Fuel costs may currently account for 65% of an airlines operating costs. Conserving fuel is therefore of utmost importance.

 

After querying the Cost Index (CI) values that should be set when flying the B777X, and being asked by Kyle to re-read the Boeing Aero article on the subject, I thought that I would give you my conclusions on setting the CI, take-off flaps and de-rates, and landing flaps and gear extension in order to conserve fuel.

1. Take-off flaps should be 5 where possible.
2. De-rates should not be used unless justified.
3. Cruise should be flown in ECON mode, with a CI of choice between 0 and 150. Some real-world airlines use quite low values.
4. Descents should be planned for zero thrust.
5. Landing gear setting should be delayed until 1500' AFE.
6. Landing flaps setting should be delayed until just prior to 1000' AFE if it is safe to do so.

Based on the Boeing article, I can explain my conclusions as follows:

 

Take-off and climb

 

The lower the take-off flap setting, the lower the drag induced. From a fuel consumption perspective, a full thrust take-off and climb profile offer the most fuel economy for an unrestricted climb. Other considerations may require other settings. Flaps 5, instead of flaps 20, can avoid a fuel burn of about 314 kgs.

 

Cruise

 

Maximum Range Cruise (MRC) setting will command a speed that will provide the minimum fuel burned for a given cruise distance (equivalent to a CI of zero). Long Range Cruise (LRC) setting will provide 99% of the absolute maximum specific range. The advantage is that 1% of range is traded for 3-5% higher cruise velocity. Since higher cruise speed has a greater number of advantages, the small sacrifice of range is a fair bargain (equivalent to a CI of ~180). LRC, though, can take no advantage of varying wind speeds.

 

ECON (economic) setting is a speed based on the CI setting. The best strategy to conserve fuel is to select a very low CI, with zero providing the maximum range. You can demonstrate this during cruise by inputting different CIs and comparing with LRC by observing the predicted fuel at destination. ECON speed is optimised for all cruise wind conditions. In the presence of strong tailwind, the ECON speed will be reduced in order to maximise the advantage gained from the tailwind. Conversely, the ECON speed will be increased when flying into a headwind to minimise the penalty associated with the headwind.

 

Descent and landing

 

Fuel burn is minimised during descent if the descent is conducted with zero thrust engines. This can only be achieved by entering the destination winds forecast into the FMC, and the actual winds encountered to coincide with that forecast. The winds forecast should be available from your weather software.

 

Delay extending the landing gear until 1500' AFE, and landing flaps delayed until 1000' AFE (if in a stabilised approach) - a potential saving of 45 kgs fuel.

 

I hope that this resume helps you to conserve fuel.

Good write-up.. my only concern is your late landing configuration recommendation. Deploying gear and landing flaps at or before FAF allows the aircraft to be stable and in landing configuration at 1500 AGL (most FAF are 1700-2000 AGL). Each operator has their own policy on this, but changing configuration at 1000 AGL makes me nervous. Hey, read a great book on EVODEVO not long ago, good stuff.

Descents should be planned for zero thrust.

Thanks! But are you sure that descend should be planned with zero thrust? This wont be possible, until you shut your engines down...not sure it would be a good idea ;-)

 

You meant "idle"?

Air Canada's policy is to decend with throttles idle

Interesting post...and I understand that using MAX thrust TO will burn less fuel, but is it worth it to put the extra wear on the engines? I think a GE90 runs upwards of $20 Million. I would think that not only would wear be higher at max thrust, but the risk of failure would be higher as well.

 

Not to mention the fact that if you scare the bejeezus out of the pax by climbing out at a 25 degree deck angle all the time, they won't likely be customers for long. Then you're bankrupt and fuel burn doesn't really matter anymore...

Based purely on the FMC predictions for a zero wind fuel burn, around M0.84 is optimum. This equates to a CI of around 400.

 

It's like people who accelerate slowly in their cars - you spend a longer time at sub-optimal RPMs than the guy who accelerates a bit harder, but gets up to speed quicker. Assuming both change gear at the same RPMs, who used more fuel? The slower guy.

 

Best regards,

Robin.

 

 

LRC, though, can take no advantage of varying wind speeds.

 

This is important. Do use CI. CI, AFAIK takes into account compensating for winds, i.e. speed up into headwind, slow down in tailwind. Makes better use of both fuel and time.

 

 

I understand that using MAX thrust TO will burn less fuel, but is it worth it to put the extra wear on the engines?

MAX THR TO will burn less fuel than a De-rated thrust ref.? How come?

Not to mention the fact that if you scare the bejeezus out of the pax by climbing out at a 25 degree deck angle all the time, they won't likely be customers for long. Then you're bankrupt and fuel burn doesn't really matter anymore...

 

25° deck angle? Scared? I'd be back for more and picking my flights by aircraft type

 

I'd be more interested in engine life. Also not busting the initial climb altitude after takeoff before contacting departures. Doesn't take long to get to 5000ft at full throttle with an empty plane doing a 30 min ferry.

 

 

MAX THR TO will burn less fuel than a De-rated thrust ref.? How come?

 

Seems to depend on the plane actually.

 

Anyway with higher thrust, there is more excess thrust and quite likely better efficiency (more thrust-per-fuel-unit) - and you also climb faster. So the fuel flow is higher, but for less time.