Boeing (NYSE: BA) and United Continental Holdings, Inc. announced today that United Airlines’ 777 fleet will be modified with a Performance Improvement Package that will result in greater fuel efficiency, along with reduced emissions.

The package, to be installed on 52 United Airlines 777s, is expected to reduce fuel spending per airplane by about $200,000 annually (assuming $100 per barrel crude oil prices), according to Boeing estimates.

“We appreciate Boeing’s continued efforts to make existing aircraft more fuel efficient,” added United Fleet Vice President Ron Baur. “We are very focused on improving fuel efficiency in our fleet as a natural hedge against rising fuel prices.”

Continental Airlines, which merged with United Airlines last year, was among the first airlines to sign up for the package, agreeing to outfit 20 777s in 2007.

“Expanding implementation to the United Airlines 777 fleet is a great testament to the value of the Performance Improvement Package,” said Dennis Floyd, vice president, Fleet Services for Boeing Commercial Airplanes. “These efficiency upgrades help our customers by making a great airplane perform even better.”

The 777 Performance Improvement Package improves the airplane’s aerodynamics through a software change to enable a drooped aileron, a ram air system improvement and the installation of improved wing vortex generators.

Eighteen airlines with fleets totaling more than 350 airplanes have ordered the 777 Performance Improvement Package, which is focused on the 777-200, -200ER (extended range) and -300 models, accounting for more than 500 airplanes. The upgrades are incorporated in the newer 777 models, the 777-200LR (longer range), -300ER and Freighter, during factory production.

Boeing press release