Message Name:

RADIO ALTIMETER SYSTEM MALFUNCTIONS

A Radio Altimeter malfunction, or Radio-Altitude Display malfunction occurs and is identified by one or more of the conditions that follow:

• Red Radio (RAD) Flag (radio altitude failure) shows on the pilot (copilot) Primary Flight Display (PFD)
• Amber RAD flag (radio altitude miscompare) shows on the pilot (copilot) PFD
• Radio Altitude Display does not show on the pilot (copilot) PFD
• Radio-Altitude Low-Altitude awareness (brown shading as the ground) is not on the altitude tape on the pilot (copilot) PFD

• Radio Altimeter Transceiver 1 (A67)
• Radio Altimeter Transceiver 2 (A68)
• Radio Altimeter 1 Transmit Antenna (E19)
• Radio Altimeter 2 Transmit Antenna (E17)
• Radio Altimeter 1 Receive Antenna (E20)
• Radio Altimeter 2 Receive Antenna (E18)
• Data Acquisition Unit (DAU) 3 (A30)
• Data Acquisition Unit (DAU) 4 (A31)
• Associated Wiring

Advisory Wire/Service Bulletin: None

Forum Articles/Infoservice/Newsletter: None

1. A metallic or reflective surface (Water, Ice, metal plates, metal grills, etc.) underneath the RA antennas when aircraft is on the ground can cause erratic readings and cause the unit to fail. The first thing that should be attempted is move the A/C a few feet and test the system again.
2. The radio altimeter (RA) supplies altitude signals to other systems. When one of these signals does not show, the smart LRU will send a fault (CAIMS) report about the RA. You can get access to the active faults on the CAIMS PMAT to find the fault messages and the fault isolation procedure. If there are more than one CAIMS fault message about a RA, the RA is the most possible cause.
3. The radio altimeter operates an internal self-test that continuously monitors the health of the unit. The unit reports the status on the digital output bus.

1. Perform a check of the radio altimeter transceiver and mounting tray for defective connectors and pins.

   1. If the mounting tray connector is defective, replace the connector as required and do close out.
   2. If the radio altimeter connector is defective, go to step 8.
   3. If the connectors are correct, continue with next step.

2. Disconnect affected RA connector (A67P1 and A68P1). Perform a voltage check between the applicable radio altimeter connector as follows:

   From	To	Expected Result	Result
   A67P1-9	A67P1-11	28 VDC
   A68P1-9	A68P1-11	28 VDC

   1. If there is 28 VDC, go to step 4.
   2. If there is no 28 VDC, continue with next step.

3. Perform a continuity check between the connectors of RA and SPDA as follows:

   From	To	Result
   A67P1-9	A14P2-J
   A68P1-9	A15P4-F

   1. If there is no continuity, repair defective wiring as required and do close out.
   2. If there is continuity, continue with next step.

4. Perform a continuity check for the RA strapping between the connectors as follows:
   For Radio 1:

   From	To	Result
   A67P1-58	GS779
   A67P1-38	GS779
   A67P1-21	GS779
   A67P1-24	GS779

   For Radio 2:

   From	To	Result
   A68P1-58	GS778
   A68P1-40	GS778
   A68P1-21	GS778
   A68P1-24	GS778

   1. If there is continuity, repair the configuration wiring of the radio altimeter and do close out.
   2. If there is no continuity, continue with next step.

5. Check the coaxial cable between the radio altimeter transceiver and the antennas. Are the coaxial connectors and cable satisfactory?

   1. If NO, repair or replace the defective coaxial cable as required and do close out.
   2. If YES, continue with next step.

6. Replace the defective radio altimeter antenna. Is the problem resolved?

   1. If NO, go to step 8.
   2. If YES, continue with next step.

7. Perform a check of the ARINC 429 Bus for open circuit and short circuit between the DAU and the transceiver as follows:
   For Radio 1:

   From	To	Result
   A67P1-2	A30CP1-H6
   A67P1-10	A30CP1-J6
   GM43-C	A30CP1-SH

   For Radio 2:

   From	To	Result
   A68P1-2	A31CP1-H6
   A68P1-10	A31CP1-J6
   GM42-B	A31CP1-SH

   1. If the wiring is not satisfactory, repair or replace the defective wiring as required and do close out.
   2. If the wiring is satisfactory, continue with next step.

8. Replace Radio Altimeter Transceiver.

   1. If system checks are good, do close out.
   2. If fault remains, continue with next step.

9. Replace defective DAU.
10. Do close out.