PPRuNe Forums - View Single Post - 757/767 LNAV behaviour in conditional waypoints
Old 4th August 2004 | 10:24
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pogop
 
Joined: Feb 2001
Posts: 18
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From: United Kingdom
The way the waypoints are written on the CDU implies to me that the FMC does not see the altitude requirements as being the CONDITIONAL part of the way point. If I remember rightly, if you are to fly to an altitude before turning then the altitude makes up the instructions on the left hand side if the CDU (such as for missed approach procedures). The waypoint position on the map display then moves about based on climb rate so that it corresponds with when the altitude is reached. On the example above the aircraft will fly to the waypoints as they are fixed points - DMEs and VOR radials - regardless of altitude. If the altitude requirements on the right hand side are not going to be met then you will receive an FMC MESSAGE stating UNABLE NEXT ALT in the scratch pad of the CDUs, but the aircraft will continue to follow the turns in the SID.
What would be evident if these waypoints are conditional (but not regarding altitude) is that the aircraft will fly over the points before comencing the turn to the next point, unlike during the cruise etc. when the aircraft will start the turn before reaching the waypoint.
Hope this helps.
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