Intermittent Radio Altimeter Fault
A Boeing 737-800 has an intermittent radio altimeter fault. The captain's radio altimeter occasionally displays incorrect altitude during approach, causing the autothrottle to fluctuate. Maintenance troubleshooting reveals that the captain's radio altimeter transceiver (No. 1) has been swapped with the first officer's (No. 2). The fault remains with the captain's display. Wiring checks from the transceiver to the display are satisfactory. The aircraft has a digital radio altimeter system that uses frequency modulation continuous wave (FMCW) technology. The AME must determine the next step.
1. What is the most likely cause of the intermittent fault and what should the AME do next?
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Since swapping transceivers did not move the fault, the transceiver itself is likely not the cause. The fault remains with the captain's display, indicating the problem is downstream of the transceiver. Wiring checks are satisfactory, so the issue may be within the display unit or its internal processing. The AME should perform a bite check on the captain's radio altimeter display unit, inspect connectors for intermittent contact, and if no fault is found, replace the display unit. Additionally, check for software configuration errors or incorrect part numbers.
2. Explain how FMCW radio altimeters determine altitude and why intermittent faults can be challenging to diagnose.
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FMCW radio altimeters transmit a continuous wave whose frequency is modulated linearly. The reflected signal is received with a time delay, resulting in a frequency difference proportional to altitude. Intermittent faults are challenging because they may only appear under specific conditions (vibration, temperature, humidity). The AME should use built-in test equipment (BITE) and consider environmental factors during troubleshooting.