Blind Landings: Low-Visibility Operations in American Aviation, 1918–1958

• 4 The Promise of Microwaves
• Chapter
• Johns Hopkins University Press
• pp. 80-103
• View Citation
• Additional Information

CHAPTER FOUR
The Promise of Microwaves

Commercial pilots’ rejection of the Hegenberger system left a schism in the aviation community over how best to achieve blind landings. The Air Corps was satisfied with its radio compass approach, while commercial pilots wanted some version of the National Bureau of Standards’ (NBS) tripartite system. In 1934, the community had not agreed on how a blind landing system should work.

Between 1934 and 1939, however, the Air Corps moved closer to the airlines’ position, agreeing by 1939 that the three-part NBS model was, indeed, the best route to a solution. The remaining dissension was over details: how long should the glide path be? Must it be straight? What frequencies should it use? With consensus around a particular model of the solution achieved, the various organizations in the aviation community had common ground to negotiate over. Nevertheless, determining the system’s details proved difficult.

The most vexing detail was the shape of the glide path beam. The Air Corps insisted on a straight glide slope, while airlines were satisfied with the “bent beam” idea and pushed for rapid deployment of such a system. The Air Corps, in turn, demanded a new system that relied on microwave radio, which was being designed in a unique collaboration between Civil Aeronautics Authority (CAA), the Massachusetts Institute of Technology (MIT), the Air Corps, Stanford University, and Sperry Gyroscope. The microwave system promised to provide the long, straight glide path that Air Corps officers believed was necessary, and they sought a way of delaying the airlines’ drive for the bent-beam system. Gen. Henry “Hap” Arnold thus deployed the classic Washington delaying tactic—he requested an independent review of the various blind landing programs. In effect, this placed the decision in the hands of President Roosevelt, who turned it over to the National Academy of Sciences. Their recommendations would become policy until U.S. entry into World War II changed everything.

In the only existing history of this subject, William Leary has argued that Arnold’s review was an “unnecessary complication” to the adoption process.¹ But Arnold had good reason, in the form of a new microwave-based system being developed for the Army Air Corps. As we have seen, the descendants of Diamond and Dunmore’s system had serious stability problems, and Arnold believed the new microwave solution, not the troubled CAA system, was the correct choice. There is a tinge of bitterness in Leary’s argument because Arnold’s review had the effect of preventing the adoption of any commercial blind landing system until well after the war, costing the airlines money—and costing pilots’ and passengers’ lives. Arnold held a vision of a microwave future free of stability problems. The National Academy’s review panel agreed with him.

DEFINING A “STANDARD” SYSTEM:
THE RADIO TECHNICAL COMMITTEE FOR AERONAUTICS

In 1936, the Bureau of Air Commerce formed a committee to help it deal with problems it had found in the use of radio technologies in aviation. Called the Radio Technical Committee on Aeronautics (RTCA), the group was composed of senior people from the technical departments of aviation-related organizations. The Army Air Corps and Navy Bureau of Aeronautics each had two representatives, as did the Bureau of Air Commerce. The Federal Communications Commission and State Department each provided one member. From outside the federal government came representatives from the Air Transport Association, the trade organization representing the scheduled airlines, and the Air Line Pilots Association, the labor union representing commercial pilots. The chair of the committee was elected and tended to rotate among the group. Private, or general, aviation was not represented until after World War II.

One important note is the RTCA’s legal basis of authority. It had none. The Bureau of Air Commerce had not sought, or received, blessing from Congress in the form of a law, or from the president in the shape of an executive order. The bureau’s hope had been that by bringing together the full range of aviation interests from inside and outside government, whatever agreement was reached would have what Osmun has called the “authority of agreement.”² The reasoning went that if everyone agreed to a standard, that standard would hold moral authority, even in the absence of legal authority. And absence of legal authority is precisely what existed: the Bureau of Air Commerce could recommend standards for airport equipment, but it had no enforcement power, and it was legally barred from funding airport improvements. It could not buy equipment to install at airports, blocking that possible route to establishing a de facto standard.

In addition to the RTCA’s lack of legal authority, no other mechanism existed to coordinate aviation standards among Army, Navy, and Commerce Departments. The RTCA provided a forum for discussion of radio issues and for the negotiation of proposed standards, but for a standard to succeed, each member had to convince his (there were no women on the committee in the first twenty years of the RTCA’s existence) parent organization to adopt the proposed standard. That proved difficult. Nevertheless, by providing a forum for discussion of needs and possibilities among various groups, the RTCA served an important role in the development of blind landing systems by establishing minimum criteria and, more importantly, by defining problems that still needed to be solved.

The Bureau of Air Commerce began to revisit the blind landing system question in 1936, after the airlines refused to use the Hegenberger system. It reinstalled the Newark equipment at Indianapolis and began once again to investigate the Bureau of Standards system as a possible solution to the blind landing problem. The rapid advancements made in the state of the art by other organizations, particularly the United-Bendix partnership, however, meant that the Newark installation was obsolete. This fact and the growing plethora of derivative systems led the bureau to ask the RTCA to examine all of the existing blind landing systems and, based on that, recommend a suitable standard. In this way, the bureau intended to capitalize on the knowledge gained and innovations made over the preceding few years by other organizations, without committing itself to any one company’s design.

The RTCA’s Subcommittee on Instrument Landing Devices, therefore, visited College Park, Maryland, to examine the Washington Institute of Technology’s Air-Track system (the civil name for the navy’s YB system), Kansas City, where United demonstrated the UAL-Bendix system, and finally Indianapolis, where on May 14 and 15, 1937, it tested the Bureau of Air Commerce’s slightly modified system. More importantly, the Indianapolis visit also resulted in an extended testing of the Lorenz system, imported from Germany by International Telephone and Telegraph (IT&T), the parent company of Lorenz A. G. This first hands-on review of the only European instrument landing system brought extensive press attention, as well as detailed scrutiny by the Army Signal Corps.

The Signal Corps’ representatives, Capt. George V. Holloman and Maj. F. S. Borum, argued that the curved, constant intensity flight path produced by the system required pilots to constantly change the aircraft’s attitude and throttle setting during landing, which was in opposition to army training.³ The army trained its pilots to land by maintaining the aircraft in a level attitude and slowly closing the throttles to maintain a powered glide into the airfield. This reduced the pilot’s workload and was relatively easy to train pilots to do.

Pilot training was not the only problem the army foresaw in the curved glide path. The curvature placed aircraft at low altitudes (around fifty feet) at a half-mile from airport boundaries. The army believed that few of its airfields, and even fewer commercial airports, were sufficiently clear of obstacles to make this a safe approach altitude. The army’s standard for obstacle clearance was to ensure a clear 20:1 slope for aircraft approaches, which was approximately a 2.8-degree glide. The Lorenz system provided a 1-degree glide from a half mile out, with the curvature increasing rapidly as distance from the airport increased. Beyond two miles out, pilots considered the path unflyably steep. They therefore had to intercept the glide path at roughly 700 feet at the outer marker 1.9 miles from the airfield, descend rapidly to fifty feet, and then level out into the final portion of the approach.

The two issues of pilot training and obstacle clearance mentioned in the two pilots’ report was likely informed by a third, unstated, concern. Although the 16,000- to 25,000-pound aircraft commonly used by the air transport industry could negotiate the curved glide path in the hands of a skilled pilot, the army had much larger planes on its drawing boards. In fact, the 55,000-pound Boeing B-17 prototype had begun flying in 1936, and by 1937 Air Corps leaders were beginning to realize that aircraft of such size were not maneuverable enough to follow either this curved glide path, or the very short overall approach that the Lorenz—and to be fair, all other existing systems—provided. As a result, the director of the Aircraft Radio Laboratory, Col. John O. Mauborgne, added an endorsement to Holloman’s report suggesting that the army needed to design, or have designed for it, a straight glide path.⁴

The two officers’ criticisms were certainly valid. The training issue is difficult to assess, but the “army way” to land a plane became the “right way” after World War II, despite the army’s complete replacement of its outdated instrument flying training program by a commercial one in 1941.⁵ The army could have trained its pilots to use a curved glide path, but it rightly perceived no benefit in doing so.

The more significant issues were obstacle clearance and aircraft size. Obstacles in airport approaches were one of the pilots’ union’s biggest heartaches, and its president agitated in Congress to get that body to grant the Bureau of Air Commerce and its successor, the Civil Aeronautics Authority, the authority to regulate airport approaches. In 1937, in fact, pilots for Eastern Airlines and American Airlines refused to fly DC-2 and DC-3 aircraft into Washington-Hoover airport, citing the dangers posed by smokestacks, power lines, radio towers, and a highway that ran through the middle of the field. They were joined by Eleanor Roosevelt, who said she feared for her friends’ lives in using the field, and a photo essay in the Washington Times showing the offending structures. The union also listed Fort Worth, Wichita, Chicago, San Francisco, Newark, Fresno, and Bakersfield as cities with seriously obstructed airfields. The Lorenz “bent beam” system would certainly not have been safe at any of these cities, until the municipalities agreed to remove the obstructions or move the airport.⁶ The army’s officers were correct in their belief that the system would not serve many airports without substantial modification of the airports’ surroundings.

It is important to note that the Lorenz system was not the only curved glide path system. All systems that had been based on the Bureau of Standards work generated such a path and at roughly the same angle of inclination. Commercial operators had considered this desirable at first because following it reduced the descent rate of the aircraft as the plane approached touchdown, thereby reducing impact. When it had tested them in 1935 and 1936, the army had criticized this aspect of all these systems in its internal reports (which were apparently released to the airlines), but because the army was still clearly wedded to the Hegenberger system, which included no glide path at all, no one seems to have taken the army’s criticism seriously. Colonel Mauborgne’s suggestion that the army campaign for a straight glide path is the first indication that someone with significant authority in the army at least recognized the unsuitability of the army’s own A-1 system. It also presaged a long-running argument between CAA, which wanted to deploy a bent-beam-type system, and the army, which demanded a straight glide path.

Following the demonstrations at Indianapolis, United Airlines hosted a meeting between representatives of the other major airlines, the FCC, the Bureau of Air Commerce, and Bendix Radio at its Chicago Office on June 23, 1937. At that meeting, a standard was agreed upon that was validated by the RTCA that September. Although it did not require a straight glide path, it stated that “study should be made of the possibility for obtaining a straight line constant rate of descent glide path.” The airlines had also recognized the eventual need for a straight path, which might well have been based on their own expectations of much larger aircraft appearing in the near future. Paul Goldsborough, president of Aeronautical Radio, Inc. (ARINC), a company established by the airlines to make radio equipment especially for airline use, forwarded a copy of this proposed standard to the Air Corps headquarters. The proposed standard was published in the August 1937 newsletter of the Air Line Pilots Association, accompanied by a statement by R. T. Freng that “personally, I feel that this is the thing that we have been looking for a long time.”⁷ It was approved by the RTCA and published in the new Civil Aeronautics Authority’s first technical report that October, along with a recommendation that CAA pursue development of the items listed under “Projected Developments,” which included the straight-line glide path.

By late 1937, then, the Air Corps and commercial aviation groups had recognized that a straight glide path offered advantages to the curved one that all previous versions of the Bureau of Standards’ system had generated. These advantages included a simpler landing procedure, greater length and consequently a longer approach from a higher altitude, and better obstacle clearance. But the ability to produce a straight path did not exist, and airlines believed that it was in their interests to adopt a curved path while a straight one was developed. It was over this point that the army disagreed, believing that waiting was better.

The response to the proposed standard within the Air Corps was mixed. The advantages of the proposed equipment were well understood, but criticism focused on the cost of the system. In a letter to the secretary of commerce, the secretary of war (very politely) pointed out that the shift to UHF frequencies that the standard imposed left Air Corps aircraft unable to use the system. Internal memos were much less polite, pointing out that the Air Corps, with more than 3,000 aircraft, faced a very expensive procurement program to outfit its planes in order to keep up with the Bureau of Air Commerce’s improvements to the Federal Airways System. Not lost on the army men was the reality that the bureau was considering the needs of only about 300 aircraft operated by the commercial airlines, whose equipment the bureau did not have to pay for.⁸ Hoping that in two or three years a straight path would be developed that might not be UHF, the Air Corps considered a temporary system unpalatable.

The major airlines were not only willing to pay for the receivers (which cost about $1,500 per aircraft) but were practically demanding the opportunity to do so. Despite believing that the equipment they wanted to buy in 1938 would be obsolete in at most four years due to the projected movement to a microwave system, the airlines were willing to install it. They wanted an instrument landing system to improve their profitability because delayed and canceled flights cost them a great deal of money. The airline demand for regularity that had led to surfaced runways also drove them to push for an instrument landing system. Airlines expected to make back the cost of receivers within a year of operations, as the additional income they expected to derive from use of the system offset the initial cost. Hence, the airlines’ economic model indicated that rapid adoption of a “good enough” system was a better strategy than waiting for perfection.

The airlines therefore pushed the Bureau of Air Commerce to let a contract to someone to design a system that conformed to the published standard. In early 1938, the bureau awarded a contract to International Telephone Development Corporation (ITD), a subsidiary of International Telephone and Telegraph, to build a new system intended to meet the RTCA standard. The company’s system was based upon two key patents held by one of its engineers. It consisted of a 110 MHz equi-signal localizer, a 93 MHz constant-intensity (curved) glide path, and two marker beacons. The installation at Indianapolis included two localizer transmitters and four glide path transmitters to allow approaches to be made from any of the four runway directions available at the field without having to move equipment. It employed a specialized array of five horizontal loop antennae for the localizer, which allowed variation of the course width by changing the amount of energy radiated by each of the loops. This invention was an important step in overcoming the problem of bent or split courses, because careful tuning of the system, combined with careful siting on the airfield surface, could provide a narrow enough beam pattern to avoid obstacles. The other significant patent was a transmitter bridge circuit that automatically compensated for aging of system components.⁹ This allowed the system to maintain a constant output automatically, eliminating another source of system instability. Neither innovation dealt with the issue of glide-path curvature, however. As initially demonstrated in 1939, the Indianapolis system possessed the same two to three mile constant intensity curved glide path as its predecessor.

In September, the Radio Technical Commission for Aeronautics’ Committee on Instrument Landing Devices met to formally evaluate the International Telephone Development Corporation’s system at Indianapolis and the MIT system. Forty-six representatives of various organizations, including six from CAA, two from the Army Air Corps, and three from the Army Signal Corps met on September 13, 1939, at the CAA Experiment Station in Indianapolis. Six airlines were represented, as were MIT, Sperry Gyroscope, Bendix Radio, Bell Labs, Aeronautical Radio Inc., RCA, the International Telephone Development Corporation, the Washington Institute of Technology, and the Federal Communications Commission. Although there was no formally appointed member of the Air Line Pilots Association listed among the representatives, five commercial pilots were present, flew the system, and submitted an addendum to the report offering specific recommendations for modifications. E. A. Cutrell, who had participated in the 1934 tests of the A-1 and NBS systems, was one of these, as was the chief pilot for United Air Lines, R. T. Freng. The senior Air Corps representative was Maj. A. W. Marriner, of the Air Corps Communications Department in the Materials Division; the senior Signal Corps representative was Colonel Mitchell, director of the Aircraft Radio Laboratory. The organizer was Richard Gazely, head of the Technical Development Division of the CAA, and the chair was J. R. Cunningham, director of communications for United.¹⁰

The report of this committee indicates that Colonel Mitchell was active in supporting the army’s interests, and in fact the chair requested his recommendation on the form in which the proposed standard was to be cast. Mitchell recommended that the standard be “left sufficiently broad to admit of many approaches to the problem, that is, that a performance specification only should be written.”¹¹

The group unanimously agreed that a three-element system be chosen (marker beacons, glide path, and localizer, the elements of the Bureau of Standards’ concept), and Mitchell suggested that the standard be written so as not to rule out use of a “heading device,” by which he obviously meant a radio compass. The group again agreed and placed that device in the “projected developments” category as desirable, but not necessary.

The vital question of glide path shape, however, proved controversial. Preston Bassett of Sperry Gyroscope Company reported in an internal memo that “the meeting devolved into a wide variance of opinions, one section of which insisted on the adoption of the IT&T system for the next step, another group insisting that the system was impossible, and no blind landings could ever be made with it.” Bassett also reported that there was a “general feeling among many of the pilots and operators that the bent beam was not the final solution.”¹² Ultimately, the group chose to defer the decision to the pilots who were then out flying the system. The pilots’ addendum to the report, accepted by the group as the committee’s recommendation, specified that “a glide path intersection shall be obtained at 1500 altitude at a distance of 6 miles from the transmitter end of the runway ... At the point of contact the glide path shall have an angle with the runway not less than 1 degree and not more than 2 degrees. The glide path shall pass through the following points—not less than 500’ nor more than 700’ altitude at a distance of 3 miles from far end of runway, and 1500’ altitude at 6 miles.”¹³

The commercial pilots had adopted an “almost straight” glide path of six miles length. This was well in excess of the two to three mile glide path that the Indianapolis system had achieved. Once again, the group placed the straight glide path in the “desirable developments” section of the standard.

Although the army members of the committee did not oppose the adoption of this standard, Mitchell was apparently not satisfied with it. He commented, in yet another attached addendum, that the Indianapolis system had merit as an experimental apparatus, especially since its ability to produce a straight glide path and a curved one would finally allow determination of which was safer. He thought that the straight glide path was too short and that the system should be extensively flown before final adoption. The army needed a ten to fifteen mile approach, instead of six miles. He obviously had in mind the Air Corps’ new bombers, which were more than twice as heavy as the biggest commercial transport in service. He emphasized the need for a common CAA/army/navy standard and pointed out that the weight of the equipment made it unsuited to the army’s smaller aircraft (which were, of course, the majority of army aircraft).¹⁴

It is important to note that the CAA does not seem to have intended the Indianapolis system to be the ultimate system. The organizer and head of the CAA’s research division, Richard Gazely, pointed out, “The only really worthwhile opinion we believe is one based on extensive pilot experience. Only after 500 or more pilots have had a chance to observe the behaviour of the equipment under every condition encountered in routine operations and only after these pilots tell us that they would trust the lives of their passengers and their companies’ equipment to the guidance which a system gives them will we permit ourselves to believe that the system is good. But to get this kind of pilot opinion requires something more than a laboratory installation. A rather costly large-scale installation is necessary.”¹⁵

He concluded with the hope that the experience gained would allow the system to be “improved out of existence.”¹⁶ The CAA intended to buy ten of these systems in 1940 so as to enable this extended testing to take place in a variety of locations. It also planned an additional purchase of fifteen for 1941.

The CAA was able to commit to a standard and buy equipment for airports in support of it because Congress had given it new legal authority in August 1938 in a major reorganization.¹⁷ The CAA’s legal authority to adopt a standard for civil aviation did not supplant military prerogatives, however, and it had to negotiate with the War and Navy Departments to achieve military/civil standardization. Widespread belief that a single standard for military and civil aviation was necessary, combined with a technical desire to eliminate the ground-induced stability problem, led it and the RTCA to view the Indianapolis system as transitional. Any adoption of it was to be temporary, to produce the sort of information regarding performance at many locations which all previous testing had not been able to provide, while continuing to investigate methods of producing a straight glide path. Following the RTCA’s recommendation, the CAA continued its research program to develop the straight, microwave path that was clearly the solution preferred by many of its members. The RTCA’s standard, then, was a means of establishing a coherent development program while providing limited service to the airlines.

THE MICROWAVE FUTURE

The root of the glide path stability problem was that Diamond and Dunmore’s “landing beam” was not a beam at all. Instead, the glide path antenna’s radiation pattern relied upon the earth’s surface to act as a reflector. That meant surface conditions affected the radiation pattern. Changes in the soil moisture content of the surface altered the soil’s conductivity and thus the glide path’s transmission pattern. That, in turn, changed the glide path angle. This was why the army’s 1936 tests found that the glide path angle changed as the ground dried out after a rain. It also meant that changes in surface conductivity caused by large pipes, underground electrical conduits, and similar artificial structures caused “bumps” in the glide path. Fixing the glide path, then, meant finding a way to detach the glide path’s radio propagation from the earth’s surface.

Wilmer L. Barrow, an MIT engineer, had discovered in 1938 that radio waves could be propagated directionally, without reflection by the ground, by using metal horns. In other words, the use of these “horn radiators,” as Barrow called them, could generate radio beams independent of the earth’s surface. This was clearly a potential solution to the instability problem, but it brought with it a substantial difficulty: the horns were very large. The size depended upon frequency in an inverse relationship.¹⁸ Higher frequencies needed smaller horns.

In 1936, CAA engineer Irving Metcalf had proposed a blind landing system based on three beams of energy, which would appear as three spots on a cathode ray tube display. Initially, he had believed that infrared energy would be the best choice, but after he consulted with Edward Bowles at MIT he decided microwaves were a better choice. Working at the Round Hill experiment station, Bowles’s research group had demonstrated that infrared would not penetrate all types of fog and mist.¹⁹ Microwave radio seemed a better solution.

Metcalf was able to convince his superiors to fund a twelve-month contract with MIT to build a prototype blind landing system based upon Barrow’s horns and microwaves at fifty-centimeter wavelength.²⁰ The horns were still large but much more manageable than they would have been at lower frequencies. Ultimately, the Round Hill group devised a system that produced four beams instead of three. By overlapping two vertical fan-shaped beams along their flat faces, the system established a plane that provided an indication of proper course. Similarly, it overlapped two horizontally fanned beams to produce a plane for vertical navigation reference. Although all four beams were transmitted at fifty centimeters, each was modulated with a separate audio frequency, which was then separated by filters in the receiver.

The resulting system had two great advantages over the Bureau of Standards system’s UHF descendants: the beams were perfectly straight, and they were nearly immune from changes in ground conditions. It therefore answered pilots’ demands for a more stable system. A successful demonstration of the system in early 1938 led to a six-month extension of the bureau’s contract, so that Bowles could finish the report he was required to submit. The system had one major drawback, however: the cluster of triodes which generated the microwave signal could only produce about three watts of power, resulting in a useful range of only about a mile. More power was clearly necessary for a useful blind landing system. Fortunately, a potential solution already existed, a new vacuum tube called a klystron.

Physicist William Hansen and two other researchers, Russell and Sigurd Varian, developed the klystron in the Stanford University physics department. Stanford’s physics program was oriented toward research into high-powered devices. In 1936, Hansen had developed a microwave tube called a rhumbatron, but this did not produce much power. Department chair William Webster had unsuccessfully sought money for a “million-volt X-ray” device in 1937, and after this failure Hansen turned toward research into techniques that might be less expensive. Russell Varian, who had been a student of Hansen’s before going into private enterprise, was also interested in this research area, and he kept in touch with Hansen. According to Russell, Hansen wrote to him in 1937 that hollow resonators seemed to be very efficient at generating high-frequency radio energy.²¹ This was the basic idea behind the klystron.

Russell and his brother Sigurd, a Pan Am pilot, had also corresponded about possible uses of microwave energy, including aircraft detection and blind landing. Sigurd had expressed concern over the concentration of power—particularly air power—in hands of European dictators. The Spanish Civil War had made clear the future importance of air power, and Sigurd believed that aircraft could easily bomb targets via “blind flying” techniques, while the targets could not defend themselves against the invisible aircraft. Microwave-based radio locators might resolve this problem. In September 1937, the two brothers went to Stanford and argued for a project to develop a resonator-based tube that might produce the high-energy microwave radiation an aircraft detector would require. Sigurd was particularly adamant about it, according to Russell, and they succeeded in persuading Hansen. Hansen got Stanford president Ray Lyman Wilbur to support the effort with a $100 grant. The university’s board of directors approved the arrangements in early October.²²

The Varians quickly succeeded at assembling an apparatus that produced microwave energy and, nearing the end of their finances, tried to produce interest in the device by visiting the local navy and CAA offices. Irving Metcalf happened to be in town, and he and Hugh Willis of Sperry Gyroscope were immediately interested in seeing it in action. Metcalf realized that combining the klystron with Bowles’s construct at MIT was the obvious route to making a stable, useable glide path. Willis perceived a new business opportunity, and he quickly arranged a deal with the Varians and Stanford to take over financial support of the klystron effort.²³

The microwave work at MIT and Stanford had also attracted the attention of the U.S. Army Signal Corps. Col. John Mauborgne, who had been director of the Aircraft Radio Laboratory at Wright Field, had been promoted to brigadier general and assigned chief signal officer of the army in early 1938. In April of 1938 General Mauborgne visited MIT, where he met with Karl Compton and Edward Bowles and witnessed a demonstration of the CAA-MIT microwave landing system.²⁴ He came away from that meeting convinced that it was the solution to the army’s need for a straight glide path, and he arranged to send people from the Signal Corp’s Radio Laboratory to MIT to learn more about it. Bowles’s assistance, in turn, led the Signal Corps to establish an in-house research program into microwave landing systems at Wright Field. At first, the army was only interested in the glide path part of the system and arranged to borrow a localizer from the CAA to use in their tests.

Mauborgne’s replacement as director of the Aircraft Radio Lab, Lt. Col. Hugh Mitchell, kept him informed of new research through correspondence with one of his staff members, Col. Luis Bender. Mitchell heard of Stanford’s klystron from Metcalf and had corresponded with Webster about it. Mitchell had then described the device to Bender in a handwritten note, emphasizing the importance of this device for blind landing of aircraft, as he knew that his boss was interested in the subject.²⁵ He believed it was the best possibility for extending the range of Bowles’s glide path from a mile to the minimum of ten that the army wanted.

Mauborgne arranged to borrow one of the prototype klystrons from Stanford for use in the army’s copy of Bowles’s glide path transmitter, believing that joining the two would provide a useable range. The army first flew the klystron to MIT, however, where Bowles was able to use it for a few weeks before the army’s equipment was ready. Initial experiments proved very exciting, but flight tests originally scheduled for May 1939 had to be postponed into June because the klystron had to be sent back to Stanford for repairs. The research projects at MIT and Wright Field suffered from frequent problems with the prototype klystrons, forcing them to send the devices back and forth between MIT, Wright Field, and Stanford, which considerably delayed their work.

The shortage of klystrons in early 1939 led the CAA, MIT, and the army’s Aircraft Radio Lab to cooperate at MIT. MIT, which expected to get a new klystron before the army’s was repaired, approved an army request to fly MIT’s system once the klystron arrived. The CAA had renewed its research contract with MIT, although its efforts in the microwave area were waning as the UHF system it had contracted for with ITD neared completion at Indianapolis. The Sperry Gyroscope Company, which had established a development laboratory for the klystron in San Carlos, California, was also informally involved in the undertaking at this point.²⁶

The cooperation was shaky, however. General Arnold had written to Clinton Hester, administrator of Civil Aeronautics, proposing a cooperative development effort in the microwave field. Hester happily agreed to participate and even to provide the equipment.²⁷ Arnold then demanded establishment of an overall policy of cooperation before agreeing to a cooperative program in this specific case, however, which Hester appears to have been unwilling to commit to. Relations between the two organizations deteriorated, while Bowles’s group continued its work at MIT and the Signal Corps prepared its own version of MIT’s system at Wright Field. But the research at MIT and Stanford had produced a new set of technologies that would satisfy the army’s requirements for a straight glide path. The remaining issue for the army was how to get everyone else to adopt what it wanted.

THE NATIONAL ACADEMY OF SCIENCES:
VALIDATING A RESEARCH PROGRAM

The completion and relatively successful testing of the International Telephone Development Corporation’s design at Indianapolis during the summer of 1939, while the army’s enthusiasm for microwaves was still growing, set the stage for what other historians have presented as an army attempt to block the RTCA’s standard. Wilson and Leary have argued, based upon CAA records, that General Arnold sought to prevent the CAA from carrying out the RTCA’s plan by calling for a National Academy of Sciences (NAS) review, apparently in the hope that NAS would recommend something else. Army records suggest, however, that Arnold’s memos were misunderstood in the White House and that Arnold had no intention to block limited commercial deployment, which was, after all, the RTCA’s own plan. Instead, Arnold sought validation of his belief that microwaves were the best solution to the glide path problem and therefore of the Signal Corps research program. Once he had that validation, he intended to insure that the CAA conformed to the limited deployment of the RTCA’s system, as he believed it had agreed to do.

Arnold, after a conversation with the assistant secretary of war for air in August 1939, had written for the secretary of war’s signature a memo to President Roosevelt. In it, Arnold requested that the NAS be asked to evaluate the existing blind landing systems. They were, he argued, a “disinterested group of distinguished scientists who would serve much as a court of justice serves in our system of jurisprudence.” The Air Corps had gotten good advice from them in the past, Arnold believed, and he thought that their national prestige would end all “quibbling or contraversy [sic]” once they had announced their decision. Arnold had begun working with members of the National Research Council in 1936, as Gen. Oscar Westover’s assistant. That experience had convinced him that scientists were willing and able to advise the Air Corps on new technologies in which it had no expertise.²⁸ The NAS was therefore a reasonable place for him to turn to resolve what he perceived as a divergence between the needs of the Air Corps and the plans of the CAA.

Arnold’s letter to President Roosevelt got an immediate response. In a letter dated the same day, August 30, Roosevelt asked Paul Brockett, the executive secretary of the National Academy of Sciences, to look into the problem of standardizing upon a common system. Brockett turned the letter over to Frank Jewett, then president of the NAS, who responded on September 1. Jewett agreed to appoint a committee to look into the problem, with the caveat that it might not be possible to attempt complete standardization.²⁹ Jewett’s initial sense, that standardization might not have been possible given the state of the art, is certainly borne out by the RTCA’s construction of an interim standard.

Jewett chose Vannevar Bush to chair that committee and assembled a number of prominent researchers to serve on it. The conference committee consisted of Oliver Buckley, executive vice president of Bell Labs; electrical engineer Dano Gunn; mechanical engineer W. F. Durand from Stanford; physiologist Joseph Erlanger of Washington University; telephone engineer Bancroft Gherardi, who was retired from AT&T; biological chemist Lawrence Joseph Henderson of Harvard; aeronautical engineer Jerome Hunsaker at MIT; and physicist Max Mason of the California Institute of Technology.³⁰ Bush also wrote to the various aviation interests, including the CAA, the Air Corps, the navy, the Air Line Pilots Association, and the Air Transport Association, seeking their positions on the issue.

In October, Charles Stanton, acting administrator for Civil Aeronautics, responded to Bush’s request with a long explanation of the CAA’s current policy. Stanton explained that the CAA intended to install ten sets of equipment at important airports, while continuing to improve the equipment. The CAA recognized the need to familiarize pilots with instrument approach procedures well in advance of attempting blind landings in routine operations. He asserted that the CAA intended to continue funding MIT’s research into a microwave-based system. A reliable, manufacturable microwave system, he believed, was three years away, and because airlines expected aircraft radio equipment to become obsolete every three to four years, he felt no concern that deploying a UHF system would prevent adoption of a microwave one once one was available. Air carriers, he pointed out, had “shown eagerness to accept the use of new equipment which tend to increase the regularity with safety of scheduled operations.”³¹ Stanton argued for the approval of ten ground stations.

Bush received similar advice from Edgar S. Gorrell, president of the Air Transport Association. Gorrell had written to each of the major airlines to get their support for Bush’s investigation, and expressed their “sincerest cooperation with the National Academy of Sciences in the subject of one instrument landing system for common adoption.” Gorrell also pointed out that the CAA had a project under way to produce a standard landing system through the RTCA and attached a copy of the September 14, 1939, proposed standard to his letter for Bush’s edification. The air carriers, he said, were convinced by their many years experience in testing various systems that the CAA’s new system was the best that they had seen, and he urged that “nothing be done to retard the program.”³² The airlines, he said, were unanimous in their wish that the CAA proceed with the initial ten installations. The airlines were clearly satisfied that even as an interim system, the RTCA’s proposal would provide sufficient economic benefit to justify their costs.

Bush’s committee had no significant funding, and it met only once, on October 14 and 15, 1939. The group saw presentations by the army, the CAA, and the navy. It drew up a draft report on the fifteenth, and the committee decided that additional information should be gathered from the various involved agencies. It also decided that Bush should personally fly each of the systems in question, particularly the CAA’s Indianapolis installation and the MIT microwave experiment. He was to keep the committee informed by letter.

At the meeting, the committee focused primarily on CAA’s system and MIT’s microwave project. The report it drafted on the fifteenth proposed continuing the CAA’s plans to install ten copies of the Indianapolis system, with the caveat that it be modified to meet the six-mile, 1,500-foot approach that the pilots had specified the month before but that ITD’s engineers had not yet managed to achieve. The draft also supported the army’s desire for a straight path and advocated continuing microwave research as the best means to produce it. In short, the NAS’s draft report echoed the RTCA program almost exactly, with somewhat more emphasis placed on microwave development than in the RTCA document.³³

As the largest unresolved issue was the straight versus curved glide path problem, Bush sought more information from various parties. He received a letter from Charles Stanton on November 3 notifying him that the Indianapolis glide path was now in conformance with the RTCA specification. Stanton also forwarded a letter from Colonel Mitchell to Richard Gazely, which stated that the army found the CAA’s specification acceptable for test purposes, but for army use a completely straight path, inclined at 3 degrees, was necessary. The colonel reiterated that he supported the installation of ten sets of the CAA’s system for pilot training and familiarization, but deployment should not go further until the system’s real world performance was well understood.³⁴

Bush also received a long letter from Bowles that discussed the Indianapolis system and the MIT microwave experiments. Bowles told Bush he had spoken with William Jackson from CAA’s Technical Development Section and had been told that the straight six-mile glide path that had been produced had not yet been thoroughly tested for straightness and “freedom from irregularities.” Further, Bowles had been left with the impression that the straight portion could not be substantially lengthened with the existing equipment because this was done by moving the transmitter equipment further to one side of the runway. The further away from the runway the equipment was moved, the greater the probability of interference with the radiation pattern from reflections and other surface conditions. Although Bowles did not discuss the issue in any detail, he stated that ITD’s engineers believed that an equal signal glide path was possible at UHF wavelengths.³⁵

Bowles argued explicitly for an equal signal glide path in his much more detailed discussion of the microwave system. “I believe that the groups interested in the instrument landing of airplanes subscribe to the idea that the ultimate system will be a microwave system and that the value of a longer wave system lies principally in the fact that these systems are now in such form that at least they offer immediate means for the training of commercial pilots in the technique of instrument approach and instrument landing.”³⁶

He added that the constant-intensity glide path at Indianapolis “must sooner or later degenerate into a curved path” if the CAA made further attempts to lengthen it. He had demonstrated already that microwave radiators could produce a straight equal signal glide path by means of a sharp ultra-high-frequency beam.³⁷ The army, he thought, agreed with his point of view and were working on such a system at Wright Field. Finally, he asserted that an equal signal glide path, whether UHF or microwave, was most likely to resolve the straightness problem and the instability problem because it was an “actual path in space” that could be made independent of transmitter output and receiver sensitivity.

In a prescient paragraph, he also argued that the “flare” at the bottom, which had been specified by the pilots to reduce impact upon landing, would be unnecessary and should be dropped because the system would only be used as an approach system, not a landing system.³⁸ If the system was only flown as an approach system, with the pilot making a visual landing, any desired flare out could be done visually. Bowles’s estimation of the unfeasibility of a true “blind” landing system was no different than that of United Air Lines, which had already recognized that blind landings belonged to the remote future, if they were possible at all. Within a year, the army also picked up on the improbability of truly blind landings and tried to change the name given to this class of technologies from “instrument landing systems” to “instrument approach systems.”

Bowles argued, finally, that the fastest path to a microwave system was a UHF/microwave hybrid, utilizing the UHF localizer of the Indianapolis system and a microwave glide path, put together by a commercial company. He pointed out that RCA, General Electric, and Sperry Gyroscope were working on equipment suitable for this task. He also contended that the “ultimate” system should be all microwave, to resolve the remaining problems with bends in UHF localizers, and should employ a ten centimeter wavelength, instead of the fifty centimeters that he and the Aircraft Radio Laboratory had been working on. He believed that this could reasonably be done in two years.³⁹ The Aircraft Radio Lab had already taken this approach at Wright Field, probably on Bowles’s recommendation.

Having digested all of the arguments presented by his interested parties, Bush flew off to try out the various systems. At the Aircraft Radio Lab, he was able to try a Bendix system, the old Army Hegenberger system, and at nearby Patterson Field, the radio lab’s prototype microwave system. He wrote the committee on November 20 that when flying the Bendix system, he was “further impressed with the desirability of a long, substantially straight path, although the system in general operated successfully and satisfactorily.” On the old army A-1 system, his pilot, whom he called “one of the most skillful and experienced,” made two approaches and missed both. Bush was “completely convinced” that this system made “too severe demands upon the pilot,” and this was reason enough for its abandonment. At Patterson Field, he found the microwave equipment to be “in experimental condition only.” He was unable to fly the system because both of the available microwave receivers broke during his visit. He was convinced of the microwave system’s “great future promise” but was clearly less optimistic than Bowles had been about its nearness to commercial utility.⁴⁰

At Indianapolis, Bush found that William Jackson’s group had done an excellent job on the equipment. His pilot made satisfactory instrument landings from all four directions. He judged it “completely satisfactory for commercial use.” He agreed that the substantially straight part of the glide path was six miles long and admitted that while the army wanted an even longer glide path, he was “inclined to believe that much more is not really necessary.”⁴¹

The issue of the length of the glide path is difficult to evaluate. The length to be chosen depended on how air traffic was managed and what sort of aircraft were using a facility. In commercial operations, especially during poor weather, aircraft were typically stacked above a radio marker beacon at some distance from the airport, with the most recent arrival at the top of the stack, and the next aircraft to be cleared to land at the bottom. Since commercial aircraft flew at a maximum of 10,000 feet (they were unpressurized), and aircraft were separated by at least 1,000 feet within a stack, having the base of the stack at 1,500 feet allowed a ninelayer stack. The width of a stack, and therefore its overall volume, was defined by aircraft performance. Faster aircraft, and heavier aircraft with poorer turning abilities, meant a bigger stack that then had to be further away from an airfield in order to prevent congestion close to the field. Because Bush was not a pilot and flew these systems in commercial aircraft with pilots familiar with commercial needs and in good weather, the six-mile glide path seemed perfectly reasonable to him. It meshed with the current suite of technologies in use by the airlines.

By 1939, however, the Air Corps was getting aircraft that flew higher, weighed more, and were faster than anything in commercial use and therefore demanded larger, more distant stacks. That in turn meant a longer approach. The need to stack those planes was reflected in the Air Corps’ demand for a longer glide path. It is true that the Air Corps could probably have lived with a localizer-only approach for the first few miles of a descent (and, in fact, it did not have a glide path until 1944), but that largely defeated its purpose in adopting a glide path, which was to reduce accidents through improved vertical guidance.

The army wanted its pilots to pick up the glide path fifteen miles out and fly the twin beams in, instead of picking up one beam at fifteen miles and the other at six miles. Since the cross-pointer instrument displayed vertical and horizontal position, it provided a vertical indication even if it was not receiving a glide path signal—the elevation needle did not disappear in the absence of a signal. Instead, no signal resulted in a “fly down” indication, which a pilot would have had to ignore until approximately six miles out, with no certain indication of when he should start receiving the signal and therefore start paying attention to the elevation needle. Hence, the Air Corps’ demand was driven by the performance characteristics it had designed into its new bombers, combined with the need to provide its pilots with a positive instrument indication. Bush could not have realized this from his own experience with these systems, but he deferred to the army’s demands in his final report.

After returning to Washington, Bush made some minor changes to the draft report, the most significant of which were a specification that the army and the CAA needed to agree on a common sensing of the cross-pointer instrument through the RTCA, and a statement that the committee did not feel that a proper solution lay in the low-frequency direction. He requested that the committee telegraph their approvals by November 24 so that the report could be submitted promptly.

Bush apparently received the approval quickly, for the report he sent to Jewett bears the date November 21. Jewett, in turn, transmitted it directly to President Roosevelt. Roosevelt had his personal secretary, Gen. Edwin Watson, send copies of it to the army, the navy, and the CAA on December 5. The CAA responded with a highly laudatory letter to Bush, thanking the committee for its efforts in investigation of this “highly important and complex subject.”⁴²

Although the report was eventually approved unchanged, the process of approving it took longer than drafting the report itself. The first stumbling block was the Civil Aeronautics Authority, which believed it had gotten exactly what it had wanted and pushed the National Academy of Sciences to make the report public immediately. The academy’s executive secretary wisely checked with the White House and found that Roosevelt considered it a confidential report until he, personally, chose to release it. The CAA had apparently not been quite so circumspect, and details found their way into the press anyway. In Brockett’s words, this had left the White House “just a little annoyed” with the CAA.⁴³

The secretary of war, Harry Woodring, responded on December 28 to General Watson’s memo. The War Department, he said, concurred with the NAS report, and he emphasized that the department considered the plan for future development it proposed “to be of material assistance in guiding the development towards standardization of an instrument landing system available to all aviation services.”⁴⁴ The records show that Arnold himself drafted Woodring’s reply. Arnold, therefore, was primarily interested in the report for its validation of the army’s research plans, which were clearly present in the report’s call for microwaves.

On the day Arnold wrote Woodring’s response to the NAS report, he also acted upon a memo initiated by the chief signal officer in October. General Mauborgne had requested Arnold’s opinion on whether to pursue the army’s microwave research by letting a contract to Sperry Gyroscope Company for a complete microwave system, based upon the klystron. Arnold had clearly held onto the memo while waiting for the NAS report. In his answer to Mauborgne, Arnold requested that the Signal Corps “expedite” its research program and then quoted the NAS report’s microwave section verbatim.⁴⁵ His holding of the chief signal officer’s request and his reliance on the Bush committee’s findings suggest that Arnold had already come to rely heavily on scientists’ recommendations in subjects outside his expertise. He sought validation of the RTCA’s program through the NAS and wound up causing a great deal more trouble than he intended.

The secretary of war’s concurrence was not enough to establish the academy’s report as a formal policy. That there was also no policy to guide its adoption led to a great deal of confusion. Because War and Navy were Cabinet departments and the CAA was an independent agency, no authority existed to approve and enforce the report’s recommendations other than Roosevelt himself. The White House did not immediately recognize this, and after Roosevelt received the secretary of war’s statement of approval, he asked his secretary, “What do we do next?” Watson was equally in the dark, and he wrote to the secretary of war asking his opinion, who then told the chiefs of the Air Corps and Signal Corps to submit their ideas on how to proceed.⁴⁶

With much of official Washington still on the traditional Christmas hiatus, neither the chief signal officer nor the chief of the Air Corps responded in person. Instead, their executive officers responded, most likely after consulting with their bosses. In any case, the executive officer of the Signal Corps, Col. Clyde Eastman, suggested that the president approve the report and “furnish copies of it to the interested agencies for their guidance.” The executive officer of the Air Corps, Maj. C. E. Duncan, concurred. The secretary of war accepted their recommendation and so recommended to General Watson.⁴⁷

By putting the decision back in the White House, the Air and Signal Corps placed a technical decision in the hands of nontechnical people. Roosevelt relied heavily upon Watson’s advice in army matters, and Watson, a retired infantry officer, did not understand the issues. A series of letters between Watson, Woodring, and Arnold ensued as Arnold attempted to explain to the others what the Air Corps did and did not want. A particular set of personal memos between Arnold and Watson had convinced the confused Watson that Arnold disliked the CAA’s plans as embodied in the NAS report, and that perception was transmitted to Clinton Hester at the CAA. The CAA’s records therefore document army resistance that did not exist.

The confusion arose from a combination of Watson’s lack of comprehension of the basic issues and an imprecise memo from Arnold. Watson had asked Arnold for his opinion of the NAS’s report in a memo dated February 6. Arnold had responded:

Dear “Pa:”

The Air Corps has no fault to find with Dr. Bush’s proposed solution. We still, of course, believe that the system which we developed is better suited to meet Air Corps needs, but we realize fully that it is an impossible situation to have different aeronautical agencies each develop their own systems, no two of which any one airplane could use. It is absolutely essential that one common system be in use by the whole aviation industry and we are perfectly willing to give and take and compromise in order to arrive at that universal system.⁴⁸

Watson seems to have understood this memo as supporting the army’s old Hegenberger system over the academy’s recommendations. That interpretation would certainly have indicated hostility on Arnold’s part towards the report’s recommendations, as Bush and his committee had clearly consigned the Hegenberger system to the scrap heap.

The context suggests that Arnold had meant to support the army’s microwave system with this memo. Arnold was well aware of the Aircraft Radio Lab’s work with Bowles on that prototype. He had sent Air Corps pilots to MIT to test the microwave system that the CAA had funded there. He had been satisfied enough with the Aircraft Radio Laboratory’s progress with the system to ask the chief signal officer to expedite a procurement contract with Sperry Gyroscope for a manufacturing prototype less than two months before. Finally, he had personally signed the Air Corps’ approval of the NAS report and drafted a paragraph especially supportive of its “future developments” section, which included the microwave straight-line glide path system being experimented on by the army and MIT.⁴⁹ If we interpret Arnold’s memo as referring to the microwave system, then there appears to be little ground for a charge of army resistance to the report. Bush’s committee had, after all, validated the army’s need for a straight glide path and had strongly preferred a microwave one.

The tone of Arnold’s memo suggests that he had some unstated concerns, however, which may well have helped confuse General Watson. What those concerns may have been are suggested in the minutes of a conference held March 11 in Arnold’s office. The administrator of Civil Aeronautics had requested the conference in order to find out why the Air Corps was resisting the report’s recommendations. At that conference, an anonymous officer recorded, it was carefully explained to Hester that neither the Air Corps nor the Bureau of Aeronautics (Navy Department) had any objections to “ten purely experimental instrument landing systems, but felt that with the microwave system so far along, it would be undesirable to invest funds in radio equipment which would soon become obsolete.”⁵⁰ The drafter reported that Hester had agreed that these were purely experimental and that the CAA would support the installation of a better system as soon as one was developed and satisfactory for general use.

Clearly, the Air Corps did not intend to adopt the CAA’s system at all, as it preferred to wait the expected two years before a procurable microwave system was available. Arnold’s concern was probably over the possibility that Hester was not negotiating in good faith. Arnold remembered clearly the Bureau of Air Commerce’s adoption and near-simultaneous abandonment of the Hegenberger system. Many officers in the Air Corps had seen that as an act of betrayal. In this case, Arnold was likely concerned that the CAA might take the NAS’s approval of the Indianapolis system as an excuse to deploy the system widely, making getting rid of it nearly impossible. Widespread commercial deployment of the system would almost inevitably force the Air Corps to adopt it, whether or not it satisfied their perceived needs.

The CAA did have larger plans for the Indianapolis system than the ten installations that NAS had approved, and it had budget authority for twenty-five in the 1940 and 1941 fiscal years. One CAA official had also suggested to the RTCA (with the army members present) that up to fifty might be deployed before a microwave system became available. That amounted to an investment of $1.25 million in 1940, if CAA’s 1939 estimate of $25,000 per installation had been accurate, all of which would have to be abandoned when a microwave system was deployed in two or three years. Because Arnold complained bitterly in his autobiography about the shoestring budget of his Air Corps, it seems likely that he did not consider such a sum easily abandonable. Congress may well have balked at any CAA plan to replace the UHF system, too, as it was already unhappy at the cost of maintaining the Federal Airways System.⁵¹ Arnold’s concern with being stuck with an unsuitable system was probably well founded.

The March meeting adjourned with a promise by Arnold’s office to draft a memo to General Watson to correct his misunderstanding of the Air Corps’ position. Arnold submitted such a memo, which unfortunately no longer exists, but it was not enough to satisfy Watson. On April 3, the War Department chief of staff requested that Brigadier General Yount, Arnold’s assistant chief of the Air Corps, go to the White House to explain the matter to General Watson in person. General Yount’s conference with General Watson resulted in a request for yet another memo, this time signed by the secretary of war, which was duly generated and sent to Watson on April 13, 1940. Watson dispatched that memo to President Roosevelt, and Roosevelt attached his “OK. Execute” on May 2nd.⁵² The RTCA’s recommendation, changed only in emphasis by the NAS committee, had finally achieved the status of an official standard.

CONCLUSION

The process of constructing a standard landing system was made painful by several interrelated factors. Each organization involved in the negotiations had a particular set of technical demands, which mostly, but not completely, overlapped. Their technical demands, in turn, were based upon the types of aircraft that they intended to operate. For example, the navy’s prewar focus on seaplanes made it in many ways the “odd man out” in these talks, playing a distinctly secondary role to the Air Corps and the CAA. The curved glide path was well suited to its seaplanes, and because no one else used flying boats, it was allowed to go its own way.⁵³ The navy’s unusual technological suite imposed unique requirements on its choice of landing system. Since it did not need to share airfields with the army or airlines, it had no reason to conform with any standard, and no one else chose to take issue with its nonconformity.

The Air Corps and commercial airlines, on the other hand, recognized that their aircraft had to be able to use each other’s airfields in case of emergency or war and agreed that a standard was necessary, but they disagreed on the timing of that standard. An “almost straight” glide path was temporarily acceptable to the airlines, until such time as the “ultimate” straight path was available. The airlines, and the government agency created to serve them, wanted an interim system sooner rather than a final system later. This was a matter of economics, as suggested earlier. A landing aid system, even one at only ten airports, was expected to pay for itself quickly though increasing the number of completed flights. Hence, the technical issue of glide path curvature was secondary to the airlines’ economic interests. The CAA’s goal was happy constituents, and the airlines had powerful friends in Congress to whom unhappiness could be expressed—and it was the following year. The CAA’s support for a temporary solution, even one costing large sums, was therefore perfectly reasonable, especially since the CAA’s investment in ground transmitters amounted to less than the airline investment in receivers.

For the Air Corps, the technical and economic issues suggested a different timetable. The Air Corps believed that its new big bombers needed a longer approach than that provided by the Indianapolis system, and it therefore wanted to proceed directly to the ultimate system, which it expected to be available at the same time that these aircraft reached full production. Without a war to provide the operational necessity to justify the expense of a temporary solution, the Air Corps’ technological needs were reinforced by its own financial analysis. With ten times as many aircraft to equip as the scheduled air carriers possessed, the Air Corps faced an enormous investment in receivers with no visible return, as well as the additional burden of having to explain to a Congress still suspicious of military spending why such an investment would have to be scrapped in a mere three years. Focusing on the ultimate system was the most reasonable approach for the Air Corps, technically and economically. The differing needs of these organizations was merely exacerbated by the lack of a policy to guide approval of interdepartmental standards.

Finally, the agreement achieved closure within the community on the proper form that the solution to the blind landing problem should take. It had agreed on the tripartite NBS system, consisting of marker beacons, localizer, and a straight glide path. This represented less an exemplary artifact than an exemplary model upon which equipment and training could be based. That model, which places the information needed to land a plane directly in the cockpit, I call the pilotcontrol model. Only two years after Roosevelt approved the NAS report, that model was challenged by a new one based on a radically different principle of operation. The result was a political crisis.