Control and Display Unit (CDU)

Control and Display Unit (CDU)

The CDU provides the major input link to the system and allows the flight crew to make inputs to obtain EPR and airspeed displays and can also be used for obtaining decision-making data in relation to an aircraft's flight profile. The CRT has a 2 in x 3 in screen and enables data to be displayed over a 13 (column) x 6 (row) matrix.

The selection of EPR and airspeed data for various phases of flight is accomplished by a flight mode select switch, the modes and associated displays being as follows:

      

• TO : Take-off EPR limits for the outside air temperature entered by the flight crew

• CLB: EPR and speeds for the desired climb profile; best economy, maximum   climb rate, or crew-selected speeds

• CRZ:EPR and speeds for the desired cruise schedule; best economy, long-range cruise, or crew-selected speeds

            

• DES: Descent speed, time and distance for best economy

• HOLD:            EPR, speed and endurance for holding

• CON:  Maximum continuous EPR limits for existing altitude, temperature and speed

• GA: Go-around EPR limit for existing altitude, temperature and speed

The standby (STBY) position of the select switch is used for data entry and for an automatic check-out of the system.

The function of the `ENGAGE' key is to couple the target command `bugs' of the Mach/airspeed indicator and EPR indicators to computer command signals which drive the bugs to indicate the speed and EPR values corresponding to those displayed on the CRT screen. If the data is verified by the computer to be valid, engageable and different from the data presently engaged, the engage key illuminates and is extinguished after engagement takes place; at the same time the appropriate light of the mode annunciator is illuminated.

The key marked `TURB' is for use only in cruise and when turbulent flight conditions are to be encountered. When pressed it causes the CRT to display the appropriate turbulence penetration data, i.e., airspeed in knots (also Mach number at high altitudes), pitch attitude and the N<sub>l</sub> percentage rpm. In the turbulence mode, the target command speed and EPR `bugs' engage automatically. This mode is disengaged by pressing the key a second time or else engaging another flight mode.

In order that the flight crew may load keyboard-selected data into the system, three push-button switches are provided above the keyboard for Selecting, Clearing and entering data. In connection with the selection and entering of data, question marks and two symbols are displayed at the right-hand end of a data line; a caret (<) and an asterisk (*). The caret signifies that the computer is ready to accept data, while the asterisk signifies that the data next to it may be entered or changed if necessary.

The keyboard primarily serves a dual function in that it (i) permits the flight crew to enter pure numeric data into the computer and (ii) permits desired performance function data to be called up from the computer for display. The data appropriate to the keys is given in Table 2.4 and is displayed in the form of pages, each page being numbered in the top right-hand corner. For example, the page shown on the CDU in Figure 2.4 is page 1 of a set of four relating to `economy fuel' in the cruise mode. In order to call up each of the remaining pages the PAGE key is successively pressed. Similarly, the PAGE key permits cycling of the pages in reverse order. When a flight mode or performance function is first selected, the first page of a set is always automatically displayed. The RCL key is used whenever a performance function is being displayed and if it is required to recall a display corresponding to a selected flight mode.

The two switches in the upper right-hand corner of the CDU are associated with auto-throttle system operation. When the A/T Annunciator switch is pressed, an internal light is illuminated to indicate connection of the auto - throttle system and at the same time an `EPR' light in the mode Annunciator is illuminated. The PDCS then adjusts the throttles to track the EPR target values displayed on the CDU and by the command bugs of the associated EPR indicators. In order for the auto-throttle system to adjust engine power in relation to indicated airspeed, the second switch JAS SEL Annunciator' is operated; the system then drives the throttles so as to track the speed target values displayed on the CDU and by the command bug of the Mach/airspeed indicator.

2.5 Computer

The computer is of the hybrid type, and the inputs, outputs and unit interfaces are as shown in Figure 2.4. Program storage is by means of a PROM and an additional non-volatile memory for retaining all entered data during any interruption of the power supply. Built-in test equipment circuits and software operate continuously to check all critical circuits of the system. The fuel summation unit that is a component of the Performance Data Computer System (PDCS,) develops an a.c. voltage signal that is proportional to the total fuel on board the aircraft; the signal being a combination of those produced by the fuel-quantity-indicating system sensing probes which are located in the fuel tanks.

Failure lights on the front of the computer indicate whether a fault is in the computer, CDU or input signals. The INDEX NUMBER switches, which are of the rotary type, are used for programming a flight index number from 0 to 200 into the computer so that maximum economy flight modes are modified according to time-related costs compared to fuel costs. The switches are guarded to eliminate the possibility of inadvertent changing of the index number.