Taking a cue from automakers, the industry giant Boeing is looking into replacing auxiliary engines on its planes with cleaner, quieter, and more efficient fuel cells. Fuel cells, which combine hydrogen and oxygen to make electricity, are already being tested extensively in cars, and all the major automakers plan to produce pilot fuel-cell models in the relatively near future. The technology is also used in some eco-friendly buildings to run heating and cooling systems. Now Boeing hopes fuel cells can be adapted to run everything from air conditioning to in-flight movies on planes. (The company has no plans, however, to replace jet engines with fuel cells, or to scrap its new high-speed plane, the Sonic Cruiser, which greenies have denounced as an environmental menace.)