Description (AMM 34-22-00)

 

A- General Description

(1)  The EFIS symbol generator receives data from the EFIS control panel and the airplane's navigation and guidance systems.  The symbol generator then processes the data and provides video signals for display on the EADI and EHSI.

 

(2)  The input data is received on ARINC 429 low speed and high speed data buses, as well as ARINC 453 weather radar high speed buses, and discrete inputs.  

             

(3)  There are two connectors at the rear of the unit.  Program pins permit selection of various displays on the EADIs and EHSIs. 

             

(4)  Two symbol generators are located on the E2-1 rack. Each symbol generator weighs 21 pounds (9.5 kg) and is packaged in a size 6 MCU case.

 

(5)  The TEST switch on the front of the symbol generator initiates a self-test of the EFIS.  The BITE (Built-in Test Equipment) circuitry within the symbol generator detects existing faults and identifies the faulty line replaceable unit.  A record of the failures detected by BITE is stored in nonvolatile memory of the symbol generator.            

 

B- Symbol Generator Computed Displays                

(1)  Most of the parameters displayed on the EADI and EHSI utilize signals that originate from various systems on the airplane. However, the display of a number of parameters requires computations based on inputs from several systems.  The computations for these parameters are performed by the symbol generator. The symbol generator computed displays are as follows (Fig. 5):  

 

(a)  Airspeed Trend Vector (Airplanes with speed trend vector disable pin not connected)

                         

1) The computed airspeed from the air data computer is monitored for speed changes. The appropriate trend vector signals are computed depending upon the rate of speed change and whether the change is an acceleration or deceleration.

 

(b)  DH Alert                          

1)  The decision height (DH) value selected on the EFIS control panel and the actual radio altitude, are compared when the radio altitude is below 2500 feet.  When the actual radio altitude decreases to the DH value, the symbol generator   provides alert signals, which cause the DH and radio altitude display to change, on the EADI. 

 

The DH circuit is reset when the RESET button is pressed on the EFIS control panel; or the radio altitude increases to 75 feet above selected DH (during go-around); or when the actual radio altitude equals zero.

 

(c)  Attitude Alert                          

1) The pitch and roll data displayed by the left and right EFIS, are compared in the left and right symbol generators. The pitch and roll data from each symbol generator is transferred to the opposite symbol generator by the cross-tie bus between the two symbol generators.

 

The symbol generator which first senses an attitude difference greater than 3 degrees, will send an attitude alert signal to the opposite EFIS symbol generator, so that both EADIs  display the alert message simultaneously.  If the pitch and roll data from IRU-L or IRU-R is invalid, or the EFI transfer switch is in BOTH ON 1 or BOTH ON 2 position, the EADI's will display a COMP flag.  If the IRS transfer switch is in BOTH ON 1 or BOTH ON 2 position, the attitude    alert displays are inhibited.

 

(d)  Curved Trend Vector 

1)  The curved trend vector is computed by the symbol generator, and displayed on the EHSI in MAP and CTR MAP   modes.  The segmented curve predicts directional trend, which is calculated based on present position, ground speed and cross-track acceleration. The dashed lines show predicted airplane position at the end of 30, 60, and 90 second intervals.  The amount of curve is based on the cross-track acceleration value.

                    

(e)  NAV/IRU Position Difference                          

1)  The NAV/IRU position difference indication is displayed on the EHSI, when the following two conditions are both met:                              

 

a)  The EHSI is displaying the MAP, CTR MAP, or NAV (if applicable) mode.                             

 

b)  The difference in present position, between one of the IRUs and the FMC calculated position, equals or exceeds 12 nautical miles.  If the difference occurs, a NAV/IRU position difference indication will appear on the EHSI. However, the indication will include the present                                   position difference between both IRUs and the FMC.                          

 

2)  The NAV/IRU position difference indication consists of a numerical readout, showing the difference between the IRU and the FMC calculated position, with an arrow pointing to the IRU position, and a letter designating the applicable IRU.

                    

(f)  Range Disagreement Message

1)  The range disagreement message is displayed on the EHSI, when the following two conditions are both met:                             

 

a)  The EHSI is displaying a mode, in which route data or weather radar data can be displayed.                              

 

b)  The range selected on the EFIS control panel disagrees   with the range of the data received from the FMC, or the range of the data received from the weather radar.

 

(g)  Course Deviation and TO/FROM Indication                          

1)  Course deviation is computed by the symbol generator, and   is displayed on the EHSI when one of the following conditions are met:                              

 

a)  The EHSI is displaying a VOR mode, and the VHF NAV receiver is tuned to a valid VOR station.                              

 

b)  The EHSI is displaying an ILS mode, and the VHF NAV receiver is tuned to a valid ILS station.                          

 

2) The TO/FROM indication is displayed on the EHSI, when the EHSI is in a VOR mode, and the VHF NAV receiver is tuned to a valid VOR station.