As fuel prices rose in the late 1970s, airlines became increasingly concerned with improving the efficiency of their operations. Jet engines were designed to operate best at high cruising altitudes, and they ate up fuel very quickly in the denser air close to the ground. Consequently, the profits from a flight could evaporate quickly if an airliner had to spend a lot of time maneuvering at low altitude as it approached its destination. Unfortunately, the growing congestion around many commercial airports meant that airline flights were often vectored around other air traffic or put into holding patterns as they neared the terminal area, wasting significant amounts of fuel.

In an effort to improve this situation, the FAA began testing a new system called Local Flow Management/Profile Descent (LFM/PD) in the late 1970s. Local Flow Management was designed to reduce the need for low altitude vectoring or holding patterns by matching the arrival rate of all the airplanes coming to an airport to number and frequency of arrivals the airport could accept. [Ref 7-1] LFM sequenced arrivals through one of four metering fixes 3040 nautical miles from the airport, at a specific time and in an order that would allow the airplanes to fly directly to the airport and land. The profile descent portion of the system allowed pilots to descend at their discretion, so they could plan a more fuelconservative descent profile to the metering fix using an idle thrust, clean (flaps and speed brakes retracted) configuration.

The LFM/PD system was first installed on an experimental basis at the Denver, Colorado, and DallasFort Worth, Texas Air Route Traffic Control Centers (ARTCC). The system was a vast improvement over existing arrival procedures, but it still had a couple of significant drawbacks. The computerized program gave the controllers the time each aircraft was to cross its metering fix, but the controllers had to manually compute how to get the airplanes to that fix at the correct time, speed, and altitude. The pilots were given the speed and altitude, or altitude range, at which they were to cross the fix, but controllers had full responsibility for modifying the cruise speed and/or descent profiles of arriving aircraft to meet the time requirements of the schedule. Using various manual calculations, controllers managed to achieve metering fix arrival accuracies as good as plus or minus two minutes. To do that, however, profile descents were often interrupted and aircraft speeds usually had to be modified. Consequently, there was still a significant gap between the fuel efficiencies obtained through LFM/PD and an optimum fuelefficient flight and descent profile. [Ref 7-2]

In an effort to close that gap, researchers at the Boeing Commercial Airplane Company and the NASA Langley Research Center in the late1970s began working on profile descent equations, using the fourdimensional, or "4D," capabilities of the TSRV 737's flight management system (FMS). Twodimensional navigation only looked at an airplane's horizontal path. Threedimensional navigation controlled the aircraft's vertical flight path as well as its horizontal direction, and fourdimensional navigation included a time element, as well. In a 4D navigational mode, the airplane would not only hold to a specific horizontal and vertical path, but it would also speed up or slow down to hold to a specific route or arrival time. The controller could simply give the pilot an altitude, speed and time to cross a metering fix, the pilot would enter those parameters into the flight computer, and the computer would calculate the most fuelefficient flight path to follow to arrive at the fix as instructed by the controller.

The software to give the 737 flight management system 4D navigation capability was developed and tested first on a fast time computer and then incorporated into the TSRV realtime, piloted simulator. But the true test of the concept was whether or not it would be acceptable to pilots and controllers in a realistic ATC environment. So the software was installed in the TSRV 737 for a series of flight tests in the Denver LFM/PD system. Denver was chosen because it was the FAA's lead center on the LFM/PD concept and had been given a certain amount of leeway to make minor changes to the ATC software as necessary to improve its operation.

The plan was for the air traffic controllers to assign a metering fix time to the 737, and both the NASA researchers and the ground personnel would monitor how accurately the airplane met the fix time and how well the profile descents used by the 737 mixed with other air traffic in the area. The NASA researchers initially expected only a basic level of cooperation from the FAA, but the Denver ARTCC personnel contributed so much assistance in planning and conducting the flights that they became, in essence, a third partner in the research effort. In fact, the cooperation and involvement of the Denver ARTCC was a critical element in the success of the flight tests.

The research flights took place June 1928, 1979. A total of 19 test runs were made from a cruise altitude to one of the metering fixes for Denver Stapleton airport. At approximately 110 miles from the metering fix, the 737 crew was given a target arrival time, which was programmed into the flight computer. From the top of descent point to the metering fix, the pilot flew the airplane at idle thrust and without speed brakes, following computergenerated path and speed cues on the electronic flight displays in the 737's aft cockpit. Once the airplane crossed the metering fix, it broke off the approach and circled around to begin another test run. The flying duties were divided among two NASA pilots and four airline pilots, and each flight carried numerous FAA and industry observers, as well.

There were a few initial compatibility problems, such as the fact that the initial metering fix times assigned by the LFM system required airspeeds slower than the 737 was physically capable of flying, but a few software changes corrected the difficulties. After that, the flight tests proved extremely successful. The 737 was able to make the metering fix arrival times with a mean time error of only 6.6 seconds, altitude error of 33.6 feet, and airspeed error of .3 knots. In comparing the FMScomputed profile descent flight paths with conventional LFM/PD approaches used by airline pilots at Denver, NASA engineers found that pilots without the advantage of the precise guidance provided by the FMS tended to descend earlier, to ensure their arrival at the metering fix at the correct altitude and airspeed. The arrival time between the two differed only slightly, but the FMScontrolled profile descent used 28% less fuel. [Ref 7-3]

Assessments of the NASA profile descent flight tests from FAA observers were very positive, as well. Their evaluations included comments such as "Fantastic accuracy at metering fix and landing time. Will definitely reduce controller work load, save fuel and much frustration on part of the controller and the pilot," and "Has potential for real capacity improvements." The FAA observers also noted, however, that although NASA's single airplane had performed extremely well, there were additional issues that widespread use of airbornecontrolled profile descents would raise. What would be the consequences, for example, of different airplanes using specific, but different, climb and descent paths and speed profiles while still making their metering fix times? How would aircraft using profile descents mix with aircraft not equipped for that kind of approach? Would airspace have to be redesigned to ensure the safety of the procedure? In addition, although the airborne system had proved itself capable of accuracies within seven seconds, the ground metering system at that time was only accurate within 30 seconds, so the system in practice could not operate as efficiently as it had in the test flights. [Ref 7-4]

The questions raised by the FAA were all valid and important issues. They also underscored the particularly wide gap between the research and operational environments with technology designed to improve not just one aircraft component, but an entire nationwide system. Furthermore, the profile descents tested in the TSRV 737 depended on an onboard flight management system, which, with the exception of a few Lockheed L1011s, did not yet exist on any production airplanes. The Boeing 767 was the first new production airplane to include an FMS as standard equipment, and the first 767 did not go into service until three years after the Denver profile descent flight tests were completed. [Ref 7-5]

In an effort to resolve some of the issues surrounding the mix of aircraft with and without flight management systems, the Langley researchers conducted some additional profile descent experiments in 198485. Engineers used a hand held calculator to compute the necessary information for a profile descent on a T39 Sabreliner jet and several United Airlines jet transports. Preliminary results showed the pilot workload was acceptable, but analysis showed the pilot had to fly the faster portion of the descent very precisely, which was difficult without a flight management system. In addition, the calculators needed to perform the computations cost around $400 each, and pilots would have to be trained in their use, creating additional costs for the airlines. [Ref 7-6]

Even if all the airliners used either an FMS or a hand held calculator, however, the issue of ensuring safety in a system where individual airplanes were controlling their own descent paths would remain. The controllers would be unable to predict the exact trajectory of the descending airplanes, and there was too much potential for unexpected maneuvers. In one example related to a Langley researcher by the FAA ARTCC staff at Denver, a pilot was asked if he could make a certain arrival time at the metering fix. He replied he could, and then proceeded to start a 360 degree turn, back into conflicting traffic, in order to delay his arrival until the correct time. [Ref 7-7]

The bottom line was that although the technology itself worked well, the ATC infrastructure was not equipped to incorporate it while still ensuring safe separation of air traffic. Even so, the United Airlines representatives who observed the test flights were impressed enough with the technology that they asked Boeing to look at the feasibility of incorporating 4D capability into the flight management systems of the 767s they had on order. At that time, however, Boeing and its subcontractors had their hands full just developing a 3D flight management system in time to meet the 767 production deadline, so a 4D capability was out of the question. [Ref 7-8]

As flight management systems became more sophisticated, Boeing did start to incorporate a type of 4D navigation capability into some of its airplanes. The Smiths Industries flight management systems in the 737300, 737400, and 737500 had the ability to compute 4D flight paths. By 1993, Boeing was also planning to offer a kind of 4D navigation, which it called Required Time of Arrival (RTA), for its 767 and 747400 aircraft as part of a suite of advanced navigation and communication functions. [Ref 7-9]

Yet even in 1993, although Local Flow Management was in widespread use, the ATC infrastructure was still incapable of integrating full 4D navigation. The FAA was finally developing a more automated system that would allow more advanced navigation and communication, however. The new technology, called the Center Tracon Automation System (CTAS), was a concept originally developed by a researcher named Heinz Erzberger at the NASA Ames Research Center. Unlike the profile descent concept tested by Langley and Boeing in 1979, it was a groundbased system, but it gave the controller the same basic information the NASA pilots had derived from the airborne flight management system. Instead of just giving the controller a metering fix arrival time for each aircraft, the system also calculated an optimum, fuelefficient descent path for each airplane that would not conflict with any other known traffic, based on current weather conditions and a database on each aircraft type's performance figures. The controller could then radio instructions for that descent profile to the pilot. The system achieved the same basic end as the 4D profile descents tested by NASA's 737 in 1979, but through a groundbased system that required no airborne equipment and took other traffic into account, as well. [Ref 7-10]

In the fall of 1992, the TSRV 737 was again flown out to Denver to conduct preliminary tests using the CTAS concept. The flight tests produced arrival time accuracies within 11 seconds, although the results indicated that the wind modelling component of the system needed improvement. The FAA planned to implement CTAS at Denver and DallasFort Worth by 1996 and then install it at 12 major airports across the United States. [Ref 7-11]

The profile descent concept tested with Langley's 737, which relied on a 4D navigation capability in an airborne FMS to design and execute fuelefficient profile descents, was never implemented. It was a good idea, but ahead of its time and perhaps untenable in the complex ATC system. Nevertheless, the research did eventually have an impact. The 737 experiments proved the potential fuel savings that could be gained from 4D navigation and precision profile descents, lending support to the inclusion of 4D navigation in some later model transport aircraft and to the eventual acceptance of the CTAS concept.