The head-up display is an optical miniprocessor that cues the commander and/or pilot during the final phase of entry and particularly in the final approach to the runway. With minimal movement of their eyes from the forward windows (head up) to the dedicated display instruments (head down), the commander and pilot can read data from HUDs located in front of them on their respective glareshields. The HUD displays the same data presented on several other instruments, including the ADI, SPI, AMI and AVVI.

The HUD allows out-of-the-window viewing by superimposing flight commands and information on a transparent combiner in the window's field of view. The baseline orbiter, like most commercial aircraft, presents conventional electromechanical display on a panel beneath the glareshield, which necessitates that the flight crew look down for information and then up to see out the window. During critical flight phases, particularly approach and landing, this is not an easy task. In the orbiter, with its unique vehicle dynamics and approach trajectories, this situation is even more difficult.

Since the orbiter is intended to be in service for several years, the addition of a HUD was considered appropriate. Most recent military aircraft include HUD systems, as do several European airliners. Additionally, since the display portion of some existing HUD systems could be easily installed in the orbiter, the HUD system requirements for the orbiter were patterned after existing hardware to minimize development costs.

While the display portion of the orbiter system could be similar to existing HUD systems, the drive electronics could not. Since the orbiter avionics systems are digital and minimal impact on the orbiter was paramount, the HUD drive electronics were designed to receive data from the orbiter data buses. Most existing HUD drive electronics use analog data or a combination analog/digital interface. In the orbiter system, the HUD drive electronics utilize, to the maximum extent possible, the same data that drive the existing electromechanical display devices.

The orbiter display device, designed by Kaiser Electronics of San Jose, Calif., uses a CRT to create the image, which is then projected through a series of lenses onto a combining glass (a system very similar to one they developed and produce for the Cobra jet aircraft). Certain orbiter design requirements, such as vertical viewing angles, brightness and unique mounting, dictated some changes from the Cobra configuration.

A HUD power on/off switch located on the left side of panel F3 provides and terminates electrical power to the commander's HUD. The same switch is also located on the right side of panel F3 for the pilot's HUD.

Each HUD is a single-string system but connected to two data buses for redundancy. It is an electronic/optical device with two sets of combiner glasses located above the glareshield in the direct line of sight of the commander and the pilot. Essential flight information for vehicle guidance and control during approach and landing is projected on the combiner glasses and collimated at infinity.

For example, looking through the HUD and out the window in the final phase of the preflare maneuver, the commander might see EAS = 280 knots (left scale), altitude = 500 feet (right scale), and orbiter heading ( + ) slightly to the left of runway centerline, which indicates a light crosswind from the left. The velocity vector symbol is just crossing the runway overrun. The guidance diamond is centered inside the velocity vector symbol. The flare triangles on the wing tips indicate that the pilot is following the flare command precisely. The lighted outline of the start of the runway zone appears at the top of the combiner. The HUD can display speed brake command and position; discrete messages, such as gear; and, during rollout, deceleration and wing-leveling parameters.

The images, generated by a small CRT and passed through a series of lenses, are displayed to the flight crew on the combiners as lighted symbology. The transmissiveness of the combiner allows the crew to look through it and see actual targets like the runway.

For instance, if the crew is conducting an instrument approach at 7,000 feet on the final approach course in a solid overcast, the base of which is at 5,000 feet, the lighted outline of the runway would be displayed on the combiner. However, when the orbiter exits the overcast at 5,000 feet, the lighted outline of the runway would be superimposed on the real runway. As the orbiter proceeds down the steep glide slope, the velocity vector is superimposed over the glide slope aim point. At preflare altitude, flare triangles move up to command the pullout. The pilot maintains the velocity vector symbol between the triangles. After a short period of stabilized flight on the shallow glide slope, the guidance diamond commands a pitch-up until the nose is about 8 degrees above the horizon, which is essentially the touchdown attitude. After touchdown, during the rollout phase, the crew maintains the approximate touchdown attitude, plus 6 degrees theta (nose above the horizon), until 180 knots equivalent airspeed and then commands a derotation maneuver.

The HUD has proved to be a valuable landing aid and is considered the primary pilot display during this critical flight phase.