“I’m becoming unbearably weary,” Chicago Herald reporter Jane Ead scrawled on her notepad as the airplane rocked through yet another storm. “With the altitude, and the rough riding, the pilot was surprised I haven’t become ill.” Ead’s harrowing flight aboard a tiny Boeing mailplane helped launch U.S. passenger air service in 1927, with all its discomforts. Early air travelers flew below the clouds, so they faced every upset the weather could offer. All the turbulence caused airsickness, yet rising higher only shifted the pain.
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Trained nurses, the forerunners of today’s flight attendants, were on every flight, and a 1928 international study, cited in Drew Whitelegg’s book Working the Skies: The Fast-Paced, Disorienting World of the Flight Attendant, explains why: “A good deal of their time is spent assisting passengers in various stages of vomiting.” During climbs to clear mountains, cabin air grew reedy, sending the beleaguered nurses “down the aisle carrying oxygen bottles, offering distressed passengers a breath from the mask.”
“My God!” exclaimed famed Navy aviator D.W. “Tommy” Tomlinson to TWA meteorologist Ed Minser on landing his Northrop Gamma from an altitude above 30,000 feet. “I had winds of 100 to 150 miles per hour up there!” Tomlinson’s 1936 flights had touched the jet stream—a kind of pilots’ heaven six miles above Earth, where turbulence falls away, flight grows serenely smooth, and wind (in the right direction) becomes an extra engine.
“Since World War I, everything pointed to the advantages of over-weather flying,” says Boeing historian Michael Lombardi. It simply required finding a way to keep passengers breathing at, say, 20,000 feet—roughly two-thirds up Mount Everest.
In 1936, those were heights where only Auguste Piccard, in a gondola balloon, or Wiley Post, in a Jules Verne-inspired pressure suit, dared venture, and only then by sucking oxygen through a tube. Yet Lockheed’s XC-35 research aircraft could reach that high without the crew needing respirators. The twin-engine XC-35, which first flew in May 1937, was the first American airplane to be pressurized (Germany’s experimental Junkers Ju 49, which had begun flying two years earlier, was the world’s first). “So elements of the technology were out there,” says Lombardi. “It was a question of who could make it a success in everyday passenger service.”
The opportunity came in the late 1930s, when Congress threatened to cancel the government contract for Boeing’s B-17 Flying Fortress (“Four engines! Too big!” went the logic). Knowing that TWA chief Jack Frye, who’d underwritten Tomlinson’s flights, wanted the speed, comfort, and safety of over-weather flying for his coast-to-coast service, Boeing president Claire Egtvedt convinced both TWA and Pan American to underwrite the gamble of trying to convert an unpressurized bomber into a pressurized airliner. Wellwood Beall, who had designed Boeing’s 314 Clipper flying boat airliner, would head the engineering team. Dubbed Model 307, the new airplane would co-opt the B-17’s wings and tail, and its four engines would offer unprecedented power and safety.
“These were Curtiss Wright R-1820 series,” says Boeing flight operations manager Mark Kempton, one of very few modern-day experts to have worked on an R-1820 exactly as found in a 307. “These use crankshaft-driven superchargers to compress intake air.” Engineers call this “boost,” a standard trick to help the cylinders breathe at altitude. Yet the real innovation was that in the 307, a similar system would “breathe” for the passengers. “Twin superchargers were mounted on the inboard engine firewall, driven by a shaft from the inboard engines,” says Kempton. The superchargers would draw hostile air (with just half the density of sea-level air and a temperature at 16,000 feet of 1.6 degrees Fahrenheit) from outside and turn it into breathable air in the cabin.
“We consulted Dr. W. Randy Lovelace at Minnesota’s Mayo Clinic, who instructed [the team] on issues related to oxygen want,” Beall recalled in Carl Solberg’s book Conquest of the Skies: A History of Commercial Aviation in America. In his patent for an airline cabin pressure control system, Boeing ventilation expert Nate Price wrote of “the need to supercharge the cabin to promote the comfort of passengers,” but noted, “if an attempt is made to maintain...an absolute pressure [such as sea level], there are produced bursting pressures, which necessitate very great strengthening of the cabin, [making] the airplane economically impractical.”
“The solution was a highly dynamic pressure system,” says Mike Lavelle, a director at Seattle’s Museum of Flight and an expert on the Boeing 307. As Price proposed it, the airplane would rise unpressurized to 8,000 feet, where medical experts believed there was enough oxygen to keep people breathing and functioning normally. Then the superchargers would introduce air to hold cabin pressure to 8,000-foot conditions as the airplane climbed. Higher than 16,000 feet and cabin pressure would fall, since the superchargers couldn’t pull in such thin air (at 16,000 feet, it would feel like 8,000 feet inside the cabin; at 20,000, like 12,000). Yet flying at 20,000 feet, the 307 would still top 90 percent of the weather and passengers would not get airsick.
“It was very well thought out,” Lavelle says. “We still pressurize most commercial airliners to that 8,000-foot standard today.” Yet Price’s invention also demanded exquisite balance. In the rarified air of high flight, the 307 would need to sacrifice some pressure in its cabin to ease the “bulging” effect high-altitude flight induced in the fuselage. But in descent, the airplane would need to repressurize to bear the inward forces its cabin experienced as the aircraft reentered thicker air. You’ve seen this effect in miniature if you’ve noticed a plastic water bottle bulge or dent as cabin pressure changes.