From the start, the researchers working on the TCV program believed that an important element in increasing the capacity of existing airports was the development of technology and procedures to allow less separation between aircraft. If airplanes could be spaced closer together, airports could handle a greater number of takeoffs and landings in any given time period. Closer aircraft spacing, however, required not only improved cockpit equipment and ATC communications, but more efficient landing and taxi operations, as well. Regardless of how efficient flight operations were, aircraft could not be spaced less than two minutes apart if it still took two minutes for an airplane to land, slow down, and taxi off the runway.

In an effort to allow aircraft to taxi off runways sooner, many airports were equipped with highspeed runway turnoffs by the late1970s. The turnoffs were vastly underutilized, however, especially in low visibility conditions. [Ref 7-31] To make the most efficient use of the turnoffs, pilots had to plan their touchdowns for a point on the runway that would allow just enough time for the airplane to slow down to a safe speed before reaching one of the highspeed exits. Aircraft autoland systems only had to achieve a touchdown accuracy within 1500 feet in order to receive FAA certification, however. [Ref 7-32] This meant that airplanes took longer to touch down and often missed turnoffs, spending a greater amount of time on the runway. Engineers at the Langley Research Center and the Boeing Commercial Airplane Company reasoned that if autoland systems could be made more precise, they could plan the touchdown spot of airplanes to allow them to get off runways more quickly, increasing the potential capacity of airports. More precise autoland systems would also reduce the operational field length requirements for aircraft, opening up more airports as potential landing sites.

The part of the autoland systems that caused such wide dispersion rates was the control law that determined when the airplane began its landing flare. The most commonly used flare control law in the mid1970s used the aircraft's altitude above the runway as the cue to begin the flare. The airplane's sink rate would be steadily decreased as the airplane got closer to the ground, until it landed. The problem with this approach was that depending on the winds, the aircraft would cover dramatically different amounts of ground between a given altitude and the point where it touched down on the runway. With a 40 knot headwind, for example, the groundspeed of the airplane would be slower, so it might cover little ground in the last 50 feet of a descent. If there was a 10 knot tailwind, on the other hand, the airplane could float far down the runway before it touched down.

The Boeing and NASA engineers researched and flight tested two possible improvements to this control law, using the TSRV 737 airplane. The first concept simply incorporated groundspeed as measured by the airplane's inertial navigation system (INS) into the flare law algorithms so the aircraft's sink rate would be arrested more quickly if the groundspeed was higher. The second approach was a more complex, but more precise, "pathinspace" flare trajectory law that aimed the aircraft toward a specific point on the runway. The pathinspace law essentially extended the glideslope all the way to landing and kept the airplane on the correct glideslope. The aircraft's altitude, sink rate and vertical acceleration were all commanded as a function of the plane's position along the runway.

The flight tests of the new control laws began in 1978, in conjunction with the microwave landing system (MLS) demonstration flights the NASA 737 was performing for the FAA. In 58 landings with the first "Variable Time Constant Flare" control law, all the touchdown points were within 641 feet, and 95% of them were within 548 feet. The pathinspace concept was then tested with three different landing guidance systems: an ILS (Instrument Landing System), a basic MLS, and an MLS with a secondary flare elevation signal. Ninetyfive percent of the landings that used the basic MLS signal for guidance touched down within a 592 foot distance. Using the MLS signal with the secondary flare elevation signal, that distance dropped to 368 feet, and all of the ILSguided landings touched down within 285 feet. Clearly, the new control laws could improve the performance of autoland systems far beyond the 1500 foot dispersion requirements set by the FAA. [Ref 7-33]

Nonetheless, convincing industry to adopt new flare control laws for the sole purpose of improving airport capacity would undoubtedly have been extremely difficult. More precise autoland systems would only increase capacity if every airplane was equipped with them, and then only if all the other separation issues could be resolved, as well. The precision flare laws offered another, more financially compelling advantage to manufacturers, however. A path in space or variable time constant flare law could make it easier for a manufacturer to meet the 1500 foot accuracy required for autoland certification, which translated to cost savings in the certification process. The Boeing 767/757 autoland system, for example, did not initially meet the FAA's 1500 foot requirement. In order to pass the FAA requirements, the design engineers ended up developing a pathinspace flare law for the airplane that was later adopted for the 747 aircraft, as well, although the production control law used different algorithms than those developed by the Boeing/NASA research engineers. In addition, the variable time constant flare law was incorporated into the Boeing 737300 model aircraft, although it had to be modified slightly because the 737300s did not have INS equipment. [Ref 7-34]

The flare law experiments showed the researchers at NASA once again that having a transport airplane with which they could actually flight test and demonstrate new technology concepts could be extremely valuable. One of the NASA engineers who worked on the flare law research remembered getting a phone call during the experiments from a colleague who told him that, for a variety of reasons, a pathinspace flare law could never work. "Why don't you come down here next Tuesday and we'll fly it for you," the engineer replied, effectively ending the debate. [Ref 7-35] The fact that a pathinspace law had been successfully demonstrated on an airplane eliminated the discussion about whether it was possible and substantially lowered the risk of incorporating it into a commercial airplane. The production engineers at Boeing merely had to decide how they wanted to design their own version.