Intermittent Radio Communication Failure
A commuter aircraft experiences intermittent VHF radio communication failures. The problem occurs only when the aircraft is flying in heavy rain and at altitudes above 10,000 feet. The radio works perfectly on the ground and in dry conditions. The avionics technician suspects a static discharge issue or a faulty antenna connection. The aircraft has two VHF radios, and the problem affects both. The maintenance manual provides wiring diagrams and antenna locations. The technician has a bonding meter, a multimeter, and a time-domain reflectometer (TDR). The aircraft is scheduled for a 100-hour inspection in 3 days.
1. What are the most likely causes for the intermittent radio failure, and how would you use the available test equipment to isolate the fault?
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The most likely cause is precipitation static (P-static) due to inadequate bonding or a faulty static discharger. Another possibility is a poor antenna connection that becomes intermittent under vibration or moisture. To isolate: 1) Use the bonding meter to check the resistance between the antenna base and the aircraft skin; it should be less than 0.0025 ohms per AMM. 2) Inspect static dischargers for damage and measure their resistance. 3) Use the TDR to check the coax cable for impedance mismatches or opens. 4) Perform a continuity check on the antenna connector. Since both radios are affected, the common factor is likely the antenna system or grounding.
2. If the bonding check reveals a resistance of 0.01 ohms at the antenna base, what is the significance, and what corrective action is required?
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A resistance of 0.01 ohms is higher than the typical limit (often 0.0025 ohms) for antenna bonding. This high resistance can cause P-static interference and intermittent communication issues. Corrective action: clean the bonding points, remove corrosion, and re-bond the antenna base to the aircraft skin. Re-check the resistance to ensure it is within limits. Also inspect the static dischargers and replace any that are faulty. After correction, perform an operational check of the radios in simulated rain conditions if possible, and document the maintenance action.