CASE STUDY 3:
Performance improvement resulting from 737-800 winglet retrofit

An airline requested assistance from Boeing to determine the airplane cruise performance improvement resulting from the retrofit installation of blended winglets on 14 of its 737-800s. To determine the magnitude of this improvement, cruise fuel mileage data collected after the installation of the winglets would be compared to data collected before the installation. The airline provided ACMS-recorded data collected on each of the 14 airplanes, before and after the installation of the winglets, to Boeing for analysis and comment.

Retrofitting the winglets is a two-step process comprising a structural reinforcement of the wing followed by installation of the winglet. Eleven of the airplanes had the wing reinforcement completed many weeks before the winglets were installed, with the airplanes returning to service with the reinforced wings. For these 11 airplanes, the nonwinglet data was based on this reinforced wing configuration. Three of the airplanes had the wing reinforced and winglet installed at the same time. For these three airplanes, the nonwinglet data was based on the production nonreinforced wing. The two sets of data were analyzed separately in order to identify any differences in the improvement based on differences in the baseline before the winglets were installed.

Boeing analyzed the data for all 14 airplanes using the same in-house software tools used to analyze Boeing flight-test data. These software tools are different from the APM software provided to airline customers, but the analysis produces basically the same results. The main difference is that the Boeing in-house software normalizes the data points to nominal weight to pressure ratios (W/δ) chosen by Boeing while the APM software does not.

The improvements were plotted versus W/δ in order to illustrate that the magnitude of the improvement depends on W/δ for a given Mach number. This dependency on W/δ is because the winglet improvement is a function of airplane lift coefficient, which in turn is a function of weight, altitude, and speed. Boeing’s analysis of the data indicated a slight improvement in drag and fuel mileage (at a fixed weight) that resulted from the reinforcement of the wing structure. The improvements were determined by comparing both the nonwinglet and winglet fuel mileage results to the nonwinglet 737-800 database. The performance improvement because of the winglet is not the average winglet deviation from the nonwinglet database; rather, it is the difference between the average deviations for the winglet and nonwinglet, both measured relative to the nonwinglet database. This same process was followed for each W/δ, and for the various sets of data (see figs. 4 and 5).

Boeing’s analysis of the data indicated a slight improvement in drag and fuel mileage (at a fixed weight) that resulted from the reinforcement of the wing structure. The results indicated this improvement to be relatively small but still worth an average of a few tenths of a percentage at normal cruise weights and altitudes. Including the effects of both the wing strengthening and the addition of the winglets, the fuel mileage and drag improvements closely matched their predicted levels.

Because the improvement in drag is a function of W/δ for a given cruise speed, the actual improvement in fuel mileage that the airline would experience for any given flight conditions depends on the W/δs flown during that airline’s operations. The change in total fuel required to fly a given route is determined by a combination of the improvement in fuel mileage offset by any increase in airplane weight. Retrofitting the winglets to the 737-800, including wing reinforcement, currently adds about 218 kg to the empty weight of the airplane, and this additional weight alone would increase fuel burn approximately 0.2 percent to 0.3 percent for an average 737-800 flight leg.

An analysis similar to the Boeing analysis could have been carried out by the airline itself using the spreadsheet output option from the APM software program. The results of analyzing the data in this manner would differ by only a relatively small amount from the analysis carried out using the Boeing in-house software. This same method of analysis could be used to investigate any type of modification to an airplane. Data collected before and after a modification would be compared to a reference database and the difference between the two sets of data would reflect the effect of the modification.

SUMMARY

The benefits of cruise performance monitoring are well known by many airlines that include the practice as part of their toolbox of practices aimed at efficient operation of their airplanes. The three case studies in this article illustrate the use of cruise performance monitoring to solve various cruise performance issues. Performance monitoring can also be used to identify flight planning and FMC performance factors and to monitor performance deterioration trends. Boeing has the resources to assist airlines with cruise performance monitoring analyses and to help them interpret results. For more information, contact David Anderson at david.j.anderson@boeing.com.

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