777 FMC: direct to with abeam points

[Puzzled FO]

Flying through Warsaw FIR recently, we were cleared direct from EGL to Lusid - about 320ish nm. We used the FMC abeam points facility - all well and good. I noticed (for the first time - but it seems to happen consistently) that at every abeam point there was a small but pronounced turn. I realise a direct track course will change following more or less a Great Circle, but the course change happened specifically at the abeam points. The FMC showed the course changing by five degrees (310-305) via about eight abeam points - that's quite a pronounced change over such a short distance isn't it? I'm sure my old type (737NG) maintained a 'straighter' (so to speak) track crossing similar abeams. Anybody else notice this? Any ideas why?

Puzzled FO

[vapilot2004]

This may have something to do with the calculated magnetic track variance between the different abeam points entered.

[slamer.]

Sounds like another good reason not to use the abeam points function.

[Cubbie]

The FMC does occasionally have difficulty in computing an abeam waypoint exactly spot on the new direct track and may give your direct track a few degrees dogleg, especially when the turn to the new direct route is more pronounced, or has a strong wind input, or involves multiple waypoints the greater the dogleg off to an abeam waypoint. Since your ATC clearance is direct the new waypoint and not an abeam point, try reselecting direct again, and then reselect abeam points it will fine tune the track and remove the abeam doglegs

[777fly]

An interesting observation and one that has never really caught my attention. I think this is what may be happening: If you select 'go direct' to a very distant waypoint and do not select abeam waypoints, the FMC will make a simple calculation and then alter the desired course imperceptibly during the leg to achieve the great circle track. If you select the abeam waypoints option, the FMC then sees each abeam point as the active waypoint, in turn, and will make the necessary adjustments at the start of each shorter leg to achieve the desired track to the next active waypoint. In other words, it is only looking at the next leg, not the whole of the original direct route..There must be something in the software that changes the way the FMC works it out over short and long legs, but Boeing never release that much detail in the manuals or the conversion course. Maybe your technical manager can find the answer?
Personally, I prefer simply to select go direct to where I am cleared so that I can see the time/distance to that point on the ND and in the FMC. Then, with a route copy performed preflight, you can look at route 2 legs and see the original planned track and waypoints ( as you pass abeam) on the ND also. An additional benefit of doing that, is that you can turn and backtrack the overflown waypoints if necessary. They are erased once overflown in Route 1. I expect you knew that.

[ea340]

Use the direct to and you build abeams on the fix page.

[lion-g]

Hi guys,

I have somewhat asked a similar question previously and the reply was fantastic.

http://www.pprune.org/tech-log/392809-great-circle-track-fmc-fmgs.html

Cheers,
lion-g

[wingtip777]

saw it heaps of time. especially when you are very close to the first turn while you select abeam. must be something to do with the FMS i think.