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
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
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.
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.
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.
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.