Flying Blind
DURING AVIATION’S HARLY DAYS PILOTS NAVIGATED BY following the railroad tracks. When the weather closed in, they would come down to low altitudes and continue onward. This practice led to such techniques as keeping to the right, to avoid collisions with low-flying oncoming planes. Hazards of the business included running into a locomotive and hitting a hill pierced by a tunnel.
“Map reading was not required,” a pilot later recalled. “There were no maps. I got from place to place with the help of the seat of my pants. If it left that of the plane, when visibility was at a minimum, I was in trouble and could even be upside down.” Another pilot, who flew the mail, was fatalistic: “I certainly had no wish to get killed, but I was not afraid of it. I would have been frightened if I thought I would get maimed or crippled for life, but there was little chance of that. A mail pilot was usually killed outright.”
Still, if aviation was to grow, it was essential for planes to fly safely in bad weather and to meet their schedules. This point was critical; the entire development of flight would hang on it. Here lay the distinction between aviation as sport and as a potentially major industry. Planes competed with railroads, after all; they offered the advantage of speed, but trains held the advantage of reliability. They could run on time in virtually any kind of weather. To find a true path to growth, airlines would have to do this as well.
Foul weather presents several problems to aviators. First of all, powerful winds can arise or shift suddenly. Though many advances have been made in detecting and overcoming such currents, they remain a problem to this day. The worst sorts of atmospheric disturbances can be too abrupt and overwhelming for any pilot, no matter how well equipped, to deal with. Landing the plane in foggy weather presents another set of difficulties, since misjudging the runway by even a few feet can be disastrous. Many schemes have been tried over the years to clear low-lying fog from runways (see “Fogbusters,” Invention & Technology , Fall 1993), and modern radio equipment has been invaluable.
Yet in the 1920s and 1930s flying in clouds or fog—flying blind—was extremely dangerous even away from airports and in windless air. Pilots by then were flying at night, by following beacon lights that marked their routes. But if an aviator could not see the ground because of fog, or lost sight of the sun amid gray daylight murk, it was almost certain that he would quickly lose control, go into a spiral dive, and crash. No one knew why this happened, but happen it did, with depressing regularity. Something about being cut off from familiar reference points made it virtually impossible for a pilot to maintain straight, level flight.
Even birds couldn’t do it. An Army flier, Lt. Carl Crane, tossed a blindfolded pigeon out of an airplane and saw it spin out of control. The bird could do no more than let itself fall with wings held high, which amounted to bailing out. That settled it; if even a bird couldn’t succeed, no pilot could be expected to fly blind if all he had was the seat of his pants. Instead, blind flight would demand a completely different approach, in which a pilot would learn to disregard his senses altogether and rely on gyroscopic instruments. Fortunately, the means to build such instruments lay at hand. Indeed, the gyro had already starred in a spectacular demonstration.
To the crowd assembled at a June 1914 exhibition at Bezons, near Paris, well aware of the sensitive character of the airplanes of the day, the flight was astonishing. Lawrence Sperry, an American, piloting a two-man biplane, flew low and took his hands off the controls, holding them high, where everyone could see them. His mechanic then climbed onto the lower wing and made his way out to a distance of several feet. The man’s weight would ordinarily have upset the aircraft, but it flew on, its wings remaining nearly level as ailerons deflected automatically. Soon afterward Sperry stood up in his seat as his companion crawled rearward along the fuselage. The plane remained level and did not nose up. For a finale both men abandoned their seats and walked out onto the wings. The airplane now lacked a pilot entirely, but still it flew on.
Sperry’s contribution was a system that used gyroscopes to manipulate the controls, keeping his craft straight and level even when he played lookma-no-hands. His feat caused a sensation. The jury of a French airplanesafety competition awarded him a prize of 50,000 francs ($10,000). William L. Cathcart, a leading aviation writer, declared: “The stabilization of the aeroplane is an accomplished fact. The human factor—to which in its history so many tragic deaths have been due— can now be entirely eliminated and replaced by an unerring apparatus.”
This particular apparatus did not go very far. The French tested it during World War I but found that it was too heavy and interfered with the rapid maneuverings that could save a pilot in combat. Nevertheless, Sperry’s 1914 demonstration was a milestone. It linked the gyroscope, a nineteenthcentury invention that had theretofore been used mostly at sea, to the problem of aircraft control. It launched a line of development that would lead to major cockpit instruments and automatic pilots. And it brought forth the Sperrys as leaders in this new field.
Lawrence was the son of Elmer A. Sperry, an inventor and entrepreneur. The elder Sperry had come up in classic Horatio Alger fashion, beginning as a farm boy near Cortland, New York, learning about machinery from neighbors in town. He went to Chicago to seek his fortune and innovated in a variety of fields: electric arc lights and generators, streetcars, electrochemistry, mining equipment, electric autos and their batteries. He achieved modest success, and his companies prospered. Still, no one placed him alongside the likes of Thomas Edison and Alexander Graham Bell, at least not at the turn of the century. This began to change when the gyroscope entered his life.
The first detailed description of the gyroscope dates to 1852 (though the British navy had experimented with an artificial horizon similar to a spinning top more than a century earlier). The French physicist Léon Foucault introduced the name, which means “turn seer.” The basic feature of a gyroscope is that while it spins, it retains its orientation in space, regardless of the movement of whatever supports it. Foucault showed how to attach a weight to a gyroscope’s suspension so that its spinning rotor would align with the earth’s axis of rotation, making it act as a gyrocompass. The gyro entered the realm of technology in 1894, when an Austrian inventor, Ludwig Obry, introduced a course-keeping system that used it to steer the straight-running torpedo.
Soon after that the gyrocompass emerged as a major focus for development. Back in 1877 Mark Twain had quoted a sailor as saying that “there was a vast fortune waiting for the genius who should invent a compass that would not be affected by the local influences of an iron ship.” Magnetic compasses became even less reliable when electricity went to sea, for a ship’s motors and wiring produced magnetic fields of their own. In submarines, which began to be built in large numbers after 1900, such compasses were totally useless; the steel hulls screened out the earth’s magnetic field entirely.
Elmer Sperry first came to the gyro believing that a large one mounted in a ship could make it stable against rolling. The gyro would swing and counteract large rolls, restricting the ship to much smaller ones. After filing for a patent in 1908, he drew support from the Navy and launched a program of development. Meanwhile in Germany the inventor Hermann Anschütz-Kaempfe was introducing the first practical gyrocompasses. Sperry believed he could build better ones and learned that the Navy was again willing to help. In 1910 he founded the Sperry Gyroscope Company, in Brooklyn, New York, to pursue these two efforts.
Then his son Lawrence introduced him to aviation. The young Sperry had been enthusiastic about flying since he was a child, building his own airplane inside the house, winning his father’s forgiveness after knocking down part of a wall to get it out, then installing an engine. His father sent him off to an Arizona prep school in 1911 before he could break his neck, but for the flamboyant Lawrence this was only the beginning. He went on in 1918 to marry a rising motion-picture actress, Winifred Alien, who said that “life with Lawrence is more exciting than anything in the movies.” Even in prep school he had no doubt about what he wanted. In April 1912 he had written home: “I want to enter the aeroplane business. … I am very determined to go into aeroplanes and I think that you should help me get started.”
THE NAVY WAS BEGIN ning to show an interest in the flying boats of Glenn Curtiss, and the senior Sperry decided that Lawrence could help in developing a flightcontrol system. It would improve on Obry’s gyrostabilizer, for while that system kept a torpedo from turning to the left or right, Sperry’s would control two types of motion: pitch (upand-down movement of the nose) and roll (rotation about a lengthwise axis). His ship gyrostabilizer featured a similar system. It used a pendulum as a sensor to detect incipient rolling; upon doing so, the pendulum would trigger an electric motor to swing the big gyro pre-emptively, before the rolling could begin to build up. This active feature distinguished it from earlier passive gyrostabilizers, which waited until a roll was under way before they took effect. In the airplane control, where a pendulum would not work because banked turns often introduce a false vertical, gyros themselves would act as sensors by defining a stable reference. When the plane banked or pitched with respect to that reference, the system would activate servomotors that would move the ailerons or elevators, restoring the plane to level flight.
Here lay the basic elements of an autopilot, able to steer a plane safely where birds would prefer to bail out. However, for the moment, Sperry and his sponsors took the view that they could offer better advantages by pursuing passive gyroscopic instruments, which would not trigger a control system but rather would present information for the pilot to act upon. Many pilots who were wary of instruments found this system preferable; it also allowed for more maneuverability and avoided weighing down a plane with heavy apparatus. The turn indicator, invented in 1917, was an early result of this new approach. The instrument amounted to an air-driven gyro attached to a spring. Left to itself, a gyro would shift position with each turn and then remain in its shifted position when the plane steadied on its new course. The spring helped by pulling the gyro upright. A needle linked to the gyro would then deflect only when the plane was actually turning; at other times it would point straight up. A pilot could fly his proper course by keeping an eye on this instrument. As long as he did this, he would have little trouble maintaining his altitude as well.
THE TURN INDI cator did not take the world by storm. Well into the 1920s it remained an experimental device, neither commonly used nor generally available, because it went against the rough-and-ready culture of the flyboys. Unfortunately, their seat-of-the-pants methods remained unacceptably dangerous when the weather closed in. The first step in devising a solution lay in finding out why. Dualcockpit aircraft made it possible to conduct tests, with one pilot under a canvas hood and the second in the clear for safety. The answer proved to lie in the fact that just as an automobile will wander off the road if the steering wheel is released, a plane will also start to turn if it is left to itself. As it turns, it banks; the wings are designed so that this happens automatically. And in a banked turn the direction down as perceived by the pilot continues to point to the cockpit floor, even if the plane is virtually on its side. In bad weather, with no visual cues to show true up and down, the illusory sense of direction provided by the plane’s acceleration is all a pilot has to go on.
As an airplane starts to turn on its own, it also dips its nose and picks up speed. A 1920s pilot would sense an unbanked turn—the kind a car makes when rounding a corner—but not a banked one. However, he would have no trouble noticing the increasing airspeed, and he would conclude that the plane was in a dive. He would pull back the control stick, which would only steepen the turn and make things worse. Soon the engine would be racing and the propeller would be snarling—and the pilot’s seat-of-the-pants feeling would urge him strongly to pull back even more. If he was lucky, he would break out of the clouds and see the ground again, realize what was happening, and level off. Otherwise he would crash, or his plane would break up in flight.
By this time designers were building planes that were inherently stable (see “How Did It Become ‘Obvious’ That an Airplane Should Be Inherently Stable?,” Invention & Technology , Spring/Summer 1988). However, that didn’t help. It merely meant that aircraft, left to their own devices, would not flip onto their backs or tumble out of control, like a World War II bomber with its tail shot away. In fog a stable airplane still could not hold its course and avoid the spiral dive. It resembled a car, which rides very stably on its four wheels but will go over a cliff if the driver can’t see out the windshield. And the airplane, like that car, would maintain its stability all the way to the bottom.
Some fliers tried to sense their turns by hanging a watch from its chain within the cockpit, as a pendulum. It didn’t work; in a banked turn the watch continued to point floorward. Others reasoned that since a ship steers through fog by using a compass, why can’t an airplane? A pilot could indeed trust the compass as long as he was flying straight and level; it then showed the same reliability as it would on a ship, which in calm seas rotates about only a single axis. But in the air, a plane could bank, turn, or dive sharply, and the compass needle, with only one degree of freedom, had to respond to motion about three separate axes. The effects could be chaotic.
The gyroscopic turn indicator was unaffected by banking and the vagaries of the earth’s magnetic field, but it offered no panacea, for it went against all the instincts that pilots had honed and relied upon. Many pilots simply ignored it. The aviation writer Wolfgang Langewiesche would write in 1943 that “in ‘contact’ [i.e., visual] flight, when the airplane turns, things scream at you: the airplane is banked; the whole world is at a slant, and wheeling. But in instrument flight, when the airplane turns, nothing screams. Only a gauge sits there quietly, no longer like this ↑ but like this ↖. And this quiet little signal must hit you like an electric shock.” Few pilots could respond in this fashion. A common reaction was that the turn indicator wasn’t something you could trust. It worked all right in the clear, but whenever you flew into a cloud, the thing would go crazy and show a turn.
Using the turn indicator successfully thus called for more than intellectual understanding; it demanded a knack. During the mid-1920s some pilots began to get that knack. One of the first was Charles Lindbergh, who was working at the time as a mail pilot. He relied on his turn indicator twice in 1926 while flying between St. Louis and Chicago, on occasions when he was caught in bad weather. Both times he could do no more than climb to a safe altitude and then bail out. But these experiences taught him the key: to believe the indicator absolutely and disregard his own senses. He drew on this experience during his transatlantic flight to Paris in 1927, in which he flew through fog and clouds for long stretches. Twice he started to fall off in a spin, but both times he recovered by trusting his turn indicator.
The thing could be done; blind flight was indeed possible. Still, the turn indicator would not stand on its own, for by itself it offered too little information. It failed to show a plane’s angle of bank, or whether it was pitched up or down. At this point the Guggenheim copper barons came to the forefront. The wealthy Daniel Guggenheim had a strong interest in philanthropy; his son Harry had flown in the war as a naval aviator; and these influences led him to set up the Daniel Guggenheim Fund for the Promotion of Aeronautics. It opened for business in 1926 with Harry as president.
Young Harry provided grants to such schools as Michigan, Cal Tech, MIT, and Stanford to build departments of aeronautical engineering. He sent Lindbergh on a forty-eight-state tour. He encouraged the growth of airlines by lending California’s Western Air Express, a predecessor of TWA, $155,000 to equip a “Model Air Line.” And in 1928 he turned to the problem of blind flight by setting up a laboratory at Long Island’s Mitchel Field, close to Sperry’s company in Brooklyn. His test pilot was James H. Doolittle.
To the general public Jimmy Doolittle was the ultimate barnstormer. He had been the first to cross the country in less than twenty-four hours, with a one-stop flight on September 4-5, 1922. He set a speed record for seaplanes of 232.573 miles per hour in a 1925 race. He was the first to fly an outside loop, in which the pilot noses over into a power dive and continues until he is flying upside down. Within the Army Air Corps he had repeatedly come close to tearing the wings off aircraft. Yet Doolittle was also a true aeronautical researcher. His Air Corps tests had given pioneering results on how a pilot blacks out when performing violent maneuvers. This work fed into studies at MIT, where Doolittle received an Sc.D. degree in 1925.
Within the Guggenheim project Doolittle wanted to do more than merely fly in clouds. He wanted to take off, fly a planned course, and land, all with a hood over his cockpit. Two test aircraft stood at his disposal, and he was welcome to equip them and the airfield with whatever new instruments he thought would help. Radio offered part of what he needed, providing transmissions that would show the direction back to Mitchel Field and indicate where he should start his descent. He would then follow a sloping approach path that he described as “flying an airplane into the ground,” relying on a strong undercarriage and heavy shock absorbers to permit a rough but safe landing on a wide, unobstructed field. But Doolittle wanted other equipment as well.
For one thing, he was dissatisfied with the barometric altimeters of the day, which were accurate to only about one hundred feet. Through colleagues at the National Bureau of Standards he learned that a German-born instrument maker, Paul Kollsman, had built an altimeter of surpassing accuracy. Kollsman had concluded that standard models had faulty gears, and he turned to Swiss watchmakers to fabricate his movements. Kollsman’s altimeter proved to be up to twenty times as accurate as the standard version, and soon one was installed in Doolittle’s main research aircraft.
By this time the turn indicator had given way to the turn-and-bank indicator. It incorporated a ball within a curving glass tube; the ball would stay centered during a properly banked turn but would shift position to indicate a sideslip. Doolittle still viewed it as insufficient, and he was displeased as well with the magnetic compass. He wanted a gyroscopic device that would show his angle of bank or pitch and a compass that would read true in a maneuvering airplane. He sought out Elmer Sperry, still going strong at sixty-eight, who had his son Elmer Jr. develop two new instruments.
The first was the artificial horizon. It contained a small silhouette of an airplane in the middle of a display, wings level, with a bar representing the horizon. That bar was linked to the gyro; it would remain parallel to the real horizon. It would tilt relative to the airplane silhouette to show a bank and would rise and fall to show pitching. Doolittle later wrote that this instrument was “like cutting a porthole through the fog to look at the real horizon.” It made instrument flying easier to accept because the display was not as abstract as .a bare needle on a dial. Instead it resembled what a pilot would see in clear weather, so the leap of faith it required was not as great.
The second new item was a gyrocompass, which had a gyro indicating direction with respect to a compass card. Doolittle could set the gyro while the plane was flying straight and level, using his magnetic compass (which under those conditions would read true). The gyrocompass then would show his true direction even while he was turning or maneuvering, in circumstar/ces where the magnetic type would be useless.
The main test came on September 24, 1929. A safety pilot sat in a front cockpit with Doolittle in the rear one under a hood. “I taxied the airplane out and turned into the takeoff position direction on the radio beam,” Doolittle later wrote. “We took off and flew west in a gradual climb.” He flew a course resembling an elongated racetrack, twice making 180-degree turns, then approached the field and landed successfully, all the while relying entirely on his instruments. The flight took fifteen minutes. The safety pilot held his hands up during both the takeoff and the landing to make it clear that Doolittle was flying the plane.
AFTER THAT TEST OTHER pilots installed Doolittle and Sperry’s instruments, and blind flight became increasingly routine. The new instruments also complemented the growing use of radio, which provided weather reports, pilot instructions, and navigational signals. Radio in the 1920s relied on low-frequency transmissions, which spread out and could not be shaped to form a true beam. But it was possible to arrange for such spread-out transmission zones to overlap, and these regions of overlap could mark a course through the air.
Radio signals pierce fog and clouds with ease, making it possible to follow them and to steer a true course. The transmission relied on a pair of simple antennas, each of which broadcast an intermittent signal. The regions of overlap were narrow, and within them a pilot would hear the signals merge into a continuous tone. He would then say he was “on the beam.”
After 1930, then, pilots had unprecedented advantages: two-way communication, weather reports, blind flying, and all-weather navigation, as well as air-traffic-control services near major airports. Nor were they optional; in 1932 the Commerce Department introduced pilot ratings that required proficiency in instrument flying and navigating by radio. Yet with all these responsibilities crowding in on him, a flier was starting to resemble a juggler with too many balls in the air. How could he fly his plane, navigate, read a map, listen to the radio, and watch his instruments, particularly at the end of a long and fatiguing flight?
Again Sperry Gyroscope came to the rescue, this time with the first successful automatic pilot. It was a direct descendant of the gyrostabilizer of 1914. That item of equipment had languished, partly because pilots expected to fly their planes themselves rather than trust to a robot. But with their workloads increasing, pilots now could see real value in the robot, for it would amount to an extra person in the cockpit. Elmer Sperry, Jr., took the lead in managing its development.
A key problem lay in the gyro’s tendency to drift. A gyro’s spinning rotor was no delicate wheel, as in a wristwatch; it had a fair amount of heft. It needed pivots that could support this weight, and these pivots transmitted frictional forces to the rotor, causing it to shift position. In an autopilot this could bring disaster.
Hence it was essential to continuously align the gyros while in flight. The solution, ironically, lay with the simple pendulum, which had been worse than useless as a turn indicator. This defect now became an advantage, for the autopilot’s pendulums would move only in response to gyro drift, which would produce an unbanked turn. This motion then would bring a corrective measure, restoring the gyro to its appropriate alignment.
In 1933 this work received a powerful boost from the aviator Wiley Post, who was preparing to fly around the world alone. He visited the Sperry factory, saw a prototype of the A2 autopilot, as it was known, and insisted on installing it in his Lockheed Vega 5-C, the Winnie Mae . He went on to complete his flight in less than eight days. The autopilot was a mainstay. It took over the controls while he was navigating; it even allowed him to snooze while in flight. During 1934 it began to see use in airliners, beginning with the Boeing 247 and Douglas DC-2. It still lacked the accuracy to keep a plane on course for hours at a time, but it was just the thing when a pilot wanted to take a break for ten or fifteen minutes.
Using instruments, airliners were able to meet regular schedules regardless of clouds or fog en route, while carrying grandmothers and babies. Thus by 1935 the problem of blind flight had reached a definitive solution. Following World War II, these efforts branched off in three new directions.
The first involved landing approaches in poor visibility. Doolittle had made true blind landings, but only by trusting in the strength of his undercarriage and having a large grassy field at hand. Postwar aircraft needed concrete runways and were too heavy to land using his simple technique of blindly following a sloping approach path. Fog could be a problem, but it was very rare for an airfield to have total zero visibility; usually the clouds were at least a few hundred feet up. Radio then offered a means to guide a plane through the murk, until a pilot could break into the clear. He then could land in the usual fashion, relying on his eyes.
The arrangement that permitted this was ILS, the instrument landing system. In contrast to the low-frequency transmissions of the 1920s, ILS used high-frequency radio. Its transmission could indeed form true beams, using specialized antennas. One such beam marked the glide slope for landing approaches, pointing above the horizon at a gentle angle. A second beam showed the direction to the airport. ILS caught on quickly after the war.
As ILS proved itself, a second new direction became apparent in guidance systems for missiles, which in essence amounted to highly sophisticated autopilots. Their genesis lay in further development of the aircraft autopilot as an electronic system. The early autopilots relied on sensitive electromechanical arrangements but used no electronics. The new ones incorporated electronic amplifiers. The difference was important, for these amplifiers could do more than accept signals generated by displacements of the gyroscopes. They could amplify signals from other instruments as well, and those instruments could supplement the gyros, making an autopilot more versatile.
This approach opened the door to tie-ins whereby a compass could also generate signals for the amplifiers. This compass could then keep the gyros aligned despite their tendency to drift. The Germans used such an approach during World War II, in the guidance system of their unmanned V-I cruise missile. After the war Charles Stark Draper of MIT took the lead in seeking guidance systems of truly great accuracy.
THE PROBLEM, AS always, was that gyros tended to drift. This inaccuracy could amount to more than ten degrees per hour; Draper wanted to reduce this figure by a factor of 10,000. A robot cruise missile—essentially an unmanned airplane—then might fly for ten hours and strike its target with an error of no more than a mile. In seeking this accuracy, a key approach lay in reinventing the gyroscope’s mountings.
Draper’s gyros were still heavy and seemed to demand the support of robust bearings, which would produce drift. He could eliminate the drift if he could use fine jeweled bearings, like those of a wristwatch. To do this, he started by enclosing the heavy gyro within a canister, floating this sealed container within a fluid. The fluid made it buoyant, supporting its weight so that the fine bearings would not have to carry a gyro’s heavy load.
In addition, Draper made the fluid viscous, to strongly damp the canister’s movements. This produced a completely different type of gyro action, one governed by the damping effect of the fluid. The result was that the gyro could drift as it wished, within its container, without diminishing the instrument’s accuracy. The instrument could then take full advantage of the much higher accuracy that the fine bearings now permitted.
Draper’s improved gyro laid the foundation for inertial guidance systems. The Air Force lent Draper a B-29 bomber, and in 1949 he began making test flights out of Bedford, Massachusetts, near Boston. Accuracy steadily improved, and in 1955 Draper showed off his progress in a dramatic flight to Los Angeles. The B-29 crew relied entirely on Draper’s internal system as they navigated. At the end of the cross-country flight, the plane was within two miles of its calculated position.
That work was classified, but a decade later internal navigation became available for use in airliners. The Boeing 747, which entered commercial service in 1970, was the first to use the new technique. It brought a complete turnabout in the role of pilots, for in an important sense these professionals would no longer fly their planes. During long stretches of routine flight they would leave the task to inertial navigation, as they watched their instruments, managed their onboard systems, and kept alert for changes that might require their attention.
Another turnabout involved the very concept of flight on instruments and represented the third new direction. Since the earliest days of flight, pilots had followed the rule of “see and be seen,” looking out for other aircraft in the fashion of motorists on a freeway. Flight at high altitude, well above the clouds, made this easy at first. After 1955 there was even the prospect of jet airliners. These would cruise at 35,000 feet, in excellent visual conditions.
Yet to knowledgeable people this prospect was a thing of horror. With their high speeds, jetliners in no way would resemble cars on a highway. Instead they would resemble the racers of the Bonneville Salt Flats, which demand twenty miles of clear surface. A jet pilot would no longer be able to see another plane in time to avoid a collision; by the time the other fellow appeared, it would already be too late.
The Federal Aviation Agency had a solution: positive control. Under this arrangement airliners would no longer enter the air at will, to fly as their pilots might wish. Instead they would fly under the watchful eyes of ground controllers, who would follow them on radar and issue directives. In 1958 the FAA took the first step, marking out three transcontinental routes that would use positive control. Within them visual flight was illegal. Over the next decade the principle of positive control spread, encompassing the whole of U.S. airspace at airliner cruising altitudes and later the lowaltitude space near airports as well.
With positive control, aviation had made a complete turnaround from its early beginnings, when pilots pressed on through clouds and fog, trusting the seat of their pants. The advent of positive control meant that aircraft would fly without visual references, even in clear weather. Indeed, they would be barred from the air unless they carried appropriate equipment. Cockpit displays now meant everything. Pilots would still look through the windshield during takeoffs and landings. But once they got in the air, the outside world would be useful only for sightseeing.
