LONG-RANGE CRUISE AND COST INDEX

The relationship between LRC speed and ECON speed is different for each Boeing airplane model. As stated, LRC is based on a 1 percent penalty on fuel mileage, while the ECON speed uses CI as an input that is based on a more detailed accounting of actual costs. However, it is possible to derive a CI for normal cruise conditions that approximates LRC in terms of the cruise speed that results. Figure 2 shows the approximate relationship for Boeing commercial airplanes.

RELATIONSHIP OF COST INDEX TO LRC FOR BOEING AIRPLANES
Figure 2

The LRC-equivalent cost index varies for different airplane models and engines.

Entered Cost Index (CI)

AIRPLANE MODEL		MRC		TYPICAL AIRLINE CI VALUES		APPROXIMATE LRC EQUIVALENT
717	0	5 to 25	25
737-3/4/500	0	10 to 30	30
737-6/7/800	0	10 to 30	35
757	0	15 to 50	85
767	0	15 to 55	70
777	0	90 to 150	180
MD-11	0	80 to 120	200
747-400	0	25 to 80	230

It is very important to note that the LRC speed is almost universally higher than the speed that will result from using the CI selected by most carriers. If faced with a low fuel situation at destination, many pilots will opt to fly LRC speed thinking that it will give them the most miles from their remaining fuel. As shown in figure 2, the best strategy to conserve fuel is to select a very low cost index, with zero providing the maximum range. Any pilot can easily demonstrate this during cruise flight by inputting different CIs into the FMC and comparing with LRC by observing the predicted fuel at destination.

EFFECTS OF WIND ON CRUISE SPEED

In addition to more accurately optimizing costs, there is one other benefit of flying ECON instead of LRC with most Boeing airplanes. The LRC speed calculated by the FMC is typically not adjusted for winds at cruise altitude but the ECON speed is. As a result, LRC is ideal only for zero wind conditions, while the ECON speed is optimized for all cruise wind conditions.

For example, in the presence of a strong tailwind, the ECON speed will be reduced in order to maximize the advantage gained from the tailwind during the cruise. Conversely, the ECON speed will be increased when flying into a headwind in cruise to minimize the penalty associated with the headwind (see example in fig. 3).

ECON Cruise Mach

COST INDEX		100 KT TAILWIND		ZERO WIND		100 KT HEADWIND
0	0.773*	0.773	0.785
80	0.787	0.796	0.803
Max**	0.811	0.811	0.811

*FMC will not slow down below still air CI=0 ECON speed.

**At maximum CI, FMC will fly at envelope limit in all wind conditions.

THE RELATIONSHIP BETWEEN OPERATING COSTS AND COST INDEX

If fuel prices increase relative to other costs, a corresponding reduction in CI will maintain the most economical operation of the airplane. If, however, an airline experiences rising hourly costs, an increase in CI will retain the most economical operation. In either case, the changing CI will result in changes to the cruise speed calculated by the FMC. Even calculating a cost index based on approximate time costs and flying ECON speed can yield significant cost benefits to the airline.

To be used most effectively, CI should be based on a comprehensive evaluation of an operator's specific operating costs. For this reason, flight crews typically receive a recommended CI value from their flight operations department, and it is generally not advisable to deviate from this value unless specific short-term constraints demand it.

SUMMARY

In order for flight crews to achieve optimum cruise operation, it is necessary to first understand the flight's strategic objectives, and then to select the cruise speed that best meets these objectives. It is equally important to recognize that real-world situ­ations may result in the need for deviations from the overriding strategy. Appropriate use varies with each airline, and sometimes even for each flight.

Boeing Flight Operations Engineering assists airlines' flight operations departments in computing an accurate CI that will enable them to minimize costs on their routes. For more information, please contact FlightOps.Engineering@boeing.com.