A320 FMGS questions...
 

I have some simple questions for the A320 FMGS experts... I hope someone could help me.

1) Supose I'm in cruise and the destination airfield is bellow minimus for landing and I wan't to resume the flight using long range power setting, how do i do that? Should I work with the cost index?

2) Talking abot cost index, just for curiosity how your airline works with that? Mine has instructed us to always set it to 30.

3) Finally, in the MCDU "cruise performace page" there is a setting for cabin pressurization, what is that for? When shall I change that?

Thanks!

1) Try Cost Index Zero

2) We Seem to use around 14

3) Change it to a lower figure if you have ear problems (or a cold) Otherwise leave it to is own devices.

In the cruise the CI wll appear in Green so you cannot change it! You have options of using the Sec FPln or changing the cruise level or using selected speed to change things.

CI is a fuel/time/'conditions of the day' type calculaton that should really give a different figure for each flight. Many airlines however simply couldn't be bothered so they use a fixed figure as a 'best average'.

You can change the cabin ROD and the aircraft TOD will be re-calculated to make sure everything matches. However, if you type in a value that cannot be achieved it will simply default back to 350fpm.

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Since initial posting it has been pointed out to me that you can change the acticve CI providing you go to Selected Speed first and don't try directly from Managed. Haven't got a 320 handy today to play with but sounds reasonable!

CI
 

Well, on my Airbus;

1. The CI IS most definitely available in the cruise to be overtyped.
2. My Company SOP is to set CI to 25 during INIT.
3. My Company SOP is to leave CI to 25 and use selected speed when a different speed/MN is needed. e.g. turbulence or ATC requirements.
Hope this helps..........

:ugh: ndsakdasdkasldkj

..obviously the software you have installed as I,m able to change the CI at anytime...climb:cruise, decent...:confused:..also I question the use of selected to managed by flaps one.....today I tried it...had a CI OF 30 at criuse,280KIAS managed.....changed the CI to 70 in the managed mode,KIAS went to 306....all in a managed mode....

becareful about changing the cabin vs in the MCDU, as it will affect the altitude constraints on the descent. (unless you do it in cruise, giving the computer enough time to give descent predictions).

pic...why would you change the cabin Vs in the mcdu,....its automatic unless your in the manual mode....maybe I,m missing something here??????

On the 330, reducing the v/s to 250 fpm can save 1 - 200 kg of fuel in the descent.

CI can be changed at any time and we use a CI roughly depending on departure fuel cost. It definitely makes a difference and most of the time doesn't slow you down too much. Ours range from 8 to 20 depends on the airframe and airfield.

You can change the CI in managed, the speed target will alter accordingly. can help if you want a high speed descent - change about 10 miles before TOD and it works well.

While we are on - if you change alt constraints to conditional - at or below - and check you are still within safety altitude, your descents will be much more efficient, also if you have a be at by x before y, when you have passed it, do direct to the next waypoint and it clears the constraint behind you, otherwise you keep the crap profile for the rest of the descent :ok:

javelin....appreciate the info and was not aware of that feature on the 330....thought he was talking A-32O specific...and if he was we do not have that feature in our box....PB

The 330 has the new Pegasus FMGEC (FMGC on 320/1) box as do late model 320's and 321's. That may be the difference.

What about a long term speed change form computed speed?

"What about a long term speed change form computed speed?"

We leave it in selected. I can't imagine too many scenearios where we fly(i.e. non ETOPS) that would warrant any different.

" ndsakdasdkasldkj"

Sorry Rejeh....... too subtle for me............

Cabin V/S
 

Hello there!

To change the CABIN VS can be useful if you are expecting a major shortcut on your descend, leading to higher sinkrates; much less Track Miles on the FMGS.
You will avoid "CAB Press" Ecams and won't be "dragging" the cabin behind you...

LNB

You will however have to be descended at quite some rate to leave the cabin behind. The only times I've seen it occur is when we had descent rates of 6000+fpm :eek:

See below...

It depends. First, what phase of flight are you in when you make this decision? Also, you want to resume flight to where? Okay, you mention Cruise, but I'm not sure if this means that you are in the enroute phase (ALT CRZ Mode) or if you have descended towards the airport, then selected a cruise mode after finding the weather is below minimums.

I'll assume that you are in the cruise phase, and have determined the airport to be below minimums prior to the activation of any arrival procedure.

Now you have two choices. First, if the weather is expected to improve, you can select a different cost index (See Airbus document "Getting to Grips with Cost Index"). Per that document, select a cost index lower than your current selection (As low as CI "0") and continue to the airport.

The selection of the lower CI will provide profiles and FMGS "assumptions" that will result in less fuel being used. However, some other items are going to experience an increase in cost. But, at this point, fuel is the important issue, so you are well justified in disregarding the cost's of labour, hourly lease costs, etc, that the cost index also considers.

Second - depending upon where you are in the cruise phase - you would divert to either your Enroute Alternate, or you Destination Alternate.

Either way, your goal is to arrive AT either of these alternates with - at a minimum - your final reserve fuel (Holding Fuel) PLUS your 5% route reserve fuel.

That said, remember that "cost Index" changes a bunch of things both in the aircraft during flight, and on the ground in the bean counters office. They include, Speed, Mach, Optimum Altitudes, Profiles, Fuel consumption, Fuel Costs, etc, AND considers the costs of labour, lease costs, maintenance, etc.


Most airlines have a "Set" cost index. However, if your airline operates sectors of vastly different distances, they would find it more economical to determine a cost index for each sector.

For example, a 3 or 4 hour sector would operate quite economically at CI 30 or 35. Furthermore, you would find that the FMGS Optimum altitude would probably be around FL 350 with an average weight and appropriate fuel load.

However, a 30 minute sector using cost index 35, would provide an Optimum altitude of about FL320. But, on this sector you would probably only operate at FL 250. So, by using cost index 35, you get profiles based on that, and FMGS "assumptions" based on the climb to FL320, but you never actually get to that altitude.

Another example. If on this 30 minute sector, you had to carry fuel for an alternate that was 90 minutes away, you could fly the trip to your destination with an appropriate CI for that sector (CI 170 - CI 300), then, if you had to divert, you could change the cost index to something appropriate for THAT sector (CI 30 - CI 50).

If, on this shorter sector, you enter a cost index of about CI 300, you will find the FMGS optimum altitude would be calculated to about FL250, so all the profiles and FMGS "Assumptions" will be based on that altitude - the altitude that you are actually flying. Therefore, operations on this sector would be more economical than with using an inappropriate CI for the given circumstances.

Either way, unless you are struggling to get extra time from your fuel tanks, your company SOP will prevail. If it say's "...use cost index 30..." and you like your job, use that cost index!

This is displaying the defaulted values of the cabin altitude descent rate used for computing the descent path (Change your cost index prior to descent and see what happens to it).

There could be any number of times when you may need to change this, but after almost 10 years flying the Airbus, I haven't had to do it once.

CJ..great info.....the default setting on our A/C...is 350ft/min..(Airbus standard) I believe...changed that to 500ft/min and as you sugest the CI did change,which makes sense....like you have never had to change that that ever.
My earlier posts suggested we dont have that feature...seems its only on some of the A/C...why? I have no idea......went through volume 2/4 to try and find an Airbus chart that would show scales and uses of changing the defalut setting...no luck....is there one???//PB

No chart that I'm aware of, only two statements in the FCOM, "...CLIMB 500 FT/MIN ... DESC 300 FT/MIN..." but those are on page 3.02.21 and relate to Manual Cabin Pressure Control. So, that information is not exactly relevent.

I'll keep looking and get back a little later...

CJ....yeah mate,would like the info if you do find anything....seems that if you are going to change a "default" there should be a reason for it???? ..I dont (normally) change things unless I have the info or a reason too ,but in this buisness there is always something to be learned.....please fwd...PB

Good attitude. Generally, the Airbus is a "Push" button aircraft. Everything - typically - will go more smoothly and safely with that thought in mind ("Selected" anything, will increase my attention in one form or another).

Airbus published a number of "Golden Rules". One of them, "Use the appropriate level of automation for the task", is too often either forgotten or missunderstood.

Fly safe, and I'll forward what I can find.