Boeing.Honeywell Box: Approach May Disappear
11-10-2017, 06:03 PM
#12
Gets Weekends Off
Joined APC: Aug 2008
Position: 777 Left
Posts: 347
I have seen this once in the Bus. Neither of us had seen nor heard of it when it happened. We spent a minute or two but got it worked out. Didn't know the Boeing would do this also. We thought it was a Fifi thing.
11-10-2017, 06:24 PM
#13
Gets Weekends Off
Joined APC: Sep 2009
Posts: 2,230
Originally Posted by galaxy flyer
Not in the Collins Bombardier types. I’ve been vectored all around and never lost the approach. But, there are trade-offs in the FMS installs and software.
GF
GF
11-11-2017, 02:51 PM
#14
Gets Weekends Off
Thread Starter
Joined APC: Mar 2007
Position: Capt
Posts: 3,215
Originally Posted by cougar
This issue is simply a function of the waypoint sequence logic. Waypoint sequencing occurs when the aircraft crosses a "line' that is perpendicular to the inbound course of the active waypoint and extends to 21 nm either side of the waypoint. In addition, the aircraft track must within 90 degrees of the inbound course when crossing this "line" to activate sequencing. For example, if the inbound course to the active waypoint is 270, then when the aircraft crosses an imaginary course line of 180/360 that bisects the active waypoint, and the aircraft track is between 359 and 181, the FMC sequences to the next waypoint.
This detail can be used to prevent the unwanted sequencing while being vectored for an approach. Either delay selecting an intercept course to the approach fix as mentioned in previous post, or instead of entering the approach course, enter a course that is 60 to 90 degrees off. This effectively rotates the perpendicular "line" such that the aircraft won't cross it and prevents the waypoint sequencing. Once base leg is initiated update the inbound course as desired.
Good idea, if I am in that situation again...
In addition, if the End of Descent (RWXX or MAXX etc.) waypoint is sequenced, the FMC phase of flight transitions from approach logic to effectively being in limbo as it assumes you are landing and is waiting to go to flight complete. The vertical deviation pointer on the ND is no longer displayed, which creates an issue if the approach is a Non-ILS approach using VNAV PTH. To "reset" the FMC, a cruise altitude (typically current aircraft altitude) may need to be entered on the VNAV CRZ page. This reestablishes the FMC in cruise mode thereby enabling transition to the subsequent descent and approach phases required for a VNAV approach.
I never thought of this. We were doing an ILS, so this wasn't a factor.
This detail can be used to prevent the unwanted sequencing while being vectored for an approach. Either delay selecting an intercept course to the approach fix as mentioned in previous post, or instead of entering the approach course, enter a course that is 60 to 90 degrees off. This effectively rotates the perpendicular "line" such that the aircraft won't cross it and prevents the waypoint sequencing. Once base leg is initiated update the inbound course as desired.
Good idea, if I am in that situation again...
In addition, if the End of Descent (RWXX or MAXX etc.) waypoint is sequenced, the FMC phase of flight transitions from approach logic to effectively being in limbo as it assumes you are landing and is waiting to go to flight complete. The vertical deviation pointer on the ND is no longer displayed, which creates an issue if the approach is a Non-ILS approach using VNAV PTH. To "reset" the FMC, a cruise altitude (typically current aircraft altitude) may need to be entered on the VNAV CRZ page. This reestablishes the FMC in cruise mode thereby enabling transition to the subsequent descent and approach phases required for a VNAV approach.
I never thought of this. We were doing an ILS, so this wasn't a factor.
Namaste...
Thread
Thread Starter
Forum
Replies
Last Post
