737NG FMS question

[pilotho]

Just a quick question. On the 737NG FMS, once the runway centreline has been extended, what is the vertical profile being shown on the ND based on? I have always thought it's the GP but it never conincides with the GS.

[WallyWumpus]

By vertical profile, do you mean the VNAV diamond on the bottom right?

If so, it is working off the point you used to extend the centreline.

Wally.

[STBYRUD]

The VNAV path for a direct intercept leg is calculated as if you were exactly on this line and uses the standard VNAV descent path, whichever that may be in your configuration (fixed speed up until deceleration or geometric descent). So youre right, it usually does not coincide with the glide path.

[shlittlenellie]

If the centreline is extended the VNAV path is calculated based on the straight line distance from the aircraft's present position to the next waypoint. The VNAV path is effectively an angle calculated from your height above the threshold and distance to the threshold. If the path angle is the same as a the glideslope then they'll be coincident. Next time you're flying look at FPA and VB. FPA is the gradient you're currently descending at and VB is the angle derived from current height above threshold and current FMC calculated distance to go.

Picture it this way: if you're on a right base and the centre fix point "CF09" is to your right and you've extended the centreline by direct intercept CF09 with an inbound course of 090 then the VNAV calculation determines the distance to go as a straight line to CF09 followed by the LNAV distance CF09 to the threshold. Hence, if it's a wide base and long final and CF09 is only a few miles out from the threshold then VNAV will be showing high when you're not (due to your actual track miles being longer than that calculated by the FMC based on your inputs).

[STBYRUD]

Ermm... thats not how I understand it. You correctly mention that it will show high in this situation, but it shouldnt according to your logic. To my knowledge the FMS doesnt care if you are on the intercept segment or 20 miles offset, it will take your displacement along and paralell to this segment (instead of a continuously computed direct path to the waypoint) to calculate the path. In your example - it shouldnt matter if you are on the 270 radial or if you are 5 miles north or south of it. Hey, thats my observation at least... :confused:

[unablecpdlc]

So, what I have been doing is to take the distance off track from the Progress page 2 and add this to the CF09 (in the example above). Say, for example, that represents 10 miles, add the distance off track, multiply by the old 'favourite number' being 3 to give your require height in thousands. So as a worked example, you are three miles away from the intercept, (10+3)x3=3900, so if I am there or thereabouts on speed then fine.

Now I have been doing this for a while, and it seems to work nicely. What do you think about that then........

[flyby797]

It depends on which lateral mode you are. Say you are 15 miles to the rwy extended centerline towards a point 10 miles from touchdown on a heading that allows you to engage LNAV. Alt 6000 ft QNH (rwy elev. is 0ft)
If you're in HDG SEL, the vertical path diamond will show that you're presently 3000 ft above the correct path. FMC assumes that you're on the extended rwy center line.
Now if you're in LNAV, he 'll assume the present position so he'll show 1500 ft BELOW correct path.
I have not taken into account speed reduction schedule.
Hope that helps!
Flyby