV path.
Lets say, I am approaching the extension waypoint at a 90 degree angle. Prior to arriving at the waypoint I am cleared for a visual approach and begin flying manually. The modes (VNAV and LNAV) are deselected and the runway centre line track is extended through the 5nm runway extension waypoint, giving me a straight line from the runway to intercept final on.
Now if I intercept final outside the 6nm waypoint the Vertical Deviation Indicator (VDI)continues to provide accurate vertical information. However if I turn inside the 6nm waypoint and now on the final track, so the 6nm waypoint is now directly behind the aircraft, sometimes the VDI calculates a path based on the aircraft still proceeding to the waypoint back behind the aircraft and then to the runway, therefore indicating the aircraft is low on profile. Other times it will ignore the 6nm waypoint and calculate the path directly to the runway EOD point and provide accurate path information on the VDI.
I have tried various options of passing closer to the waypoint and also making the speed/ altitude BIG associated with this 6nm waypoint, but sometimes it bypasses the waypoint and other times it still uses it for path calculations...
Question I have is what parameters does the FMC use to decide when to bypass a waypoint and proceed to the next waypoint in these situations?
Thanks Mangatete

[Kit d'Rection KG]

I've always found using Direct/Intercept (colloquially 'extending the centreline') to be potentially troublesome, and gave up doing it some years ago. Much better to build a path that reflects what you expect to do, and simply advance the legs page in the old-fashioned way. Often, putting in the runway/-.1 waypoint and hardening the right side of the page is very helpful too. I'm not pontificating about the technological side of it here (others will do that, no doubt), just offering practical ways out of your problem, and others.

[BOAC]

With KdR KG on that one too. I NEVER use it. The number of times I have used anything like LNAV on arrival are few. Either 'on field' NDB and DME or Fix page on the field for 'spatial awareness', plus map showing where the elusive airfield is. On the 737 the runway centreline is displayed anyway.

Whenever I see F/Os (and Captains!) doing it they seem to be prone to losing the basics of 'wtf am I'. If I have anything there I have the 'worst case' descent profile displayed in case I get an early turn in (yes please:) )

[Permafrost_ATPL]

Couple of days ago a Cpt showed me that if you hit LNAV on your base leg (after having extended the centerline and coming out of HDG for a few seconds) you get a pretty good VDI indication (as opposed to what you get in HDG mode after extending the centerline). You still need to factor in your intercept vector, but overall it seems like a pretty good tip. Will try it out a few times and see if I like it.

Cheers

P

[mangatete]

I have found that when I build the planed arrival using waypoints on the legs page that all goes well until you need to turn inside one of the waypoints, (normally a waypoint on the final approach which may have been at around 5 or 6 miles.) What I am trying to establish is why sometime the FMC will bypass the waypoint that the aircraft has turned inside of and other times the FMC will still look at the waypoint you have bypassed even through is is back behind the aircraft, and continue to calculate a VNAV path based on going back to the waypoint behind the aircraft and then to the next waypoint.

Cheers Mangatete.