The helmet mounted display research with the TSRV 737 stemmed from research the McDonnell Douglas Corporation was conducting on the X30 National Aerospace Plane (NASP). Engineers working on the X30 faced a number of formidable design challenges. The plane was supposed to take off from a conventional runway, go into space and travel at hypersonic speeds, and then reenter the earth's atmosphere and land once again on a runway. The performance requirements of such an vehicle required the design to be as light, streamlined and as simple as possible. One potential strategy for saving significant amounts of weight and drag was to design the aircraft without any cockpit windows. Under normal conditions, it was argued, automatic landing systems and advanced flight displays could safely guide the aircraft without any direct outside visibility for the pilots. In order for such a revolutionary design to be acceptable or even considered, however, it still had to afford the pilots some way of flying and landing the aircraft visually in case of an emergency.

The helmet-mounted display being tested on the ground at Langley Research Center in 1989. Dr. Jeremiah F. Creedon, director for Flight Systems at Langley, in the middle looks on.

McDonnell Douglas researchers who were working on the X30 cockpit design came up with an idea for a helmetmounted display that would use television cameras or sensors in the nose of the airplane to provide the necessary visual display of the outside world for the pilots. A helmet mounted display would be more flexible than a fixed cockpit display, because it could track the pilot's head movements and show him the specific portion of the outside scene where his eyes were "looking." Helmet mounted displays were already being used in military helicopters, but they had never been tested for basic control of a transport aircraft in a cockpit that had absolutely no outside visibility.

The TSRV 737 was a far cry from the X30. But the McDonnell Douglas engineers knew the 737 was involved with stateoftheart cockpit displays, and the windowless aft flight deck on the airplane seemed like the ideal place to test the viability of a synthetic vision system. The research systems on the aircraft were also set up to incorporate new experiments with a minimum of effort. So the McDonnell Douglas researchers approached NASA with a proposal to test a helmet mounted display system on the airplane, and the managers at Langley agreed. McDonnell Douglas would provide the experimental technology, and NASA would provide the testbed and flight time.

Since the purpose of the flight experiments was simply to test the basic viability of the helmet display concept, the test system was constructed from crude, offtheshelf components. Two television cameras were mounted in the nose of the 737, giving the pilot a total field of regard 80 degrees across and 30 degrees high. Within that area, the pilot's field of view at any given moment was 40 degrees wide and 30 degrees high. The images conveyed by the cameras were displayed on two eyepieces in the pilot's helmet. Because the display was to be used for basic aircraft control, critical flight data including altitude, airspeed, flight path angle and and flight path acceleration were superimposed on the televised image. [Ref 8-17]

Radome of the Boeing 737 testbed aircraft, used to provide information to the helmet mounted display.

Although the McDonnell Douglas engineers had initially envisioned using the aft cockpit of the 737 for the test flights, they wanted to be able to compare the results of the helmet mounted display landings with conventional landings using outside visual references. Visual landings had to be made from the 737's forward cockpit, and the controls and displays in the aft flight deck were dramatically different from those in the front. In order to make a valid comparison of landing accuracies with and without the helmet display, therefore, all the landings had to be done from the forward cockpit. Since safety considerations precluded the front cockpit windows from being covered, the visor of the helmet used for the synthetic vision display was painted a flat black color and kept down over the pilot's eyes during the helmetguided landings.

The helmet mounted display system was tested by two NASA research pilots and two pilots from McDonnell Douglas. In addition to practice time in a ground simulator, each pilot made a minimum of 14 visual landings to NASA's Wallops Island Flight Facility and observed an additional 14 landings through the helmet mounted display before attempting any landings with the synthetic vision system. In evaluating the system, the McDonnell Douglas engineers wanted to compare both the pilots' perceived workload and the accuracy of the landings with and without the helmet display. While the Wallops Island airport was equipped with laser tracker that could provide data on the accuracy of the landings, the researchers wanted to give the pilots immediate feedback on how close they had gotten to the target landing spot. To obtain that information, ground personnel ran out onto the runway immediately after each landing and found the sticky rubber patches that marked where the 737's tires had just touched the runway. The distance between those marks and the target point was then quickly measured and radioed to the flight crew on the airplane.

Measuring aircraft skids during the runway friction tests.

The test flights were conducted throughout May and June of 1989. A total of 67 landings were completed using the helmet display system, with very favorable results. The accuracy of the landings with and without the helmet display were almost identical, although pilots reported that their workload was higher for the landings using the synthetic vision system. There were also some difficulties caused by the limitations of the test equipment, such as an image processing time lag that made pilots reluctant to move their head once they were on a short final approach. Still, the flights showed that a helmet mounted synthetic vision system could be used to land a transport airplane. [Ref 8-18]

Although the helmet mounted display tests had been very successful, the X30 manned vehicle program was abandoned in 1992. McDonnell Douglas continued funding some helmet display research for military applications, since windowless cockpits could potentially help shield aircraft from both radar detection and laser weapons. The McDonnell Douglas engineers also believed the concept might one day find its way into another advanced transport aircraft design. [Ref 8-19] But while the future of the helmet mounted display system was still uncertain, there was no question that the TSRV 737 had been useful in its initial development. The 737 allowed the McDonnell Douglas engineers to test the basic viability of the concept, economically and in actual flight conditions, before investing large amounts of company funds and staff time in fully developing the technology.