FPA V/B and V/S on B737NG!

[slam dunk 4]

Hi all,

on the FMC DES page you have FPA,V/B and V/S. Can someone explain in very plain language what the vertical bearing (V/B) is for?

Correct me if I am wrong please:

For a given waypoint, I can insert a speed and level restriction. And to hit the waypoint, I can use the vertical speed which is displayed on the DES page. The flight path angel to hit the waypoint is displayed there as well...right? for example 2.3deg to meet the waypoint altitude...right? but what for or how are you guys using the vertical bearing information? Boeings' airplane system is not very helpfull I think

Thanks

[STBYRUD]

Whats not helpful about it? Yes, it shows you the FPA angle required to meet the most restrictive altitude target - and yes, you cannot directly set it on the MCP (unlike the newer Boeing models) yet you can just hit VNAV and it will take care of it... There is a lot of info displayed on the box which you cannot directly use but that doesn't mean its useless ;)

[ImbracableCrunk]

The V/S that is displayed is the required rate to maintain the displayed V/B. The waypoint is also selectable. You can enter the runway if you like, but I find it useful to enter the FAF.

[Kirks gusset]

If you use the VB in conjunction withe the VSD, the terrain awareness can be improved.

[TlomasLjung]

FPA is the actual flight path angle of the aircraft. It is typically 3 to 4 degrees in a descent.

V/B is the required vertical bearing to reach the WPT/ALT. This would be an altitude restriction in the LEGS page that you could enter (or delete) manually or with the ALT INTV button on the MCP.

V/S is the required vertical speed to make good the V/B.

As you approach your ToD the FPA will remain at zero (because the aircraft is in level flight), but the V/B and V/S will both increase until the V/B is at about 3 to 4 degrees, or whatever the FMC has calculated is the optimum value. If VNAV is engaged the aircraft will descend with a FPA equal to the V/B. The actual V/S will however be slightly different to the computed V/S because V/S changes during the descent.

Read more about the FMC:
http://www.b737.org.uk/fmc.htm (http://www.pprune.org/FPA is the actual flight path angle of the aircraft. It is typically 3 to 4 degrees in a descent. V/B is the required vertical bearing to reach the WPT/ALT. This would be an altitude restriction in the LEGS page that you could enter (or delete) manually or with the ALT INTV button on the MCP. V/S is the required vertical speed to make good the V/B. As you approach your ToD the FPA will remain at zero (because the aircraft is in level flight), but the V/B and V/S will both increase until the V/B is at about 3 to 4 degrees, or whatever the FMC has calculated is the optimum value. If VNAV is engaged the aircraft will descend with a FPA equal to the V/B. The actual V/S will however be slightly different to the computed V/S because V/S changes during the descent. Read more about the FMC: http://www.b737.org.uk/fmc.htm)

[Skyjob]

TlomasLjung, not quite correct these statements below:
but the V/B and V/S will both increase until the V/B is at about 3 to 4 degrees
If VNAV is engaged the aircraft will descend with a FPA equal to the V/B

VB shows you the direct line of sight constant angle from the aircraft the the next point in space (Waypoint) which has a defined altitude (at, ignoring the above or below).

So, be careful with VB (Vertical Bearing):
When the next waypoint is coded as an A (above) or B (below) VB does not recognise the use of them thus does NOT use crossing altitude/level but coded altitude/level only.

VB for CDA is great as it gives e.g. constant angle to FAF or other point used in procedure;
But when using it on an approach with minimum or maximum restraints, it does not reflect the path to be flown.

Use it wisely, and it becomes VERY USEFUL;
Ignore its limitations and be in for a surprise one day being high on approach or flying a level segment instead of a CDA.

VB is a calculation of distance versus height only!
VS as on the DESC Page indicates CONSTANT V/S required to achieve VB