General

The radio altimeter system has two receiver/transmitters (R/ Ts). Each R/T has a transmit and a receive antenna. The radio altimeter R/T makes a frequency modulated continuous wave RF signal that is sent to the ground and reflected back to the airplane. The time that it takes for the signal to travel from the transmit circuit of the R/T to the receive circuit of the R/T is changed into absolute altitude. The number one system altitude shows on the captain display and the number two system shows on the first officer display.

The altitude data and signal validity is sent on two ARINC 429 data buses.

ARINC 429 data bus 1 sends data to these components:

· Flight control computers (FCC)

· Autothrottle computer.

 

ARINC 429 data bus 2 sends data to these components:

· Ground proximity warning computer (GPWC)

· Traffic alert and collision avoidance system (TCAS) computer

· Flight data acquisition unit (FDAU)

· Weather Radar (WXR)

· Common display system (CDS) display electronic units (DEU).

 

The receiver/transmitters get discrete inputs from the proximity switch electronics unit (PSEU) used to count flight legs for fault recording.