747-400 Fmc Vnav Path

[TENTOTEN]

Can someone explain to me how the VNAV works when you do a runway extension from the Final Fix.
If you are tracking for straight in appr and do a runway extension from a point on the ILS the VNAV seems to calculate a 3 deg slope back from the way-point you have extended from.
However if you are approaching the runway from a reciprocal position (downwind leg) and you do the runway extension from the same fix the VNAV seems to draw a path from the aircraft's current altitude to the height displayed on the fix. Obviously this path is of no use to anyone.
Why is it so????

[flite idol]

Better explaination below...I hope!

[TENTOTEN]

Thanks for your input. I understand how the slope works from 50' to FAF Alt.
The area I am confused in is from FAF and beyond. ie FAF is 10 nm how is the path generated when the aircraft is say 20nm from the FAF??
For 1) straight in appr
2) joining downwind

tks

[brentmarcus]

VNAV PATH before the coded approach

If you are downwind on a heading and you execute a course intercept to the initial approach fix ( two ways to do that using the APP/DEP page or Legs page FF twice and add a course) and the initial approach fix has a hard altitude not a 4000A for example the VNAV path will be flown providing the SPD window is closed and the FMA is VNAV PTH. For this to happen nicely you need to input speeds and remove A from the altitude constraints, there are a number of variables to this and it would take some time to explain, the variables are reduction in speed, if the aircraft is flying VNAV SPD then it is speed on elevator and may require thrust to stay on profile being generated by FMC. There is lots more to VNAV but when understood works really well.

[flite idol]

When you select and execute an approach ext on DEP/ARR 6R you will have an altitude constraint over that fix. VNAV will still provide path information so as to arrive over that fix at the constraint altitude. You will be off the LNAV track while on a vector and as the FAF is now the first fix with a constraint then I assume the VNAV path is constructed as an idle thrust descent with applicable deceleration segment to that fix along the extended LNAV track. As such the information is still of use, you will still intercept that path at some point and still need to be at the FAF at the intercept altitude so it is still valid as a guide. Use all available info as always, DME , drag circles etc.

[TENTOTEN]

If you are 20nm (say at 4,000') on a sraight in approach and the FAF is 10nm with hard ALT 170/3000. You execute a course intercept via legs page the VNAV PATH will show you immediatley low on profile as it seems to generate a 3deg path from the FAF to your present position. So if the aircraft was 6,000' it would show you on slope.
Now if you are in the downwind position at either 4,000' or 6,000' after course intercept is executed in both cases VNAV PATH will show you on slope.
Do you have any info on the above two scenarios???