VS CLimb?

[YankeeGolf]

Type: A321

Was flying the other day and someone told me that using VS +2000 ft climb and selecting speed below which the engines will produce less than climb thrust will save fuel?

I told him that the most economical way to get to cruise level is on climb thrust and let the plane manage the energy by herself.

what say the bus drivers here?

:confused:

[Down Three Greens]

+2000fpm in an A321 with less than climb thrust ???? confused: Is that possible?

Generally, the longer it takes to get to cruise level, the more fuel you use. Using climb derate in an A330 uses 25kg more in D1 (5% less) and 50kg more in D2 (10% less than CLB)

[IFLY_INDIGO]

sorry but it sounds very stupid to me :ugh:... What I learnt and noticed is that once you select V/S mode, selected vertical speed governs the engine thrust..

On economy climb, it depends what is your company policy.. economy level required is set by the airline and is indicated by CI.. depending upon CI, OAT, weight etc most economical climb would be achieved if you fly the ECON speed with thrust levers in CL detent...

individual tricks are useless and does not serve the purpose..

at my best

cheers

[javelin]

In heavy 320's and 321's - the last couple of thousand feet can be a challenge.

Because the aeroplane is very speed sensitive - i.e. it wants to get to or stay at the selected or managed speed quickly, you can get into a level, climb, level, climb scenario.

In this instance, selecting an achievable and conservative VS - 500fpm is a good start and selecting the mach HIGHER to keep climb thrust on, you will find that the climb is maintained, the speed may vary over an acceptable range and a smooth end to the climb is achieved. Getting to the last 2,000 foot point with extra energy (speed) is also a bonus and helps.

The big caveat is if you see a constant speed trend vector downwards, vary the VS to suit.

Other than that, I agree with others, managed climb and descent are usually the best unless you are late in which case its gear up, flaps up - 340kts until you reach your econ cruise mach, climb on that until IAS is the same as in the PERF page, then back to managed for climb and cruise. Repeat on descent and you will save about 8 - 10 minutes irrespective of leg length. Fuel will be unchanged for the sector length.

330 is different however, the D2 climb is pants - TWOFT :ok:

[YankeeGolf]

Agreed totally with all of u here~! :rolleyes: the longer u fly the more strange things u see......i guess...many thanks~!:ok:

[forget]

Is this relevant, or not? VS selected for climb - and Airspeed/thrust didn't quite keep up.

http://www.airdisaster.com/reports/ntsb/AAR80-10.pdf

AIRCRAFT INCIDENT REPORT. AEROMEXICO OVER LUXEMBOURG, EUROPE.
NOVEMBER 11,1979. DC-10-30, XA-DUH

3. CONCLUSIONS.

1. The aircraft was maintaining a constant Vertical Speed during the period of time immediately preceding the incident.

2. Thrust from all three engines was at an auto-throttle limiting value for several minutes during which pitch attitude increased and airspeed decreased.

3. The relationship between aircraft attitude changes and flight control commands and the minimum airspeeds recorded indicate that the aircraft was in an aerodynamic stall.

4. The autopilot system was commanding aircraft pitch attitude and, preceding the stall, the auto-throttle system was controlling thrust during the climb.

5. The autopilot system was in a Vertical Speed mode rather than an Airspeed or Mach command mode during the climb contrary to prescribed normal operating procedures and recommendations, AEROMEXICO's procedures and contrary to the manufacturer's recommendations.

6. The autopilot commanded an increasing angle of attack while attempting to maintain a pre-selected Vertical Speed which exceeded the limit thrust performance capability of the aircraft at higher altitudes.

[javelin]

On a non FBW aeroplane yes. FBW aeroplanes have reversions that will amend the climb or descent mode if parameters are exceeded.

So, in the case of an Airbus, if you overselect VS and the airspeed falls to min safe, it will revert to open climb (selected speed) to protect you.

[EMIT]

Quote (from I Fly Indigo)
"sorry but it sounds very stupid to me :ugh:... What I learnt and noticed is that once you select V/S mode, selected vertical speed governs the engine thrust.."

OOOPPPSSS, I hope that is a shortcut only in writing, not in understanding, because what does your FMA tell you when you select V/S?

It says SPEED - V/S

This means that AutoThrust controls speed and Pitch controls Vertical Speed.
Now, depending on the Vertical Speed that you select, thrust will be anywhere between idle (usually for a descent) and climb thrust (for a 'good' rate of climb). If you select a very high rate of descent, the speed will increase above command speed, even at idle thrust. If you select a very high rate of climb, the speed will drop below command speed, even at climb thrust. That is why V/S is not a fire and forget mode, as pilot you have to stay closely in the loop.

As far as fuel economy is concerned, like others stated already - max (i.e. CLB) thrust, at best speed to best level and than cruise at best speed to top of descent, which should bring you to short final at best speed all the way in idle thrust. Just dreaming, doesn't happen often in the real world.

[ppppilot]

If you want to save fuel, select 0 Cost Index and let the FMS manage the speed.
It is very difficult to explain all the relationships between IAS, V/S and fuel.:}
This is a very good study in airbus document:
getting to grips with fuel economy (http://iata.org/NR/ContentConnector/CS2000/Siteinterface/sites/whatwedo/file/Airbus_Fuel_Economy_Material.pdf)

Found at page 5:
This time difference plot has the same characteristics for all Airbus aircraft,
with the best time being obtained at the highest climb speed and max climb Mach number. Note that although a slow climb speed gets the aircraft to cruise altitude earlier, this requires more acceleration to cruise speed and more cruise to a given distance, making it slower overall.
The fuel difference plot characteristics vary with aircraft type.
The A310, A321 and A330 show similar characteristics to the A300 with a best fuel climb speed of about 290 to 300 knots.
The A318, A319 and A320 show better fuel burn at the lower speed range
(260 to 280 knots)
The A340 shows better fuel burn at the higher speed range (310-330 knots).
The A310 and A340 are similar to the A300 in showing minimum fuel at a
max climb Mach number of 0.78. In fact 0.8 is better for the A340-500/600.
However the A320 family and A330 benefit from the lower Mach No of 0.76.

In fact the more important to the fms at 0 cost index is to maintain an specific IAS, not a V/S. If you leave the second segment of the slope lift/drag and goes behind green dot (best ratio) you will burn more fuel even with reduced thrust.
Here you have another airbus document (http://www.icao.int/env/workshop/joselzon.ppt) :ok:
Tailwinds

[barit1]

If you're not familiar with the limitations of VS mode, you MUST read about the Pinnacle CRJ accident (http://www.ntsb.gov/ntsb/brief2.asp?ev_id=20041015X01633&ntsbno=DCA05MA003&akey=1). :=

[guiones]

Airbus in Normal Law has a protection called mode reversion to protect pilots from themselves in situations like this. V/S climbs, specially at high altitude should be avoided.

What happened to the Pinnacle crew would not hapen to an Airbus FBW in normal law, first you would have mode reversion, if it is overriden by the pilots(manual flying) then you have alpha floor, if that is disconected; when you reach V alpha max, the protections will not allow the speed to decrease any more and regardless of sidestick position, the aircraft will pitch down to maintain at least V alpha max.

G

[Dream Land]

If you're not familiar with the limitations of VS mode, you MUST read about the Pinnacle CRJ accident.What do CRJ'S and DC10"s have to do with the A321? :confused:

[airfoilmod]

I read that CRJ AR some time ago. What about that flight should be used for example except don't screw around and grab ass to join the FL41 Club in an aircraft that shouldn't be there to begin with?

[PA38-Pilot]

What do CRJ'S and DC10"s have to do with the A321? :confused:

That they both have wings?:}s

Back on topic: Just do it the way it was meant to be done... don't try to invent new ways of flying it... there is probably a reason why it it isn't on the FCOM.

[Wingswinger]

V/S in the climb saves fuel? Sorry - only CLB or OP CLB will do. the most efficient climb speed is ECON determined by your trusty FMGC and the CI.

The only reason for selecting V/S is if you are climbing at a high rate (still early in the climb) and there is proximate traffic above you, selecting V/S and reducing it to +1500 or less should avoid a TCAS RA.

Now, the descent, on the other hand, is a different matter because following the managed path is NOT the most fuel-efficient thing to do.

The reason for the 'bus-struggle in the last 2000 ft to cruise level is usually because you have just found the tropopause and as everyone who has done proper high altitude training as some point in their career will tell you, that is indicated by a drop in engine performance as the air outside is no longer getting colder. The way round it? Select MACH, it will usually be .77 or .78 at this stage, reduce it to .75 or .76 and watch your little 'bus leap up to FL 370 or whatever your desired level is. Don't forget to push managed speed when you go into ALT*.

[guiones]

Wingswinger:

Quote:

"Now, the descent, on the other hand, is a different matter because following the managed path is NOT the most fuel-efficient thing to do.

The reason for the 'bus-struggle in the last 2000 ft to cruise level is usually because you have just found the tropopause and as everyone who has done proper high altitude training as some point in their career will tell you, that is indicated by a drop in engine performance as the air outside is no longer getting colder. The way round it? Select MACH, it will usually be .77 or .78 at this stage, reduce it to .75 or .76 and watch your little 'bus leap up to FL 370 or whatever your desired level is. Don't forget to push managed speed when you go into ALT*."

Now, I would like to know when you did flight testing and data gathering to proove that the calculations and tests made by the Airbus Engineers, Test Pilots and Flight Test Engineers are all wrong for managed descent, please send your data to Airbus Flight Test so they can correct their mistakes.

Also Airbus ST would be very intersted in your new procedure for the last 2000' in climb, I notice it does not make a difference if your cruising altitude will be 310 or 390, just a general statement.

Just let the airplane do the work it is designed to do by people who not only have vast knowledge to do it, but the means to test their findings. If you are so sure about your findings, talk to Airbus, send your sugestions in, they are very receptive, and usually give you a great reason for how they design the systems.

There is a link in one of this posts for getting to grips with fuel economy, read it and understand it, it is very good.

G

[OPEN DES]

Now, I would like to know when you did flight testing and data gathering to proove that the calculations and tests made by the Airbus Engineers, Test Pilots and Flight Test Engineers are all wrong for managed descent, please send your data to Airbus Flight Test so they can correct their mistakes

Managed descent is not the most fuel-economic descent because:

1. managed descent flies a geometric path in between altitude constraints
2. managed descent even in nil wind has some residual thrust above idle
(to account for engine anti-ice and statistical probability of reduced trackmiles to touchdown/early slowdown)

Even disregarding the above, Managed descent will only be the most fuel economic when flying Cost index=0. (commands green dot during descent and thus reduces time spent in cruise)


regards

[javelin]

And easy to demonstrate.

Fly a descent with an intermediate alt constraint such as the vile and odious FL270 by LIFFY. After LIFFY go direct to the next waypoint and the vertical profile goes from 0 to -3,000' or more.

Over the years you get to play with the machine and find out where it's strengths and weaknesses are - that is what makes a routine profession interesting again.

Striking a balance between ultimate economy and professional experimentation provides hours of endless pleasure and enables me to outwit our technical officer time and time again :E

[NoNameRecord]

speaking of VS climb:

http://nl.youtube.com/watch?v=eRiCHgQnf9s :ooh:

:E:E

[guiones]

OPEN DES:

Your choice of names speaks volumes about your prefered descent mode!

Jokes apart,

1. Managed descent does fly a geometric path between constraints, but what you are not saying, is that if necessary it will fly it at idle thrust same as open descent.

The path of the managed descent is calculated:

DES MODE
The DES mode guides the aircraft along the descent path computed by the FMGS.
The system computes this flight path backwards from the deceleration point up to the top of descent (T/D) with respect to the speed and altitude constraints .
Internally, the computer divides the descent path into various segments, depending on the relative positions of the constraints. It starts at top of descent (T/D) by setting up an "idle" segment that takes the aircraft down to the first constraint, and follows this with "geometric" segments between constraints.


DESCENT SPEED PROFILE
The descent speed profile is usually the ECON speed profile, amended by any speed constraints and speed limit contained in the flight plan.
Before the descent phase is active, if the pilot does not intend to fly the ECON speed/Mach profile, a different speed or Mach can be entered to amend the speed profile :
The pilot may enter a Mach number and/or a speed in the MANAGED field of the PERF DES page (3L key).

2. Idle for descent will be the same in DES or OP DES, the idle will increse for anti ice regardless of descent mode

Also on the last paragraph, you are contradicting yourself, if you select cost index of 0 your speed for the descent will be slower(not green dot, but closer to 250KTS on the A320) which means you will spend less time at altitude, that is really where you want to be for fuel savings.

I don't know what your airline's sop's are, but Airbus procedures call for managed DES to have a priority over selected DES whenever possible, for economy reasons and another big factor: constraint compliance.

Here is some more info:

FLIGHT OPTIMIZATION
The optimization function computes :
The best target speed for climb, cruise, and descent (ECON SPD/MACH).
The best descent path from the cruise flight level to the destination airfield.
An optimum flight level (for pilot's information).

BEST TARGET SPEED
It is computed by the FMGS (except for V2). The best target speed (ECON SPD/MACH) is the basis for the managed speed profile.
ECON SPD/MACH is a function of :
Cost index (CI)
Cruise flight level (CRZ FL)
Gross weight
Wind and temperature models

The computer processes the ECON SPDs for the climb and descent phases before the initiation of the flight phase, and freezes the values once the flight phase becomes active.

G

[guiones]

Javelin:

There as as many ways of doing a managed descent as there are doing a selected descent to meet the profile you desire, there is no limitation to select speed in managed descent or eventually if necessary(but defeating th purpose of fuel savings) the use of speed brakes or early configuration, while assuring yourself of at least altitude constraint compliance.

I leave "playing with the machine" to test pilots without passengers, with flight test engineers and data gathering to tell me what the ACTUAL more efficient way of flying the aircraft is, not the PERCIEVED one.

You might be a highly skilled pilot that finds that "striking a balance between ultimate economy and professional experimentation provides hours of endless pleasure and enables me to outwit our technical officer time and time again" is the proper way of flying, but myself as more limited pilot, will stick to sops and keep flying the airplane like the people who designed it and tested it recomend that I do, in my case Airbus sop's.

G

[OPEN DES]

Hello G,

A quick reply from my side since I need my rest asap.

but what you are not saying, is that if necessary it will fly it at idle thrust same as open descent.

Yes, when necessary it will fly engine idle thrust in DES mode but only IF you have a significant tailwind which was not accounted for or when high on profile and also at lower levels. (below 10000´, ballpark figure).

In a nil wind managed DES on profile the proposed idle segment is not really idle. It´s more around 40 to 45% N1 while the FMA will read THR IDLE, but there is no idle flashing on your ewd. If you pull for OP DES at this stage you will notice the N1 decreasing while the FMA continues to read THR IDLE. The idle segment of a managed descent profile is more shallow than a true idle OP DES. This is as I said to account for reduced trackmiles/early reduction/anti ice requirement.

I personally always descend in managed descent as this is SOP and it´s their trainset, also being conservative (not being true idle) it leaves your options open.


Also on the last paragraph, you are contradicting yourself, if you select cost index of 0 your speed for the descent will be slower(not green dot, but closer to 250KTS on the A320) which means you will spend less time at altitude, that is really where you want to be for fuel savings.


My concept was that the longer the engines are operated at idle and not at an 80% N1 cruise setting the better. Green dot will maximise your time in descent thus reduces the time the engines are running at high N1. However the engines will be less efficient at lower levels. So I guess there is a bit of a trade off there. I stand corrected.


S.


PS javelin is not really doing testpilot stuff. he is just forcing geometric descents between constraints to become level with idle segments. this is more efficient. the same way a boeing behaves in VNAV. even if you object to this, you can do it by accident by re entering the QNH on the perf app page for example.

[sooty615]

There is another interesting thread similar to this Tech Log related to NDB approaches and I added my tuppenceworth! You guys have endorsed my theory that not enough of us fully understand the technicalities, capabilities and software design parameters of the equipment we are operating nowadays.

Food for thought....or rather should it be more focused training?

[Tree]

Climbing at 2000fpm in the wingless wonder. What were you carrying, ping pong balls?

[Dream Land]

it leaves your options open.
??? Not really really sure what that means, I usually fly past TD (CI35) and pull open descent, works fine without the engine spooling up or down, I have never felt like I was reducing my options :confused:, maybe you can explain. Where I fly there are not very many complicated STARS, if an arrival does have several constraints I will let the magic do it all in DES mode.

[OPEN DES]

Hi Dreamland,

Where I fly we have lots of speed limits which are overruled by ATC. Then subsequently being reduced to 220kt 50nm out doesn't help. Welcome to Madrid ATC, which should be in a completely different thread. We could even dedicate a forum to them. :(

Regards,

S