757/767 LNAV behaviour in conditional waypoints

[teropa]

Hello !

B757/767:

In the procedure below the FMC manual states that if 4000ft hasn't been reached at LAS280 then the plane will continue without turning.

What would happen in this case, if the altitude is not reached ? Will the FMC give up at some point and drop out of LNAV?

Also, will this behaviour be consistent for all the conditional waypoints ((INTC), Vor and DME intersects, etc...) ? Ie. will LNAV be dropped at some point and if so, when and what AFDS mode will take over (HDG HOLD?).

http://pilots.svil.fi/teropa/jako/cond.jpg

cheers
Tero

[HPSOV]

Tero, my guess would be that the waypoint will remain active and the plane will just keep flying its heading. I dont see why LNAV would drop out, it will continue to do as its told... fly the heading until 4000ft.

[slamer]

How about the "Hdg Sub-mode"

[teropa]

The What ?? Never heard of that.

[pogop]

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.

[FunctionedSatis]

I think i understand the question allough i may go off half cocked here.

On the left of the CDU is the LNAV profile, so long as u are on a good intercept heading LNAV will engauge and the aircraft will follow the profile.

On the right is the VNAV profile, 4000A indicates 4000ft at or before the waypoint, the FMS will calculate the required climb/decent rate to attain this requirment, climbimg the FMS will recalculate through the Thrust Management Computer the best thrust required from the engines to get you there. On decent the FMS will prbably command idle hold and may be more drag wich will appear in the scratchpad of the CDU. If the calculations dont add up the the scratchpad will show unable next alt. Meanwhile LNAV will continue with its profile the 2 modes are totally independant.

Now this if off the top of my head i could be inacurate in some areas but i think thats the gist of it.

Stu.