Manufacturers of aircraft engines have, like many other industrial manufacturers, try to make engines that are more economical to operate and maintain while still providing the power needed to fly large$-$eapacity aircraft. The Boeing $747,$ long considered the workhorse of the jumbo jet fleet, is powered by four engines, each capable of $44,700$ pounds of thrust. Because the $747$ has been in operation since $1969,$ maintenance costs are now relatively fixed. The goal for any airline is to plan for consistent, predietable maintenance costs.

The new era of more efficient planes include the Boeing $777,$ introduced in $1995,$ has only two more efficient engines, each capable of $84,000$ pounds of thrust. Not only are the newer engines capable of greater power, their efficiency also should provide lower maintenance cost variance. The following engine maintenance cost data $($in $ millions$)$ from a sample of the United Airlines fleet is provided: