A320 VDEV Question

[B737SFP]

Hello guys... I flew the 737 for the last 5+ years and recently finished my line training on the A320.

If anyone could help me with the following question, I would really appreciate:

Let's say you're on your descent procedure using managed modes (DES, NAV) at ECON speed (300ish kts) and you're on profile.

The ATC requests a speed reduction to 230kts and you pull speed to comply with it.

Right away, you'll get high on the profile...

Now, here comes the question: is the VDEV information on PROG page related to the speed you pulled and selected, or to the previously used speed from managed mode (ECON) ?

I know (by looking at the F-PLN page), that all predictions (at least those related to time) of a specific phase will be based on the selected speed, but don't know if the same applies for the VDEV calculation.

If possible, please, give the FCOM reference.

Thanks in advance.

[safelife]

The VNAV path remains calculated for the managed speed.
The predictions are for the speed you selected, though.

[MD83FO]

been flying the plane for 10 years and stopped looking at the vdev a long time ago, haven't found it relevant unless complying with an altitude constraint. still altitude times 3 is the best, if vertical speed is less than half your ground speed you need either drag or speed. if no atc speed, expedite mode works wonders if available, if speed approaching vmo to fast a little touch of speed brake raises the elevators immediately to keep the speed in check.

if not complying with constraints, relevant track miles to touch down is what counts, frequently the don't match the flight plan sequence. this way the speed brake can rest and passengers are happy.

[giggitygiggity]

Originally Posted by safelife

The VNAV path remains calculated for the managed speed.
The predictions are for the speed you selected, though.

Yup, this is right. It will give you the progress VDEV for the originally planned managed descent speed.

You can get it to recalculate a managed descent path at your new ATC speed although it is a little fiddly. Reset a lower cruise altitude in the progress page if you're able to level off. This puts it back into ALTCRZ and then you're able to reenter a new speed in the Descent PERF page according to the one you've been given by ATC. It will then assumedly tell you to DECELERATE and get descent clearance etc, then the PROG VDEV figures will be accurate for your new shallower descent profile.

Level Off and reset ALT CRZ to the current level.
Modify managed descent target on the Descent PERF page to the ATC speed.
Now you have the new profile in VDEV according to your assigned speed.

It's probably in FCOM-PRO-NOR-SUP somewhere if that still exists.

I personally find it very useful and saves the calculating for somebody that has always been crap at maths. It's only as useful as the info you put into it. Try using the secondary flight plan to plan your minimum track milage and work out the height you'd expect to be at your most likely short-cut divergence point and then stick that in as a hard height against that waypoint in the primary and then the VDEV info is useful.

[B737SFP]

Thank you very much for the answers guys !

We never stop learning...

Safe flights for all of you and a happy 2018.

All the best.

[A320ECAM]

I know youve got the answers you need but a useful tip for any aircraft...

(Current ALT - Target ALT) * 3 = TOD
+1 for every 5kt of tailwind

FL390-3000 = 36*3 = 108
Wind is 103/30 = +6
TOD = 114nm

[B737SFP]

Hello A320ECAM...

Thanks for the tip, my friend ! That's pretty much what I was taught on the 737 and what I did during my time flying that type... x3 always worked fine for me, and I gotta say that the energy management on the 737 is a little bit more tricky than on the A319/320.

Recently, though, I've read in a "Getting to Grips" that the Airbus recommendation is to do (Current ALT - Target ALT) / 3 or miles to go x 3 + the appropriate margin for speed reduction.

For your example, that would be:

360/3 = 120 + 6 ~= 126nm

For big altitude changes the result is a little bit different, but not so much for smaller changes...

Let's say I want to be at the IAF at FL050 with green dot speed (200kts) and I'm now at FL100 with 250kts:

50/3 = 16 + 5 nm =~ 21nm

Or

5*3 = 15 +5 =~ 20nm

What do you think?

[vilas]

A320ECAM forgot to add 10 miles to the calculation otherwise it works out to the same figure.

[Fursty Ferret]

Honeywell VDEV always seemed more accurate than Thales, IMHO. Making sure you've got up-to-date descent winds in helps. If a Thales FMGC you can modify the managed target during descent which will start giving relevant guidance again.

However, at low level and intermediate configuration it's very good, easily sufficient to approximate the vertical guidance you'd get from final approach mode and handy for visual approaches.

[easymxp]

Another quick way would be changing the des speed lim, usually 250/FL100 with your new speed.
In the example above, if you are descending at 270 passing FL180 and Atc tells “speed 230”, simply change the speed lim in 230/FL180. This way your profile is updated with your speed. At high levels it’s quite useful. Below fl100 I usually just go open des and use hockey sticks to fine tune my descent

[giggitygiggity]

That's a good idea, does it only work when you need a slower descent speed than the one you had originally set in the descent perf page?

[easymxp]

Yes, it's correct. I use this technique when I get given a lower speed than the one originally planned

[radical_100]

The VDEV is with respect to the FMGS Computed Vertical Profile (DSC-22_20-50-10-28 P 131/162). This is the managed speed shown in the PERF DES page.

On the Thales systems (and newer standard Honeywell systems), you will be able to modify this speed on the PERF DES page directly. In which case you don't have to resort to some of the old tricks that existed (i.e. re-enter CRZ FL, modify Vert SPD LIM etc).

Remember, the VDEV is not going to be very useful unless you are getting down to your final altitude at high speed & decelerating from that point. If you are configuring your aircraft earlier due to an ATC speed limitation and trying to keep a continuous descent going, the VDEV is going to show you deviation with respect to levelling off and decelerating.