A few Instrumet flight question

[mmrassi]

Hi,
Please someone help me with these terms.

1) VDP

2) Difference between FAP and FAF

Thanks

[brianw999]

Visual Descent Point, or VDP. This is a point on the MDA (Minimum descent altitude) of a non-precision approach profile, from where the aircraft would be able to continue its descent to the runway threshold while maintaining a standard 3 degree descent angle.
FAP The point, applicable only to a nonprecision approach with no depicted Final Approach Fix (FAF) (such as an on-airport VOR), where the aircraft is established inbound on the final approach course from the procedure turn and where the final approach descent may be commenced. The final approach point (FAP) serves as the FAF and identifies the beginning of the final approach segment.

So to answer the second point an FAP is a point where the final approach is deemed to start in the absence of a precision approach system where there is a beacon on the airport from which a distance to run can be calculated.

[mmrassi]

Visual Descent Point, or VDP. This is a point on the MDA (Minimum descent altitude) of a non-precision approach profile, from where the aircraft would be able to continue its descent to the runway threshold while maintaining a standard 3 degree descent angle.
FAP The point, applicable only to a nonprecision approach with no depicted Final Approach Fix (FAF) (such as an on-airport VOR), where the aircraft is established inbound on the final approach course from the procedure turn and where the final approach descent may be commenced. The final approach point (FAP) serves as the FAF and identifies the beginning of the final approach segment.

So to answer the second point an FAP is a point where the final approach is deemed to start in the absence of a precision approach system where there is a beacon on the airport from which a distance to run can be calculated.

Thanks for VDP description.
I would like to know why do they use VDP? Doesn't enough to have a MAP? On what occasions they use it?

[Gabriel]

Thanks for VDP description.
I would like to know why do they use VDP? Doesn't enough to have a MAP? On what occasions they use it?

The MDA, or minimum descent altitude, is a "floor" that you must not penetrate unless you have the runway on-sight. In the typical "dive and drive" approach, you descend to the MDA and, if you don't have the runway in sight by then, you keep flying at the MDA until either you see the runway and land or you reach the MAP (missing approach point) and go around.

So the MDA is not a point but a plane.

The "dive and drive" approach is pretty unstabilized, requiring a descent, then a level off at the MDA, and then another descent if you see the runway, with the corresponding significant changes in power.

On the other hand, the VDP is a point in the intersection of the MDA plane and the line that takes you to the runway threshold with a 3 degree glide slope.

In airplanes with vertical navigation capability you can fly a 3deg slope straight to the VDP and then, if you see the runway you land and if not you go arround, similar to what you do in a precision approach.

The VDP is important because the navigator needs its 3D coordinates (along the runway extended centerline, how far from the threshold, MDA) to make the vertical navigation towards it in an ILS-like fashion from which you can smoothly transition into a visual final approach and landing without changing the descent slope.

[brianw999]

...and going on from this I believe that some airports are considering using a constant descent rate on approach to help noise abatement. The reasoning behind this is that there is a constant noise level during a constant descent as against increased noise each time the aircraft levels off during a stepped approach when power is reapplied to level off.
During the constant descent the aircraft will arrive at the Minimum Descent Altitude where the final decision to continue and land would be made. Basically, if the field is in sight then carry on with the approach and landing. If the field is not in sight the it's go-around time for another try.

[3WE]

I believe that some airports are considering using a constant descent rate on approach to help noise abatement.

Semantics here- but it's not really that airports are DECIDING to do this.

It's more that ever-more-affordable technology is becoming available on more aircraft to generate a "virtual glideslope".

Without a fancy computer, Gabriel's "dive and drive" is a perfectly safe option (as long as you do it correctly) that works wonderfully with 1950 to 1980 technology!

It's more that airplane OWNERS are deciding to get newer and fancier GPSs.

Now, for an interesting twist: if there are broken clouds, there are certain instances where "dive and drive" can get you in where a constant "glideslope" would not.

You level off and fly along at the MDA and hit an area of higher clouds and identify the ground, you could do some visual maneuvering, but if the VDP coincides with a lower cloud, you can be SOL.

That being said, with a bigger, faster aircraft, there's something to be said for flying down an imaginary glideslope in a nice, stabilized configuration and power setting as Gabriel alluded to.

[MCM]

3WE,

It can be safe - but it is not the safest option, as you alluded to particularly in large aircraft.

The problem is that you need to level off at the MDA, and then when you get to the missed approach point you have to commence descent at the appropriate "stable" rate again - not ideal.

The "glideslope" style approach, with the whole thing done at 3 degrees, is the safest option.

You don't need fancy electronics to do it either - you just need some way of measuring your distance. You can do a stable descent with a VOR and DME only. Thats how they are designed. Sure, fancy electronics make it easier - but you do NOT need them. The only time it is impracitcal to do it is when you do not have a distance measure.

Brian - airports are instigating Constant Descent Arrivals - but its a slightly different thing. The CDA is from an intermediate altitude down to the commencement of the approach - it isn't the approach itself.

For example, at LHR it is from 6000ft down - the "CDA" part is commonly thought of as the 6000 to the glideslope intercept... (although technically its the whole way - but we will be on constant descent from glideslope intercept). Basically, CDA was introduced to stop us from descending to and flying level at 3000ft 10nm before we intercept the glideslope and making unnecessary noise.