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atpcliff
11-05-2017, 11:31 AM
We did not get this info in our training, and many pilots at my airline, including a senior Check Airman, had never heard of this:

This is a Boeing product with the Honeywell FMS. I don't think it matters if you are using the old type of box or the NextGen FMS:

We were vectored off of the STAR, and so had extended the approach course from an approach fix, as per standard procedure.

There was a lot of traffic, and, for the first time in my career, we were vectored past the airport on an upwind vector, on the side of the airport away from the approach. Since we passed the fixes on the approach, within 22 miles, the approach vanished from the ND and the FMS.

We had a pilot in the observer seat who had 9+ years of experience with the Boeing.Honeywell product, and had seen this before (for both myself and the PM, we had not heard of this).

The third pilot noted that our approach had disappeared, with only the MAP still on the ND/FMS. We re-loaded the approach, and then were vectored crosswind/downwind/base (about a 25 mile base) and were back to ops normal.

To fix this:
The 22 mile zone is appropriate for sequencing the fixes during enroute navigation. But, in the landing phase, it is too far from the airport. I think there should be a change, so when you close-in to the airport, and are manuevering to land, that the 22 mile zone should be removed, or the zone should be reduced to something like 1 mile, for example.

Namaste...


Adlerdriver
11-05-2017, 11:49 AM
People are unfamiliar because it rarely happens and isnít worth the extra expense to fix. Make everyone aware and simply pay attention.

About 4-5 key strokes plus an execute? With a 25 mile base, sounds like you had plenty of time for that.

atpcliff
11-05-2017, 11:55 AM
We had plenty of time to fix it, but if we hadn't had the third pilot who had seen that once before, it could have become more of a problem.

It's kind of like when you get the "Route Full" notification on the FMS. I have had it three times. Two of the three times, the Boeing FCTM fix didn't work (our airline manual doesn't cover the Route Full situation at all). But, after I had seen it happen once, the second time the official fix didn't work, I knew to switch to Route 2 as a workaround.


Twin Wasp
11-05-2017, 04:49 PM
Or just wait a bit to extend the the course fix.

atpcliff
11-05-2017, 05:32 PM
Most pilots at my airline extend the course fix after they are taken off the arrival, and are sure they won't be vectored back onto it. The situation above was very abnormal...never had been vectored on a long upwind before...

galleycafe
11-05-2017, 06:09 PM
Airbus 320 series box does this too.

2old2fly
11-06-2017, 07:53 PM
I've had it happen in the A320 just once, never saw it in a Boeing.

atpcliff
11-07-2017, 12:41 PM
I wonder if it happens on a box used for the nice biz jets, like the Falcon 7x...?

My understanding is that the automation/FMC for the Gulfstream/Falcon/Bomardier/Emraer biz jets is much, much better than the Boeing or Airbus systems...

cougar
11-07-2017, 10:50 PM
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.

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.

galaxy flyer
11-09-2017, 01:23 PM
I wonder if it happens on a box used for the nice biz jets, like the Falcon 7x...?

My understanding is that the automation/FMC for the Gulfstream/Falcon/Bomardier/Emraer biz jets is much, much better than the Boeing or Airbus systems...

Not in the Collins Bombardier types. Iíve been vectored all around and never lost the approach. But, there are trade-offs in the FMS installs and software.

GF

awax
11-09-2017, 02:25 PM
I've never experienced this, thanks for the heads up.

Absent the observer, my guess is it would have been about a one potato count before you guys figured something out. ;)

FastDEW
11-10-2017, 06:03 PM
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.

trip
11-10-2017, 06:24 PM
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

It does a similar trick in the CRJ but you don't lose the last fix, being the runway. If you see it sequenced through all the fixes just reload it, wait until your back on the approach side of the airport to execute it, then it has all the fixes back.

atpcliff
11-11-2017, 02:51 PM
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.


Thanks a tonne for the info!
Namaste...