A320 VDEV symbol

[redout]

I was studying about the PFD and came across this. "Upon reaching TOD and down to final approach guidance a magenta VDEV symbol is displayed. It indicates the aircraft vertical position in relation to the FMGC vertical profile". So my question is, does it just act basically like a kind of large glide slope ?

[SoundLesS]

yes it does...

[Chrome]

Basically yes.

But unlike being on the glide slope of the ILS, you don't really have to religiously have your VDEV on the dot on the DEV indexes. Since VDEV is calculated taking into account managed descent speed, current wind data and your lateral/vertical flight plan if you expect some of those parameters to change while descending then you can still just have it just as that, as a guidance as you descend.

[Cardinal]

Yep. The symbol also happens to ride along the altitude that the aircraft "should" be at. This limits the resolution to the plus or minus 500 feet displayed on the PFD. If one want's information outside this range you can pull up the Prog page on the FMGC and read the vertical deviation in Feet.

[A4]

Hello Redout.

Yes in essence it can be thought of as a sort of Glideslope. It is often reffered to as "the doughnut" because it is a small magenta circle that slides down the side of the PFD Altitude tape.

Whilst it is useful for watching a managed profile there are some caveats - remember the FMGC calculated information is only as good as the information put into the FMGC - keep it up to date!

For example, lets say you have an intermediate constraint at a point in descent i.e FL200 at VOR "ABC". So you leave FL350 in "managed" descent and you're "on profile" so the doughnut is centred. You achieve the required constraint of FL200 and now the doughnut starts to indicate you are going "high" (moves down the PFD Tape) backed up by the PROG Page deviation. The FMGC Software does this irrespective of how many track miles you have left - it's actually not that clever. So you maintain FL200 and your deviation now indicates you are 3000 feet high on the "FMGC Profile" (whereas in reality you may be 8000 feet low based on the 3 X table rule). So if ATC now clear you to FL 180 and you choose Managed descent the aircraft will set Idle and "dive" to try and get back on its (incorrect) profile.
There is a cure for this. If you re-enter any constraint further down the plan (perhaps 220 knots approaching the hold), this will force the FMGC to "look ahead" and recalculate the profile. So when you reach your first level off i.e. FL200 and the doughnut starts to slide to the bottom of the PFD - just re-enter ANY constraint downroute and hey presto, a recalculated profile of perhaps -9999 feet. Now when ATC clear you to FL180 the FMGC says - ok I'm below profile, so I only need to go down at 1000 FPM....not 3000 FPM!

A "DIR TO" has the same effect (recalculte) but if you're in HDG you MUST pull HDG again (so as not to enter NAV) so I always teach guys to re-enter a constraint - much safer and covers all cases.

Hope it helps.

A4

[redout]

Thanks lads the help was much appreciated.

[Chrome]

A "DIR TO" has the same effect (recalculte) but if you're in HDG you MUST pull HDG again (so as not to enter NAV) ...

A4,

Does it make any difference in anything if we select 'Abeam Points' for a 'Dir To' on a descend?

[A4]

Chrome,

Any DIR TO will force a recalc. Just remember if in HDG a DIR TO will put you in NAV...... careful :eek: The FMGC should look ahead to the next constraint or all the way to touchdown. My previous description was about Idle Segments and Geometric segments....... a recalc effectively creates another "Idle Segment".

[Chrome]

Yes the TRIs were very particular in getting that to stick to our minds during training :)

If a 'DIR TO' in descend involves to an FAP, would the FMGS calculate the new Idle Segment based on the altitude constraint at FAP or the start of the decel point?

[Dream Land]

Direct to is fine, just make sure the GREEN line reflects what you are actually doing, in our NAV data base going into Cancun, the green line went to the VOR on the field before it back tracked out to the Center fix for the ILS, don't get caught in this trap.
D.L.:ok:

[A4]

Chrome,

I don't know the definitive answer to your question..... but if you do a DIR TO the FAF/P and it has an ALT and SPD constraint then the FMGC should take into account a DECEL point so as to achieve the constraints.


Dream - you make a very valid point which reinforces why it is so important to check tracks and distances whe briefing through the FMGC. A good example is the approach into LPFR/FAO (Faro, Portugal). The VOR to R/w 10 has a track reversal in the final approach. i.e. the approach terminates closer to the runway than the final descent point for the runway. It's only about a mile or so and when looking at the displayed plan it looks ok. However if you do not correct it, as you turn onto final a huge 10 mile orbit then appears to sequence the points. If you've just let the aircraft calculate its descent profile you are now about 3000 feet to high.....:ugh:

I first rasied this issue about 8 years ago, but it doesn't seem to have got through to the database providers yet.......

Remember - check the FMGC in the first place, keep it sensible (TO W/POINT) and back it up with 3 x table and you won't go far wrong.

A4

[Essential Buzz]

If you don't wish to play about with "DCT TO's", just re-enter the temperature on the PERF APP page and hey presto ... descent path is re-calculated! (It doesn't even have to be different, the same temp will do.)

Very handy with geometric segments, particularly if you have troubles with your 3x tables.