But operating fuel cells at high altitudes can be tricky. Indeed, the sole purpose of Boeing's fuel cell propeller plane, says Boeing engineer Michael Friend, is to let researchers take the system up to altitude to study the fuel cells' performance. Inside fuel cells, hydrogen fuel and oxygen from the air react to produce an electrical charge. In the thinner air at 35,000 to 50,000 feet, a compressor will be needed to ensure a constant supply of oxygen. This compressor will also maintain a high internal pressure under which fuel cells run more efficiently. Boeing hopes to get its test plane off the ground by 2004.