The overall goal of the TCV experiments with electronic flight displays was to examine how well the displays worked and how they could be used in a transport cockpit. In addition to validating the benefits of the basic equipment, several different display concepts were tried and evaluated to see if they would improve pilots' situation awareness and their ability to compensate and correct for flight path errors.

Much of the development work was actually conducted in the Transport Systems Research Vehicle (TSRV) simulator at the Langley Research Center, but promising display concepts were then incorporated into the 737 airplane's aft flight deck (AFD) for operational testing. There were a number of research flights dedicated specifically to evaluation of display concepts, but because the electronic flight displays were an integral part of the AFD, researchers were able to gather information on the displays in the course of conducting other research, as well.[Ref 3-8] During two years of flight demonstrations of the U.S. microwave landing system between 19761978, for example, the TCV program engineers gained valuable information on the impact of electronic displays on pilots' ability to fly complex, curved approaches to airports.[Ref 3-9]

Original configuration of the aft flight deck of the NASA 737 with monochrome flight displays. Note the "Brolly Handle" flight controls.

The initial displays in the 737 aft flight deck were 5" x 7" monochrome CRTs. Each pilot position had two displays: an electronic attitude director indicator (EADI), and an electronic horizontal situation indicator (EHSI). The EADI contained the same basic information available on a conventional attitude indicator instrument; that is, pitch attitude, roll or bank angle, and raw vertical and horizontal (localizer and glideslope) tracking information for an ILS approach. Because of the flexibility offered by the electronic display format, however, the EADI also offered some additional information and options. In an effort to provide the pilot with an improved awareness of the airplane's situation, the screen also displayed the radar altitude of the airplane, symbols that showed if its airspeed was slower or faster than a selected target speed and if the plane was accelerating or decelerating, and an indication of the airplane's actual flight path.

The TSRV 737 research simulator at Langley.

On a electromechanical indicator, the airplane symbol was oriented around a "horizon" line that was drawn across the center of the instrument. If the airplane was level, the symbol would be directly on top of the horizon line. With the additional information being evaluated on the electronic display, however, researchers were concerned about whether they would be able to see individual elements clearly. In order to unclutter the middle of the display screen, the airplane symbol was biased up five degrees, so the symbol appeared five degrees above the horizon line when the airplane was actually level.

The aft flight deck of the NASA 737 in 1987. An upgrade in 1986 replaced four original 5 x 7 monochrome displays with eight 8 x 8 color monitors.

The EADI display also came with three pilotselectable options. A perspective runway and an indication of the airplane's track angle to the runway could be included on the display. A second option was the addition of flight director symbols, which would show the pilot what pitch and bank angles to follow to stay on a predetermined flight path. The pilot could also select to superimpose all the display symbology on a realtime image received from the Lear Siegler Forward Looking, Low Light Level Television camera mounted in the nose of the 737. With this option, the pilot in the aft flight deck could actually "see" the airport and runway during an approach. The TV camera also served as a method of verifying the accuracy of the runway symbology used in the display.

The EHSI displayed the same navigational reference information provided by an electromechanical horizontal situation indicator (HSI), but in a much more integrated, pictorial format. The EHSI was a map display that showed a diagram of the airplane's preplanned flight path annotated with navigational waypoints, geographic reference points such as airports and navigational radio beacons, and the location of selected terrain and obstacle hazards along the route. The display also showed the airplane's current horizontal position on the flight path, represented by a triangle on the map, and dashed lines that indicated what the airplane's location would be in 30, 60, and 90 seconds if no further flight control inputs were made. The actual magnetic compass heading and track of the airplane were indicated at the top of the display. The map could be oriented in either a conventional "northup mode," or a "trackup mode" which showed the direction the airplane was flying at the top of the screen, regardless of its actual compass heading. The display could also be set for numerous scales, ranging from one to 32 nautical miles/inch . Additional options included the ability to display altitude or speed targets associated with waypoints, and even time reference information for experimenting with "4D" navigation.

The aft flight deck of the NASA 737 after a 1988 modification. The pilot position's "Brolly Handle" flight controls have been replaced with a McFadden sidestick controller while the co-pilot position has not been modified.

As with the EADI, the objective of the EHSI display was to provide pilots with integrated, intuitively understandable information that would give them a more accurate picture of the airplane's exact situation at all times. Armed with this information, pilots were expected to be able to monitor and control the airplane's progress much more effectively and precisely in both manual and automatic flight modes.

Manual flight from the aft flight deck was accomplished through a flight mode called "Control Wheel Steering" (CWS). Instead of a direct linkage from the control yokes (or "brolly handles") to the airplane's flight controls, CWS took the pilot's control inputs and processed them through the airplane's digital flight computer, which, in turn, operated the flight controls. This allowed the pilot to command changes in the airplane's pitch or bank angle while delegating the actual stabilization functions to the computer. One of the concerns often voiced by pilots and human engineering specialists about automated cockpit functions was that they would eliminate the pilot from the control loop entirely, leading to an undesirably low level of activity for the pilot and even a decay in his flying skills.[Ref 3-10] The idea behind CWS was to reduce the pilot's workload without automating the flight control function entirely, so that the pilot would remain "in the loop."

Even in 1974, control wheel steering was not an entirely new concept. The Douglas Aircraft Company's DC10 widebody airliner had a CWS mode that allowed the pilot to control the autopilot through inputs to the control yoke. The TCV setup, however, had a couple of slightly different twists. The digital flight control computer that drove the autopilot in NASA's 737 allowed the research engineers to experiment with different control laws and algorithms. So in addition to a more standard "attitude" control wheel steering mode, the 737 could be operated through a second control law concept, called "velocity vector control wheel steering."

Velocity vector control meant that the pilot's inputs commanded changes in the airplane's flight path instead of its attitude and bank angle. The rationale was that the end result a pilot was trying to achieve through attitude and bank angle changes was, in fact, control of the airplane's flight path. With velocity vector CWS, the pilot's control inputs told the computer what flight path he wanted the airplane to follow, and the computer would make whatever attitude and bank changes were necessary to achieve that flight path. With a combination of the CRT displays and velocity vector CWS, the NASA researchers thought it might be possible for pilots to manually fly complex approach maneuvers with a high degree of accuracy and success, even in low visibility conditions.[Ref 3-11]

The aft flight deck of the NASA 737 with both sets of "Brolly Handles" replaced with McFadden sidestick controllers and color displays.

The NASA research pilots encountered some problems with the velocity vector CWS at first because of the way the control law was implemented and the EADI displays were configured. The displays initially showed the actual flight path of the airplane. When a pilot commanded a change in the flight path, there would be a slight delay as the computer and the airplane responded to the command. During that lag time, however, the flight path on the display would not have moved. So the pilot had to guess at how much of a control input would result in the correct amount of flight path change, and the frequent result was a series of oscillations as the pilot hunted for the correct flight path angle. This made precision control very difficult, so the displays were changed. Initially, the displays were modified to show both the commanded flight path angle and the actual flight path angle. Eventually, the actual flight path angle symbology was removed altogether, because when the commanded flight path symbols reached the position the pilot wanted, he could simply neutralize the controls and the computer would hold that path.

The initial EADI displays were also oriented around the nose of the airplane, regardless of which CWS mode the pilot was using. In cruise flight or in a nowind situation, this was not a problem, but on approaches with a crosswind, the nose of the airplane would be pointed to one side to compensate for the wind. So although the ground track of the airplane might be straight toward the runway, the display would show the runway off to one side. Pilots found this somewhat disorienting, so the displays were modified. In the attitude CWS mode, the displays were still oriented around the attitude indicator symbol and the nose of the airplane, but in velocity vector CWS, the displays were oriented around the flight path of the airplane.[Ref 3-12]

One of the more significant display concepts the Langley researchers tested in the TSRV simulator and on the 737 was the addition of the perspective runway, extended center line and track angle to the EADI. In essence, this gave the pilot a visual, 3D picture of the approach on a single display, instead of just the raw localizer and glideslope data. Research experiments tested pilots' performance using the velocity vector CWS, with and without the added symbology on both straightin approaches and 130 degree curved path approaches with final legs as short as one mile. The results showed that with velocity vector CWS and the pictorial horizontal situation information provided by the runway and track symbols, pilots were able to manually fly the airplane on both types of approaches with the same precision achieved by pilots following conventional flight director commands for a Category II low visibility landing.[Ref 3-13]

The Electronic Attitude Director Indicator (EADI) on the NASA 737 research aircraft. Attitude Centered EADI format orientation in a crosswind approach.

The Electronic Attitude Director Indicator (EADI) on the NASA 737 research aircraft. Flightpath centered EADI format orientation.

The pilots' mental workload was lower and their performance was significantly better with the added pictorial information than with the basic EADI display. Research pilots commented that the integrated format gave them a better understanding of the airplane's position and trajectory to the runway, which allowed them to more quickly recognize and recover from large course deviations with confidence.[Ref 3-14]

Unlike many research projects conducted by NASA, the electronic flight display work was not a formal research experiment with formal beginning and ending dates. The research began when the airplane arrived, and experiments on better display formats continued throughout the aircraft's 20 year history at the Langley Research Center. By 1978, however, the researchers had proven the viability of the basic concept and had demonstrated some significant potential benefits that integrated electronic flight displays, control wheel steering, and velocity vector control could offer.[Ref 3-15] Consequently, researchers at the Langley Research Center were optimistic that some of the technology would be integrated into the next generation of commercial transport airplanes.[Ref 3-16]

Of course, the TCV program was not the only effort to develop new technology for transport aircraft. Even before 1978, the commercial aircraft industry had begun to incorporate some advanced equipment into transport designs. The LockheedCalifornia Company, for example, certified an operational flight management system (FMS) for some of its L1011 widebody airliners in 1977. The system offered operators approximately 3% fuel savings by providing automated 3D navigation and power management for fuel efficient flight profiles. That kind of improvement might not seem all that significant, but for a fleet with 10 L10111 aircraft a 3% savings translated to 1,750,000 gallons of fuel a year. [Ref 3-17]

An order of L1011200s sold to Saudi Arabian Airlines even incorporated an 8" square monochrome map display with the flight management system. Unfortunately, the display proved unreliable, the company that manufactured it was unable to support it, and the displays were eventually removed from the L1011s. Flight management systems, on the other hand, were offered on all L1011500 series airplanes.[Ref 3-18]

Flight management computers (FMCs) had a couple of distinct advantages over electronic flight displays when it came to gaining acceptance among airframe manufacturers, however. First, they were seen as having a lower technical risk than the CRT technology. Flight management computers were also perceived as having a more compelling benefit, since they offered operators concrete fuel savings in an era of rising fuel prices. Since the financial advantages of electronic flight displays were not as concrete or obvious, the work the TSRV did in developing CRT display formats and demonstrating their benefits in a transport aircraft environment played an important role in gaining acceptance for the "glass cockpit" concept.