Attitude Director Indicator

The commander's and pilot's ADIs are supported throughout the mission, while the aft ADI is active only during orbital operations. They give the crew attitude information as well as attitude rate and attitude errors, which can be read from the position of the pointers and needles. Each ADI has a set of switches by which the crew can select the mode or scale of the readout. The commander's switches are located on panel F6, the pilot's on panel F8 and the aft switches on panel A6.

The orbiter's attitude is displayed to the flight crew by an enclosed ball (sometimes called the eight ball) that is gimbaled to represent three degrees of freedom. The ball, covered with numbers indicating angle measurements (a zero is added as the last digit of each), moves in response to software-generated commands to depict the current orbiter attitude in terms of pitch, yaw and roll.

The ADI attitude select switch determines the unit's frame of reference: inrtl (inertial), LVLH (local vertical/local horizontal), and ref (reference). The inrtl position allows the flight crew to view the orbiter's attitude with respect to the inertial reference frame, useful in locating stars. The LVLH position shows the orbiter's attitude from an orbiter-centered rotating reference frame with respect to Earth. The ref position is primarily used to see the orbiter's attitude with respect to an inertial reference frame defined when the flight crew last depressed the att ref push button. It is useful when the crew flies back to a previous attitude or monitors a maneuvering system thrusting period for attitude excursions. The two forward switches are active during ascent, orbital and transition flight phases but have no effect during entry, the latter part of a return to launch site or phases when the backup flight system is driving the ADIs. The aft switch, like the aft ADI, is operational only in orbit.

Each attitude director indicator has a set of three rate pointers that provide a continuous readout of vehicle body rotational rates. Roll, pitch and yaw rates are displayed on the top, right and bottom pointers, respectively. The center mark on the graduated scale next to the pointers shows zero rates, while the rest of the marks indicate positive or negative rates. The adi rate switch for each indicator unit determines the magnitude of full-scale deflection. When this switch is positioned to high (the coarsest setting), the pointer at the end of the scale represents a rotation rate of 10 degrees per second. When the switch is positioned to med, a full-range deflection represents 5 degrees per second. In the low position (the finest setting), a pointer at either end of the scale is read at a rate of 1 degree per second. These pointers are ''fly to'' in the sense that the rotational hand controller must be moved in the same direction as the pointer to null a rate.

ADI rate readings are independent of the selected attitude reference. During ascent, the selected rates come directly from the solid rocket booster or orbiter rate gyros to the ADI processor for display on the rate pointers. During entry, only the pitch rate follows the direct route to the ADI display. The selected roll and yaw rates first flow through flight control, where they are processed and output to the ADI as stability roll and yaw rates. (This transformation is necessary because, in aerodynamic flight, control is achieved about stability axes, which in the cases of roll and yaw differ from body axes.)

Three needles on each attitude director indicator display vehicle attitude errors. These needles extend in front of the ADI ball, with roll, pitch and yaw arranged just as the rate pointers are. Like the rate indicators, each error needle has a background scale with graduation marks that allow the flight crew to read the magnitude of the attitude error. The errors are displayed with respect to the body-axis coordinate system and, thus, are independent of the selected reference frame of the attitude display.

The ADI error needles are driven by flight control outputs that show the difference between the required and current vehicle attitude. These needles are also ''fly to,'' meaning that the flight crew must maneuver in the direction of the needle to null the needle. For example, if the pitch error needle points down, the flight crew must manually pitch down to null the pitch attitude error. The amount of needle deflection indicating the number of degrees of attitude error depends upon the adi error switch for each ADI. In the high position, the error needles represent 10 degrees, med represents 5 degrees and low represents 1 degree.

At the aft flight station on panel A6, the aft sense switch allows the flight crew to use the aft ADI, RHC and translational hand controller in a minus X or minus Z control axis sense. These two options of the aft ADI and hand controllers correspond to the visual data out of the aft viewing (negative X) or overhead viewing (negative Z) windows.

Each ADI has a single flag labeled off on the left side of the display whenever any attitude drive signal is invalid. There are no flags for the rate and error needles; these indicators are driven out of view when they are invalid.

The ADI contractor is Lear Siegler, Grand Rapids, Mich.