The Total Energy Control System research was, essentially, an effort to make an autopilot/autothrottle system perform more like an actual pilot would. In most transport aircraft, the autopilot, which operated the aircraft flight controls, and the autothrottles, which managed the engine power settings, were designed as totally independent systems. If the pilots wanted to fly the airplane through its automatic controls, they would set a desired speed, heading, and altitude into a control panel. The autothrottles would correct deviations in speed by increasing or decreasing the power settings of the engines, and the autopilot would compensate for deviations in either altitude or heading by changing the pitch or bank angle of the airplane.

The systems were designed separately so that if one failed, it would not affect the operation of the other. The approach was not very efficient, however. The autopilot functions always had priority so, for example, if the airplane hit turbulence, the autopilot would be making constant pitch corrections to try to keep the airplane on the correct altitude. The autothrottle had no way of knowing that the pitch changes were temporary, so it would continually increase and decrease the throttle settings to try to keep the speed constant. The problem was even worse in other situations, such as when the airplane was trying to stay on a descending glideslope into an airport.

If the airplane was high and slow, for example, the autopilot would pitch the nose of the airplane down to get back to the proper descent path, and the autothrottles would increase the power settings to correct the speed. As the airplane pitched down, however, the plane would speed up, and the speed would increase even further because the autothrotttles had raised the engine power levels. Suddenly, the airplane would be too low and too fast. The autopilot would then pitch the airplane up, and the autothrottles would retard the engine settings. But that combination would cause the airplane to slow down too far, so the autothrottles would have to increase the power settings again. As the engines came up in power the airplane would start to climb, so the autopilot would tell the airplane to descend, and the whole cycle would start over again. The net result was that in automatic modes, the autothrottles were moving constantly, and the autopilot had to make an excess number of pitch adjustments. It was a system that was not only potentially uncomfortable, but also extremely wasteful of fuel.

A human pilot intuitively took a more coordinated approach to controlling an airplane. Upon realizing he was a little high and slow on a descent path, for example, he would simply point the nose of the airplane down, correcting both the alitutude and speed errors without adding power. In doing so, the pilot would be using the stored energy in the airplane's excess altitude and converting it into speed as he descended, instead of relying solely on the energy of the engines to adjust the airplane's speed.

The Total Energy Control System was an attempt to design a more efficient, integrated autothrottle/autopilot system that would make better use of an airplane's stored energy. Studies at the Langley Research Center had indicated the potential advantages of such a design, and in 19791981, NASA contracted with engineers at the Boeing Commercial Airplane Company to develop the control laws the system would require. After pursuing several design strategies that did not work, the engineers finally focused on how the different controls affected the energy of the airplane. The throttle, they realized, controlled the energy state of the plane, and the elevator controlled the distribution of that energy from flight path to speed.

Within six months of that realization, the Boeing engineers successfully completed the design of the Total Energy Control System (TECS). When making decisions about how to correct errors in the aircraft's flight path or speed, TECS would first look at the energy state of the airplane. If it was correct, TECS would simply use the elevator to redistribute the energy to achieve the desired flight path or speed. For example, if the airplane was high and slow on an approach, TECS would recognize the potential energy in the extra altitude, and use the elevator to pitch the airplane down, just as a human pilot would. When a maneuver demanded a significant climb or descent rate and/or a very large speed change, of course, it might exceed the energy capabilities of the airplane. In that case, TECS would first insure that the engine settings were within the preset limits programmed into the system. It would then give priority to meeting either the speed or flight path target, depending on which mode the pilot had selected. In an aborted landing and goaround situation, for example, speed would be critical, so that would take precedence. If the pilot selected flight path priority, the airplane would give up speed to maintain the flight path target within the safety limits of the airplane's stall and maximum speed limits.

TECS was first tested in the realtime B737 fixed base simulator at the Langley Research Center. The concept worked well in simulation, so the NASA engineers then programmed it into the TSRV's flight computers and conducted 20 hours of flight testing with the system in 1985. The objective was not only to test the performance of the technology, but also to give pilots an opportunity to fly and evaluate the integrated autopilot/autothrottle system. The flight test results showed that TECS met or exceeded all of its performance objectives, and pilots liked the system.

Yet by 1993, TECS had not been incorporated into any of Boeing'scommercial transport aircraft, and the decision had been made not to include it in the new B777 airliner the company was building. The primary reason was that implementing TECS required the entire redesign of the automatic control system. While the control laws the Boeing engineers used to develop TECS had been kept generic on purpose, to make the concept easily adaptable to new aircraft, designing a new control system from scratch would still entail significant costs for Boeing. In order to make that investment worthwhile, there would have to be a compelling need for the change, or significant cost savings involved. While TECS would result in fuel savings, the fuel crisis of the late 1970s that had helped spur the start of the research had subsided by the mid1980s. The bottom line was that while TECS was an excellent concept, there was not yet a great enough need for it for Boeing to justify the expense of developing it into a commercial application. Nevertheless, Boeing engineers who worked on the project remained optimistic that a commercial transport application for it would eventually develop.

Actually, an application for TECS had already developed, although it was not in a commercial transport airplane. After the TECS research was completed, Boeing began building a high altitude, long endurance, unmanned airplane, called the Condor, for defense and/or other uses, such as patrolling the antarctic for holes in the ozone layer. As it was unmanned, the aircraft had to be flown entirely on autopilot. Since it was also designed for maximum endurance, that autopilot system had to be operating at peak efficiency, at altitudes where the speed envelope between its maximum speed and its stall speed was only two or three knots.

By a fortunate coincidence, one of the Boeing engineers who had worked on the early TECS research was assigned to the Condor project. The other engineers had heard of TECS, but having someone on the project who knew the technology intimately helped convince them to pursue it. It was easier to introduce new technology to the Condor project, of course, simply because of the aircraft's nature. It was unmanned, so there was no concern about having to retrain pilots. There were no human factors problems to consider. It was already being designed from scratch, so incorporating a new control system did not entail so many additional costs, and it was not a commercial airliner, so the new control system did not have to be recertified by the Fedeeral Aviation Administration.

Nevertheless, TECS still might not have been incorporated into the Condor if it was only a paper theory. The fact that the technology had already been successfully flight tested in NASA's 737 gave the Boeing designers a crucial level of confidence in its performance and reduced the risk level enough that Boeing was willing to use the new technology in an operational aircraft. [Ref 6-8]