Fuel Savings

[voice_of_peace]

Hello everyone,

I am writing about fuel savings, and wanted to ask the wider airline pilot population about their own operators' procedures and, what they have found, to work best in the following:

It might be safe to assume that on Boeing or Airbus most of us arrive at the CDU/MCDU respectively, and input an OFP and company derived cost-index for the mission that will take into account cost of fuel and cost of time and accordingly adjust parameters for cruise climb, cruise, and descent.

Recent economical considerations got me thinking again...

Whatever is takeoff configuration we use (flap, derate, assumed/flex, derate plus assumed/flex, EAI/Packs and so on) and however we are prescribed the initial climb, NADP1 NADP2 and so on, that all takeoffs until 10,000AGL/FL100 roughly follow the same 250 kias speed regime unless otherwise required by performance (WAT) or ATC.

I am more interested in the bit from FL100, or greater than 250 kias, until ToC.

Most of use tend to fly this in VNAV or Managed Speed at an FMC/FMGC derived high-speed climb, until crossover to Mach is achieved somewhere near to or around FL300 (type specific).

It has been rumoured that a number of airlines use this technique, and I have trialled on A320,A330 and B777 types, the benefit of climbing from FL100 to ToC at "best rate of climb" speed (Vy). Airbus=Managed speed with cost index '0' set in either OPEN CLB or CLB, Boeing = Vref30+140 in either of VNAV or FLCH.

In many cases, the results have been quite pleasing. In others, the results decidedly inconclusive. Speaking to a mate of mine, this procedure of Vy climb from FL100 to ToC, will actually burn more fuel on the four-engined B747 or A340/380. This I do not understand (confused?)

Naturally we might consider some the following:
- less time spent with THR CLB, CLB power
- less time spent in the lower, more dense air
- an increase in deck/body angle
- an increase in time spent at CRZ thrust
- an increase in total flight time

Does anyone out there have any empirical data on using 'Vy' as a climb strategy on narrow -vs- wide body, and two-engined -vs- 3 or 4 engined wide body aircraft? Similarly, does anybody know of a library where this kind of data is available? Or would anyone be willing to share their own experiences?

Of course this may seem very academic, especially when up at MTOW/PLTOW and Vy and CI=x climb are nearly coincidental, and when one considers the minimal fuel savings that (may) result. However, if this works, then very real savings might be realised on a fleet-wide application.

Anyone with similar interest, it would be great to get engaged on this topic.

[Capt Scribble]

Cost Index gives a speed that puts fuel against time costs for min cost as you have said. Some cleverer people than I must have thought about it, so I fly CI in the climb as the best solution.

[voice_of_peace]

after a little digging

thank you for the reply. as it happens, some data is out there after a search of the internet.
however i was looking for more information from those have tried or operated this climb strategy.


British Airways

Airbus Fleet:

A319/A320/A321
Climb at: Cost Index 0
Cruise at Cost Index 20

Boeing Fleet:

Boeing 737-300/400 and 500 Series
Climb at: Cost Index 0
Cruise at Cost Index 28
Boeing 747-400
Climb at: Cost Index 0
Cruise at Cost Index 90
Cost Index 0 (Sometimes used on East Coast USA to UK Flights and less often on UK to Singapore/Bangkok flights)

Boeing 757-200 and Boeing 767-300
Climb at: Cost Index 0
Cruise at Cost Index 40
Boeing 777-200
Climb at: Cost Index 0
Cruise at Cost Index 100

[fpuentegomez]

Question: how can they do variable CI on Airbus fleet? If I recall correctly, there is no such option during pre-flight preparation of the MCDU and if you modify during the flight your fuel initial consumption calculations will be wrong.

[speedrestriction]

You can adjust CI at any stage on the FMGC Perf page. Sometimes set it to 0 in the cruise/descent if we are early for our landing slot. Makes naff all difference in the cruise above 360 due to the relatively narrow range of cruise speeds available but on certain routes which have a low level cap it can make a reasonable big difference to the optimum cruise speed.

[voice_of_peace]

Right you are, fpuentegomez.
The MCDU last time I flew airbus only allowed for one holistic 'mission' CI to be entered and this will cover CLB, CRZ and DES phases of the trip.

So what we did at briefing stage, was to enter OFP/FPLN CI into the MCDU and brief accordingly from those predicitions, and then after departure and SID complete, whilst still in 250/FL100 regime, change the CI to "0" approaching the high speed climb regime at FL100 or equivalent point. Thereafter, from FL100 until ToC or next most restrictive clearance, we would conduct climb at CI=0 Airbus with managed speed set (Vy) -or- Boeing Vref30+140 (80+60). Finally upon reaching ToC and before ALT*/VNAV PTH becomes the active speed mode, enter again the dynamic cost index that was briefed on the ground, from the OFP/FPLN and conduct the remainder of the cruise and descent speed portions I.A.W. the OFP FPLN CI.

The purpose of this entire thread was to garner opinion, and see if there is more data out there from operators who (do?) use this technique. So far as I know it was used at British Airways and Alitalia. Surely a few others have tried it. Why did they try it? Did it work? Are they still doing it? Why didn't it work?

In my own trials, I have found that savings on a wide body twin can be pushing upwards of 300-400kg at times, and slightly less on a narrow body. But, this does not always hold true (ISA dev, Weight, step climbs {and what to do with that extra vertical energy during a step climb: turn it into forward speed}, differences airframe by airframe and so on.}
I would be delighted to hear from anybody who has tried this on a tri- or four-engined aircraft, or from anyone who has aircraft performance expertise in these aircraft also.

[wiggy]

Originally Posted by voice_of_peace

thank you for the reply. as it happens, some data is out there after a search of the internet.
however i was looking for more information from those have tried or operated this climb strategy.

British Airways .....

Boeing 747-400
Climb at: Cost Index 0
Cruise at Cost Index 90...


Boeing 777-200
Climb at: Cost Index 0
Cruise at Cost Index 100

FWIW that is no longer the way it is done at BA now on the 777,and I suspect it is no longer standard on the 744. Given the emphasis the company has given to fuel saving over the last few years I suspect that is because somebody has decided it's not the most efficient way of handling the CI.

[hunterboy]

Pity they don’t ensure the stand guidance is switched on so that we don’t burn the 300 kg we just saved on the ground

[vilas]

Many airlines do not permit to keep changing the CI. They work out overall CI and it is left undisturbed because there are many factors especially ATC constraints which are difficult to predict and can undo any advantage gained. If there is a requirement to reach a certain FL at the earliest then CI 0 can be used to do that.

[wiggy]

For the OP...TBH I'm not sure you can draw too much of a conclusion on best technique from discussion here but I guess it makes for interesting debate.

Aside from the tactical stuff which will screw comparisons ( vilas point) from what I'm hearing there are so many different "sub-techniques" in play regarding FMC use across the operators, especially on long haul ops e.g. wind usage ( which levels do you load?) and frequency of wind up dating ( 6 hourly? After every level change?) ..and there's increasingly thought that FMCs simply no longer have the processing "grunt" to really finesse the levels and speed profiles to produce really optimal fuel useage.. in short I'm not sure you can decide whether method A is better than method B just on the basis of FMC predicted fuel, but I guess you could at least spot a technique that was an outlier.

....and then as hunterboy rightly says when you get back to base having saved 300 kg by climbing at CI 0 to FL 100 rather than CI 57 and find "no one home"...then..................

[Uplinker]

Airbus do not recommend changing the CI tactically during different phases of flight to save fuel.

They only suggest changing the CI:

[From A330 FCOM, A320 similar]
It is recommended to modify the CI in flight:

In case a fuel problem is encountered, CI = 0 may be selected. The ECON SPD profile is then computed to ensure minimum fuel consumption. (I have done this on occasion).

In case the aircraft is behind its schedule, CI = 999 may be selected. The ECON SPD profile is then computed to ensure minimum time.

Note:
The airline’s operations department usually defines the cost index to optimize each company route. The flight crew does not ordinarily modify the cost index during a flight. (my bold)

I suspect that any perceived savings made in one phase of flight by modifying the CI are probably lost overall?

A better way to save fuel might be to input winds above and below the planned cruise level, to enable a more accurate forecast of advantageous step climbs - something I have seen very few pilots do.

[OPEN DES]

Hi VoP,

Nice subject.
Academically the minimum fuel scenario notwithstanding wind/temp gradient and engine wear would be:

Single Pack ops on ground
Delayed APU start
Single Engine Taxi with delayed engine start
Rolling take-off with TOGA thrust (keep for 5 min), packs OFF and minimum flaps setting
Acceleration at minimum (400ft by PANS OPS)
Acceleration to Vy
Continuous Climb with Vy (CI=0) till ToC
Cruise at OPT CRZ with CI=0 ---> Vmax range (min SFC)
Idle Descent with Green Dot (below CI=0, as Airbus FMGS defaults to 250 or 270 minimum iso GD)
Decelerated approach meeting stable criteria 500/1000ft
Idle Rev
Single Eng Taxi
Single Pack ops on ground
no APU start -> GPU

[voice_of_peace]

Originally Posted by Uplinker

Airbus do not recommend changing the CI tactically during different phases of flight to save fuel.

They only suggest changing the CI:

[/I]

I suspect that any perceived savings made in one phase of flight by modifying the CI are probably lost overall?

A better way to save fuel might be to input winds above and below the planned cruise level, to enable a more accurate forecast of advantageous step climbs - something I have seen very few pilots do.

thanks Uplinker. some very valid points and food for thought. Like you, I have tried using winds and have found that using: 1 level below, actual level, and then the remainder of available FMC wind calc levels above works quite nicely.

Neither Airbus nor Boeing recommend this practice of changing CI during operation, but neither is it expressly forbidden. The manufacturers have learned a lot from operators' experience in using their aircraft.

naturally, a saving of fuel in one part of the mission will cost time in another part of the mission.
as such, the value and importance of time becomes the governing criterion if "saving fuel and spending time"; as in the case with with expediting climb at Vy and losing 1-2 minutes on a sector of 1-2 hours or 15-16 hours, for that matter.

would it be safe to assume that if running ahead of schedule, as function of a day that went your way, or seasonal wind and/or optimal routing, that then it makes sense to employ a "fuel saving time spending" strategy as mentioned?

the fight mechanics are a bit above my head as to why this strategy saves fuel on a light twin (A320) or heavy twin (eg B777), but actually burns more fuel on a 3 or 4 engined beast.

I do not work in the office. been there done that. It is not so much about cost saving or even the environment (though I do care a great deal), as we have earlier made mention, it is nearly impossible to get it right 100% of the time due to other external restrictive bottlenecks (and shortcomings) in our collective operations (arriving at stand and nobody is home). seen this movie before?

I enjoy pushing myself on fuel economy and thrust idle descents without spoiler or thrust until stabilisation criteria so require, because it is a personal challenge that keeps me on the ball and I find it enjoyable to do things well, or, try to do so anyway.

hauling it around with handfuls of thrust, level at low altitude, uncomfortably nose high, and after the use of speedbrake because FMGC said so, is very unsatisfying and unpleasant to watch.

[gnarlberg]

the cost index isn't just a number to play with guys.
The best thing is to insert the wind and try to feed the computer the best you can.
The company will calculate a Cost index which suits you the best.
What matters saving 200 KG of fuel when you fly 3 minutes longer on every flight and thus exceed engine wear limit more early and engine time limited parameters?
Cost index isn't just Fuel Time. It´s also about engine overhaul, salary, and stuff.
We need to accept that we aren't more intelligent than the CostIndex.
Whats important is that your company has a good fuel saving and performance Team calculating all this. Our company thought about reducing the thrust reduction altitude to 800ft but then they faced the problem that the government wanted to implement noise bills for actual measured noise. And at this part it would've been more expensive to reduce thrust on A320 at 800ft AGL than 1000ft.

The last month we flew with 50, quite fast, and as your speed is high you get a different number for arrival and more often get directs. but yeah on descends I always modified the speed down to have some buffer.

So talk to your performance staff, do fuel saving courses, and stop trying being more intelligent than the airbus.

For example all people switching the Vent blower off. How often do i need to reset that **** and how much delay do we produce to such small things... The Vent Blower button and Vent isn't made for switching on off that often...

[autoflight]

fuel vs time

Lets not get too hung up on saving fuel. For a start, the bean counters have zeroed in on fuel because the cost is easier to identify. Its too easy to say "no extra fuel was used due to best CI practice". Possible, but more difficult to define is the hourly cost of operating the aircraft. Some might simply argue that the company published cost per hour is paramount

Not so simple. Part of the hourly cost involves APU time, ground operation, increased costs due short sector cycles, ATC and other traffic delays and more. Standard fuel use with on time or late arrival vs slightly greater consumption with early arrival might mean a great departure slot time rather than a very ho hum 2 hour turnaround. Arriving quite early might mean additional airport parking or aerobridge costs. So there is nothing very simple about cost per hour.

As captain, what can you do if your aircraft departs late and will therefore delay the next sector? Try to catch up some time? Good direct tracking (ie direct Casa Mhd V to right base 08 Orly), a little faster climb/cruise/descent, visual approach, advise company that you would like fuel (or arrive with through fuel) and other ground services more promptly as you are trying to make up some time. When turnaround progress indicates, suggest to company that an earlier slot time would be manageable.

Ongoing delays caused by avoidable late arrival of operating aircraft have a real cost, including crew duty problems, curfews and missed connection.

[Intruder]

Personally, I don't think dispatchers plan to a specific CI for fuel savings or other cost savings, but only to meet the schedule imposed by others. Hence, unless you have good personal or professional reason to change it, use what is given on the flight plan, and let the outfall be somebody else's problem.

[Skyjob]

Consider that each time you (or colleague) use start a slightly early descent at 1000'/min, or when V/S is used for whichever reason and the engines keep adjusting thrust for the rate selected, or more than IDLE revers is required on landing, or Engine Out Taxi procedures cannot or are incorrectly followed, or ATC levels you off for some time or mandates a different speed,... FUEL IS WASTED
The slight savings based on the technique suggested may work fine on some flights, but equally have no bearing on most others.

[wiggy]

Ongoing delays caused by avoidable late arrival of operating aircraft have a real cost, including crew duty problems, curfews and missed connection.

+1.

I guess a lot of this argument about whether you should always aim for maximum fuel saving might depend whether you are "networked" or not.

We are heavily encouraged (for fuel saving/cost/environmental/political reasons) to reduce burn, especially on the ground at base, but we also have an ops manual requirement to try arrive at the gate at base on time and changing the CI to try and achieve this is most certainly allowed (though doing so is most usually followed by the frustrations hinted at earlier).

One look at the list of transfer pax details on a typical Longhaul flight will show why - possibly 25% -50 % of pax will be connecting onto one of the company's short haul or long haul onwards flights. If you are late arriving, especially on an evening arrival, those pax are headed for hotels at company expense, followed by other complications for the company the next day. So if we depart a few minutes late downroute for whatever reason tweaking the CI by a few digits on a long haul sector to arrive at base on schedule makes more sense for the company (economically ) than blindly flying a CI that was planned in the office half a day ago.

[FullWings]

I think some of this issue is that there is not much in the way of “low hanging fruit” any more when it comes to fuel saving initiatives, therefore we enter the realm of dubious procedures which may or may not produce positive results but certainly complicate the operation.

As wiggy points out above, intelligent on-the-day management of the flight profile in response to up-to-date information is in reality much more important.

My airline has, at various points, brought in SE taxi, delayed APU start, lower flap landings, variable CI, fuel saving apps, etc. A rough calculation showed that if I did a visual circuit every 6 months as opposed to a 12-mile ILS, it would save more in that period than all of the above procedures combined...

[Mikehotel152]

There is an airline that strongly encourages pilots to take a highspeed exit at an airfield into which I fly regularly. This is to shorten taxi times by a minute or two on as many flights as possible to save fuel and allegedly provide the base captain with an annual fuel saving bonus.

In terms of performance, you usually need a higher flap setting with its related higher drag and fuel burn, and taking the exit relies on the length of the highspeed exit being incorportated into the landing roll which I regard as poor airmanship. And then there's the terrifying experience for the pax of braking hard, which is reprehensible.

After vacating the runway and giving each other smug high-fives, you will be held on a taxiway due to an occupied stand or arrive abeam your stand and wait for up to 15 minutes for a ground crew, merrily burning fuel all the while.

I'm in favour of good practice leading to fuel savings, but joined-up thinking is needed.