FMS abeam points

[RAT 5]

A question which may result in more opinions than facts, but I hope for the latter. You are cleared DCT XYZ. This could be e.g. >200nm. Use LNAV. To avoid having an empty screen and following a magenta line to who knows where, you select ABM's and clean up the route to e.g. 60nm segements. Will the a/c indeed fly DCT XYZ or a sequence of shorter straight lines? I've always used ABM's for situational awareness, time/fuel keeping etc. It is not unusual to have DCT followed by more DCT's and never overfly a wpt for checks. I've heard some operators forbid the use of ABM's and wonder why. You could enter a significant wpt in FIX for cross checks; you could even use the ABM function of the fix page to create a wpt for checks. What's the difference between that and using the AMB function of the DCT to LEGS page in the first case. I've asked various trainers and heard many opinions. Where's the truth, and what would be the danger of selecting ABM's such that it is forbidden?

[MarkerInbound]

Boeing has a bulletin out that there have been cases when abeam points are selected the FMS changes the OAT on the wind page to 0. This can affect the predictions and lead to an "insufficient fuel" message. Boeing says before you execute the abeam points you should check the wind page and if it changed to 0 pull up the SAT and enter it for the next fix. It will propagate to all the down track waypoints and be close enough for government work. Maybe some carriers don't want to be bothered and just say no abeam points?

[linedriva]

As well as the Boeing bulletin, another issue we've come across is regarding CPDLC. Sometimes using the abeam points causes coding problems with the CPDLC which causes some ATC problems. If I need to put in a fuel check, I put an abeam position on the fix page to use.

[RAT 5]

Agree with much of what has been said, and thank you for it. But, the point is, that Boeing has offered the Abeam Points function as an added upgrade, presumably, to be a 1 button work saving method. By using all the piano playing you mention you are going back to the type of FMC that was in use years ago before the Abeam Points function was added. if we're not allowed to use it, or if it causes problems, then it is all a waste of an option and could be removed.

[Mikehotel152]

RAT 5: That's a good question and one that puzzles me too.

A quick example: We were coming back to the UK last night and got a good direct from the Polish border to OTBED. After selecting 'ABM' on the legs page and tidying up the mess, I noticed that the track between the remaining waypoints varied by up to 3 degrees.

FCOM2 explains: Data and status corresponding to the parent waypoints is passed to the abeam waypoints. If abeam distance is less than 100 nm, only the wind data is passed to the abeam waypoint. I haven't noticed the effect of abeams points on the SAT but will have a look this afternoon. Oh wait, my mistake, we're flying to Spain so we're unlikely to get any shortcuts... ;)

[stormyweathers]

Use ABM PTS, thats what its there for. The only time you might not want to use it is in a ADS-WPR (contract) environment. That is, after you log on to a specific ATC unit, the aircraft system sends automatic position reports to the ATC facility at each waypoint (this replaces manual position reports). So ATC will receive a position report at each ABM PT which they dont want. But for this you would have to specifically log on to the CPDLC facility, as in the NAT region for example. Other than this, use it. If at all you have minor differences in track, ATC wont know. You are using a state of the art aircraft for cryin-out- loud. They're not gonna violate you when you're on a 200nm direct course.. I would suggest that you do not "clean up" the direct route. Leave it like it is and ignore ABM PTS that you dont need. It's when you start messing around with cleaning up abeam points that you're looking for trouble. Leave it like it is.