Pan American was the first U. S. airline to operate pure Jet Aircraft. Most planes previously operated by United States and worldwide airlines were piston-engine aircraft. Some airlines have turboprop-powered planes in service (primarily the British-built Viscount and Britannia). Turbo-propeller aircraft do have some speed advantage over piston-engine planes, but they are not jets.
Many of the larger pressurized piston-engine planes are capable of flying high enough to reach the so-called jet stream. In essence the jet stream is a huge circumpolar whirl of air, circling the globe from West to East. Its course varies from twenty-five to fifty thousand feet above Earth’s surface; and when its extreme velocity is at the lower level, planes flying within it may add as much as 200 miles per hour to their normal cruising speeds.
Pan Am Clippers frequently take advantage of the jet stream on Eastbound flights over the Atlantic and Pacific. However, the speed benefit of the jet stream is not consistent enough to allow reflection in published schedules. For the jet stream is apt to be erratic, on rare occasions even reversing itself with East-to-West winds up to 50 m.p.h.