Climb predictions on ND vs MCDU page (A320)

[pineteam]

Hello guys,

Once again, I'm getting confused and frustated not to clearly undestand: Imagine you are in NAV mode, speed managed in managed climb to FL320. You must reach your cruising altitude before waypoint Bravo. But you won't make it with a managed speed of 300kt. The TOC arrow is like 15 miles ahead of the waypoint on your ND. You decided to use Vertical speed ( eg VS+2000) instead of selecting a slower speed. Now the aircraft flies rougly at 260/270kt to maintain the vertical speed. My question is: I noticed the TOC arrow on the ND was not really accurate. Like I reached FL 320 way before the actual prediction on the ND. and also the prediction on the ND (TOC arrow) and the TOC predictions on the FMS are not always exactly the same... Can someone explains please? Does the FMS predictions assume selected or managed speed only? Looks like it gets confused when using vertical speed and the actual IAS is away from for the target speed... please bring some lights on this case. Aircraft was a brand new A320 with Thales FMS.

Thank you.

[767-300ER]

The MCDU time predictions for waypoints will be based on the climb profile and the cruise profile and it wil be the most accurate. Assuming accurate winds have been programmed.

The waypoint predictions in the top right of the ND are based on current G/S so they are not reliable for ETAs.

The Airbus TOC and descent arrows are notoriously inaccurate and are slow in updating as the vertical trajectory changes.

[FlightDetent]

There is a certain logic how the arrows get created. I've seen it once surely, my memory only retained the idea not to look into it again.

Various inputs are factored but not necessarily all to catch what's going on. E.g. during descent to FL 100 the arrow only indicates the point where the aircraft would have slowed down to 250 kt after reaching that level.

The scenario when climbing VS+2000, speed has dropped to 260ish and the managed target is still 290 kt, for instance, would be a one when the hockey-stick is surely meaningless.

[vilas]

pineteam
Unfortunately in FCOM the words of wisdom are scattered in many places so it becomes difficult to fully understand a system especially like the auto flight. This is what I found in one place.

The flight crew should rely on the ND for short-term predictions. It indicates what the aircraft will do under the currently engaged modes (selected or managed). The flight crew should rely on the MCDU for long-term predictions, when managed guidance is active or about to be reengaged.

[pfvspnf]

V/S is not a good mode to try and do that fyi:|

[pineteam]

Thank you for your answers guys.
Vilas, thank you. I read that also. Not much from the Fcom..
I tried Yesterday quickly during climb. I pull V/S: +2000 ( it was in CLB with a VS of about 1100') And the TOC arrow on the ND moved backward and the distance to TOC was pretty accurate. On the MCDU performance page on the other hand, the TOC distance remained unchanged.

V/S is not a good mode to try and do that fyi:|

I don't want to open a new debate. I heard that so many times during line training. I do agree at high altitude on an heavy aircraft with tigh buffet margin, definetely not a good idea. But when you still below FL300 in smooth condition, using vertical speed is not a problem as long as you monitor carefully your PFD. Just keep a safe margin from GD speed. I quite like it actually especially when you have to reach an altitude in a certain distance or time.. If I set 1000'/min passing Fl220 climbing Fl320, I know in 10 min exactly I will be at Fl320. Add 1 or 2 min for safe margin in case you will need to reduce it to keep speed above VLS. Cause in Manage speed at 300kt+ how many times the plane pitch up and down to maintain the speed and V/S often falls close to zero. Kinda annoying. = )

[pfvspnf]

You must be a real charmer during your line training

[wiedehopf]

@pineteam

i'm not even a pilot but as far as i understand you should look up your Vx (best climb angle speed) if you are concerned about reaching a level. don't know if it's as low as VLS but i doubt it somehow.

going below Vx would be very uneconomical and reduce your V/S. autopilot would then pitch up further to keep the V/S and you only reduce it reaching VLS meaning you are wasting fuel and not even achieving your max angle of climb.

but i guess you normally set your V/S trying to keep it near your managed speed and not even going close to Vx or VLS.

[hph304]

Vx for Jet aircraft is around (at?) minimum clean speed(green dot). Especially when the AC is heavier, VLS can be at green dot.

Note that Vx and Vy are very different from prop to jet AC.

Gotta love the guys that are afraid of using V/S on the bus... I don't really think the constant pitch changes are very comfortable.

[pineteam]

You must be a real charmer during your line training

That was during my line training as fo when I did not know anything about Airbus. I never argued with any instructors or any line captains. My training went smooth thank you. I'm now captain and Instructors with common sense will never say anything if I'm using V/S during climb when the buffet margin is wide. I'm a pilot not a robot. I use the automation and all the tools Airbus has to offer while fully undestanding the consequence if something goes wrong. Yes using Climb mode is safer but does not mean using vertical speed is dangerous.



Gotta love the guys that are afraid of using V/S on the bus... I don't really think the constant pitch changes are very comfortable.

Oh my god thank you. At least one guy to support me. Tired of people afraid to fly close to VLS or green dot speed. If you inadvertently goes into VLS you won't die, you will have a reversion mode. You don't have to do anything. Lol. Gotta love the guys also afraid to use expedite mode at low altitude cause the target speed is Green dot. Funny how those people forget than when they are flying in the sim on single engine they fly at green dot speed....

[Hawker 800]

using vertical speed is not a problem as long as you monitor carefully your PFD. Just keep a safe margin from GD speed.

I really hope you are not a Captain or pilot of any sort.

[pineteam]

Sorry for you then. I am. And got upgraded after 2 years and 3 months only so I guess I'm not doing too bad. :)

[Hawker 800]

That's Air Macau off my list then. I'd rather have a crew with a bag full of experience up front.

[pineteam]

What do you know about my experience? Air Macau is not my first aviation job. Don't judge a book by his cover. Since now my thread is spoiled, I'm happy to listen to your concern about using vertical speed during climb. Maybe you can write to Airbus also cause as far as I'm aware it's not written in the Fcom or Fctm that using vertical speed is dangerous.
And I have flown with captains who fly Airbus for 10 years + and I won't trust them to fly my family. That's how bad they are. Just because they fly for a long time does not mean they are amazing pilots...
Vilas if you read this I would like to have your opinion also since you are a master on Airbus.

[pfvspnf]

Wow they must be really short on manpower .

To not have some of the basics down and coming to pprune for clarification while you are commanding an a320 is shocking

[pineteam]

Give me a break man. You have :mad: to say such a thing when you are the one who come on my thread and said that Vertical speed should not be used. :mad: Where do you get that from? From some captains who don't know what VLS means? Nowhere in the Fcom or Fctm there is a restriction about it. So check your sources before insulting Air Macau and I. The questions I asked are usually not written in Airbus manuals or are ambiguous. I do study a lot before asking questions here. I'm very far from knowing everything but I try my best to improve my knowledge. And it does not hurt to ask questions on Pprune. Lots of guys here like Vilas and Open Des and many others are very knowledgeable and I really like hearing from their experiences. It's with this attitude that I managed to get upgraded so quickly with no previous jet time experience.

[Hawker 800]

It's with this attitude that I managed to get upgraded so quickly with no previous jet time experience.

No jet time then 2 years and three months a skipper. What's this industry come to?

Air Macau is still off my list. If it's not good enough for your family, it's not good enough for me.

And I have flown with captains who fly Airbus for 10 years + and I won't trust them to fly my family. That's how bad they are. Just because they fly for a long time does not mean they are amazing pilots...
Vilas if you read this I would like to have your opinion also since you are a master on Airbus.

[pineteam]

I joined Air Macau with 1900hr PIC... And was upgraded with 4500hr total time including 2000hr+ on type. What more do you need to be confortable on an aircraft?

Concerning the bad pilots, they are everywhere... You should open more your eyes and maybe the FCOM also. Please stop trolling my thread. Thank you.

[vilas]

pineteam
First the VS mode. Nothing is forbidden off course but some things are recommended. Aversion to VS mode may have it's origin in the fact that in VS mode the speed(global as it is called) protection is a later development and wasn't their from the beginning. Secondly the priority of this mode is VS and not speed so sometimes not an appropriate mode.
As far as command upgrades are concerned there is regulatory minimum, beyond that it is on requirement. Without proper attitude and aptitude mere thickness of logbook is like growing old it just happens. Experience can be what Oscar wilde had said "Experience is merely the name men gave to their mistakes".

[CaptainMongo]

Agreed Vilas

V/S (very seldom as we like to say) mode use by the PF increases the PM work load. I can count a dozen times where the PF (FO in my case) has used that mode and forgot to monitor the airspeed - in a climb and/or descent. I have seen pilots use it incorrectly in managed climb and descent WRT appropriate altitude set in the FCU. I have occasionally seen incorrect use of speed brakes with V/S which results in a discussion of what V/S is and is not.

At some airports noise requirements may necessitate use of V/S to maintain a constant descent, some descents may require the use of V/S but personally, I don't believe V/S is an appropriate mode in most cases - my opinion only - not procedure.

[hph304]

If ATC gives you a descent rate, do you use V/S or open descent?
Same question for climb rate, what mode do you use?

A good pilot should ALWAYS monitor the energy level of the aircraft, especially in climb or descent. This means eyes on thrust, PFD, etc. Normally you wouldn't have any problems in V/S mode if you apply this correctly.

And to the guys getting personal and offensive because they disagree with a certain point of view: really mature.

[vilas]

pineteam
Instead of experimenting with VS there is more accurate and safer way given in airbus document of getting the best climb performance from the FMS. Since climb is the most fuel expensive phase if you change cost index to zero it operates at a speed which makes this phase shortest. But don't forget to change the CI at TOC otherwise it will cruise at snail's pace.

[Vessbot]

If climbing in VS increases the PM's workload, then the PM wasn't doing his/her job in the first place.

[underfire]

vilas... that is correct, for CI=0.
Unfortunately, in many cases, not an option with airspace/noise constraints..

All;

While it is a higher workload, understanding the ac parameters does have value, for when to use the settings, the associated parameters, and understanding the variables, albiet brute force.

For TOD, optimized descent is a much different animal.

[underfire]

Jimmy, thanks for the input. As you note, the cost is relative.
Saving fuel has been a marketing ploy for quite some time (dont forget the carbon you save!)
TOC is far more difficult to show benefit, for all the reasons you mention, with all of the real time variables, especially winds and bleeds.
Time of flight that must be considered to see potential value if cost is the issue vs time.
TOD has far more value, especially in a coded procedure. The original 'idle descent' sales pitch was crap, especially mixing RNP and non-RNP traffic in the terminal environment, with ATC managing the flow.
Quickly learned there was no value, and actually penalties.
TOD management to FL10, now that is much different. A simple waypoint on the STAR to the system to calc and one at FL10 will optimize TOD for the ac real time (we can add a time constraint), will provide the optimum fuel, time and flow management for the flight.
Driving as fast as you can, to sit in a hold pattern?

[jimmyg]

A bit more expanded from my previous post:

The best Rate of Climb occurs at a speed where there is Maximum Excess Power. As jet engines directly produce Thrust, not Power, it is necessary to consider Thrust multiplied by speed (Power = Force X Velocity). Thus, for a given Thrust setting, Power increases as TAS increases. Thrust actually ‘dips’ as speed increases, but then there is significant Ram recovery at higher Mach Numbers, thus further increasing Power at higher speeds. If we examine a Power Required and Power Available graph for a jet aircraft (about the only time that we’re interested in Power for a jet), it is observed that at the higher speeds, Power Available Vs Power Required (thus excess Power) is close to parallel with minimal divergence over a fairly wide speed range. Speed variation of the order of about plus and minus 20 to 30 knots from the optimum, shows only a slight reduction in excess Power with this speed variation.

For a fairly Light jet aircraft such as the A320, a fairly typical best Angle speed is 250 KIAS, best Rate 280 KIAS, and a typical Economy climb speed is 300 KIAS. Best Angle speed is at a somewhat lesser Rate than best Rate speed, so, if the two are compared, the low speed climb will take longer and consume more fuel to Top of Climb. Then, for a climb time of, say, 30 minutes at a difference of 50 KIAS (about 80 KTAS), there will be an incremental cruise of 40 nm required to just achieve the point where best Rate of Climb would have ended. Therefore, in the comparison between climb at best Angle and best Rate, the lower speed will cost you both time and fuel. The best Rate of climb can be refined further to optimize time and fuel. Remembering that up to 20 to 30 Knots ‘Off Optimum’ have only minimal effect upon excess Power, an Econ Climb speed a little above best Rate (+20 KIAS for the A320) will have a negligible effect upon Rate of Climb, but put the aircraft 30 miles or so further down track at Top of Climb. The very slight increase in climb Time and Fuel is more than off-set by the extra distance covered in the ‘fuel expensive’ climb, and elimination of the incremental cruise. If we consider operations for a Maximum Range profile (absolutely minimum fuel), it’s not at all uncommon to see a Climb speed in EXCESS of the initial Cruise speed.

[FlightDetent]

You only know little, Jon Snow. What a confused post.

[jimmyg]

Perhaps you would care to enlighten us with a intelligent response other than obnoxious Games of Thrones sardonic nonsense.

By the way;

The quote was originally said by Ygritte in Chapter 15 of A Storm of Swords. In the chapter, Ygritte says the line to Jon Snow in anger. This happens because Jon asks why she was crying because of a song about “the last of the giants” when he had just seen hundreds of them.

[Vessbot]

You only know little, Jon Snow. What a confused post.

If this is a response to Jimmyg's post above yours, I'm also curious what found incorrect (or "confused") about it. It all checks out, as far as I can tell.

(Except this part: "As jet engines directly produce Thrust, not Power, it is necessary to consider Thrust multiplied by speed (Power = Force X Velocity). Thus, for a given Thrust setting, Power increases as TAS increases." also applies to piston engines.)

[pineteam]

Vilas, Jimmyg and the others, thank you for your valuable inputs.
I was aware of the CI O as equivalent to best ROC but it looks like the reality with the ATC restriction and the wind always changing, an heavy A321 with a high IAS of around 280/290kt often gives you a very poor sometimes zero ROC for few seconds as the aircraft is pitching down to keep the speed. Flying in Hong Kong airspace every day, we have to reach cruising altitude before exiting their airspace. Sometimes, especially in winter with very strong tail wind increasing as we climb, even flying at Green dot speed the whole way we can't make it. But with a fairly light A321 or A320, I like using vertical speed for the last 5000 feet or so as you can aim to reach your cruising altitude exactly at the waypoint required by ATC. No need to workout what is the Best IAS, just set the required vertical speed with a safety margin as you might need to reduce the ROC at higher altitude. Works pretty well. On A319 I always fly managed speed unless restriction from ATC as its climb performance is never a concerned.

[vilas]

It can also be done by reducing flying select speed. PERF page will give you the ETA and distance required to reach. This is actually the preferred method by airbus and demonstrated in type rating.

[pineteam]

Hi Vilas. I do agree with you but the Perf page is sometimes so unreliable lol.

[jimmyg]

Dear Pinteam,

Absolutly, applying an AOC with VS and managing the slow bleed off of airspeed to TOC is IMHO a decent technique a very acceptable way to manage your climb profile. Back in the day AOC was/is the primary instrument. We tend to get lost in push the button pull the button flying for dummies process now adaays.

This old adage that V/S is taboo in climb in nonsense. Sure do you have to manage your A/C, of course. Chasing a strict speed around the sky can at time be a unreasonable way to manage you A/C.

I say when, I say where, I say how much :ok