At 35,000 Feet, air pressure outside an airplane is so low that, should explosive decompression occur, the atmosphere inside the craft would instantaneously turn to blinding fog. Passengers would hear a tremendous explosion. In such a catastrophic loss of cabin pressure, expanding air would blast meals, luggage, crew, and unbelted passengers into the thin and frigid air outside.

The pilot would have to put the aircraft into a dive at once, since the passengers would be threatened not only by the loss of oxygen but also by the drop in pressure. At this altitude, pressure is so low that dissolved gases in the blood fizz like the contents of an abruptly opened shaken bottle of pop. This causes aeroembolism, or the bends, bringing excruciating joint pain and sometimes paralysis. Life-threatening bubbles in the blood enter the heart, lungs, and brain.

Seventy years ago, none of this was well understood. What was clear was that military airplanes, especially fighters, were going to have to fly higher than they usually did during World War I. German zeppelins, ghosting in from the fog and clouds of the English Channel, had presented the first challenges of flying and fighting at high altitudes, about 20,000 feet. Few fighters, and even fewer pilots, operated well at that altitude. Fortunately for England, German crews also suffered; frostbite and hypoxia, or oxygen hunger, were constant companions of the zeppelin crews, though some primitive oxygen devices were used.

By 1918, the US War Department was well along in creation of the Air Service Engineering Division and its Flying Section, a flight-test organization, at McCook Field in Dayton, Ohio. At McCook Field, predecessor to one of the world's largest military research facilities (Wright-Patterson AFB, located about ten miles from the site of McCook), the earliest experiments in high-altitude flight took place.

The first of the high-altitude pilots at McCook Field was Capt. Rudolph W. Schroeder, known as "Shorty." At six-foot-two, his physical appearance belied his nickname. He was an intense, wiry man with the soul of an explorer. As early as 1914, Captain Schroeder had made a name for himself in air racing. In 1919, he was handpicked by his commanding officer to compete in a landmark New York-Toronto race. Schroeder and Mc- Cook's engineers and technicians resurrected a moribund Vought VE- 7. In this plane, Shorty flew to victory.

The X-15 of Its Day

Captain Schroeder's high-altitude work commenced in earnest with a series of flights beginning in 1918. Late in World War I, a new biplane, the LePere type C-11 entered the inventory of the US Army Air Service. In production trim, this craft was powered by a normally aspirated Liberty engine with a twelve-cylinder Vee configuration. Soon after the war, this aircraft became America's first dedicated research aircraft and was, in a sense, the X-15 of its day.

Captain Schroeder's early high-flying attempts netted altitude marks of 24,000 and 27,000 feet, and with this background of experience, another record attempt was made on September 8, 1918. By the time he had reached 23,000 feet, Captain Schroeder was experiencing symptoms of hypoxia: He felt sleepy, tired, cross, and hungry. The symptoms were relieved by oxygen.

On earlier flights, Captain Schroeder had used a face mask and an oxygen bottle with a "very positive regulator." However, the mask fit so tightly that it numbed his face. On this flight, he used a rubber hose and pipestem connected to an oxygen flask, with a valve he controlled manually to regulate the flow. Having no flow regulator or indicator, he checked to make certain that oxygen was flowing by putting his tongue over the pipestem opening to feel the pressure. By the time the flight was well under way, the hose and oxygen flask were covered with a quarter-inch of frost.

Continuing to climb, he again experienced hypoxia symptoms at 25,000 feet and cranked up his oxygen supply, noting in his log that the temperature was minus sixty degrees Celsius. At 27,000 feet, he could not see through the frost on his goggles and raised them to read the aneroid altimeter. The air was so cold that his eyes watered excessively, but he saw that he had reached an altitude of almost 29,000 feet.

At this point, however, he ran out of fuel and the plane began to spiral down to 20,000 feet, where he recovered from most of the adverse symptoms. Captain Schroeder continued his descent through clouds and snow and finally broke into the clear over Canton, Ohio, about 200 miles from McCook Field.

He was forced to make a landing on rough ground, breaking off the tip of his propeller in the process. It was a good landing, considering that his hands and face were entirely numb and his lips and four of his fingers were frozen badly enough to require immediate medical attention.

On February 27th, 1920, Major Schroeder (who had been recently promoted in recognition of his high-altitude work) set a new world altitude record of 33,113 feet in the further-modified LePere, which by now was equipped with a Moss turbosupercharger built by General Electric. This historic flight took an hour and forty-seven minutes, logged an average speed of 119 miles per hour, and came close to costing Major Schroeder his life.

A Blind Landing

As he neared the top of his climb, Major Schroeder discovered that falling ambient pressure caused what is described in official records only as a "valve problem." The valve probably was the one on his oxygen regulator, since Major Schroeder later stated that he was losing consciousness.

Whatever the case, he raised his goggles in an attempt to see and deal with the problem, but the intensely cold blast of the slipstream instantly froze the moisture in his eyes. Now blind, and on the verge of passing out, Major Schroeder put the plane into what he thought was a steep spiral; it was actually a dive.

The fabric-covered LePere screamed earthward for six miles at speeds of up to 300 miles per hour. At an altitude of only about 2,000 feet, the crippled pilot managed to bring the plane back under control. Despite his impaired vision, Major Schroeder landed safely at McCook Field with a new world's altitude record and a wealth of valuable new information on the winds, temperatures, and turbulence that one could expect to encounter near the stratosphere.

Following Major Schroeder's pioneering work, the torch was carried further by 1st Lt. J. A. Macready. Between early 1919 and 1921, intensive work went on at McCook Field developing a new propeller for the LePere. A propeller was needed that would not overload the engine in the "thick" air at low altitudes but would allow the engine to develop full turbocharged performance in the "thin" air at altitudes higher than 35,000 feet. (In those days, the variable-pitch propeller was just a gleam in an engineer's eye.) The final design was a large two-bladed propeller, which proved superior to earlier four-bladed designs.

By September 28, 1921, Lieutenant Macready and the modified aircraft were prepared. Lieutenant Macready described the day as being perfect for the flight. He suited up in his long woolen underwear, topped by an all-leather flying suit filled with down and feathers, coupled with similarly insulated boots. On his head he wore a helmet-like head "mask" made of leather and lined with fur. Attached to the head mask was a fitted oxygen mask, which, though not described in detail, does not appear to have contained any valving and was probably connected by a hose to a manually operated flow valve and regulator. The goggles were separate pieces of equipment; the insides of the lenses were coated with a "secret gelatin" which was supposed to prevent ice from forming on them.

The on-board oxygen system consisted of five flasks loaded at 2,300 pounds of pressure to supply the oxygen mask. There was an additional emergency flask, pressurized at 1 ,500 pounds. By ripping off a patch of tape on the side of the oxygen mask, Lieutenant Macready could, in the event of trouble with the main oxygen supply, push the hose from the emergency supply through the exposed hole and into his mouth.

Following takeoff, Lieutenant Macready flew in circles over McCook Field in order to be within gliding range of the airstrip. By the time he reached a record altitude of 36,750 feet, his circle was about seventy miles in diameter.

"Dim and Shaky"

Lieutenant Macready began using oxygen at an indicated altitude of 20,000 feet. He noted that he observed a significant "slowing up of the senses" starting at around 30,000 feet, and the effect steadily worsened as he flew higher. He also observed that any physical effort at these higher altitudes caused his vision to become "dim and shaky," and when this happened he increased the flow of oxygen.

Fortunately for the unknowing pilot, his slow ascent and use of 100 percent oxygen allowed his body time to rid itself of the normal nitrogen dissolved in his blood. As a result, he remained unaffected by the bends.

At an indicated altitude of 39,000 feet, Lieutenant Macready's breath froze in the oxygen mask tube. Fortunately, he was alert enough to notice the restriction in flow and immediately began an effort to clear the blockage, achieving only a slight taste of ice for all his trouble. He ripped the tape off his mask and inserted the emergency supply tube, opened the emergency valve, and felt immediate relief.

An hour after takeoff, Lieutenant Macready reached an indicated altitude slightly higher than 41,000 feet. On the basis of preflight engineering studies, he expected to be able to exceed this altitude by 7,000 to 8,000 feet, but he could not. Inasmuch as he was breathing pure oxygen at what was probably ambient pressure, it was fortunate for him that the aircraft could not struggle higher. In modern systems, the regulators supply pure oxygen at positive pressure at these altitudes since, at that height, the partial pressure of water vapor in the lungs is the same as the partial pressure of oxygen. Without positive pressure oxygen, Lieutenant Macready would not have survived at a higher altitude.

As the biplane struggled along at maximum ceiling, Lieutenant Macready found it almost impossible to control the aircraft. Its aerodynamic surfaces had virtually lost control authority in the thin air. After about five minutes, he became convinced that the aircraft had topped out and reduced throttle to begin his descent. At that point, he later reported, "the bottom seemed to drop out of the plane and down it went quickly."

The aircraft was completely out of control. The speed of descent was so rapid that Lieutenant Macready was unable to adjust the engine and radiator to maintain cockpit heat. Because of this, his goggles iced over and, as his plane fell to earth, he became weak, groggy, and effectively blind in his struggle to regain control.

Lieutenant Macready knew that, if he could fall to thicker air, he could survive and regain control of the aircraft. He hung on grimly and finally got the LePere flying again at 30,000 feet. He then descended to 20,000 feet (where, he noted, he felt comfortable) and flew there for around twenty minutes to make certain everything was working properly. His return to McCook Field was routine.

Postflight analysis yielded various estimates regarding his record-setting altitude. Measured by the method used by the Fédération Aéronautique International (custodian of aviation records) in 1919, he had reached 39,730 feet. Calculated by the FAI method used in 1920, it was 34,563 feet. The probable (best estimate) altitude was 36,750 feet. In any case, Major Schroeder's record had been broken by a decisive margin.

In the course of his many research flights, Lieutenant Macready set three world altitude records, the highest officially logged at 38,704 feet. For this work he was awarded the McKay Trophy three times. He had set the stage for modern high-altitude flight in piston-engined, turbosupercharged aircraft.

Partial-pressure and full-pressure suits, cabin pressurization, and sophisticated oxygen regulators were more than ten years in the future at the time of Lieutenant Macready's flights. His heroic work, and that of Major Schroeder, laid the foundation for all that we now take for granted and paved the way for safe and reliable high-altitude flight.