HF Radio Noise and Bonding After Repaint
After repainting a twin-engine turboprop, the HF radio has very high background noise and reports from a remote station are weak and distorted. The paint shop removed and refitted the HF antenna coupler and several access panels. Bonding straps were reinstalled, but some were painted over. On the ground, the AME measures bonding resistance from the HF antenna coupler base to nearby primary structure at 0.6 ohm. The manufacturer's maintenance manual calls for a bonding resistance of less than 0.01 ohm for the HF installation. The antenna coupler tunes on some frequencies but not on 5.6 MHz; the SWR at the coupler is 3.5:1. The aircraft is used for maritime patrol over water, and HF is required. The AME has a bonding meter, paint stripper, new bonding straps, and an HF test set. The operator wants to depart on a task in four hours.
1. Interpret the bonding reading and the radio symptoms. Why can paint over a bonding strap matter at HF?
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The measured 0.6 ohm is far above the manual limit of less than 0.01 ohm, so the HF installation does not have the intended low-impedance bond to structure. At HF, the airframe is part of the antenna system and the ground return. A high-resistance bond, or a bond that is insulated by paint, forces RF return currents to take unintended paths. That can produce high noise, unstable tuning, poor radiation efficiency, distortion, and arcing or heating at poor contacts.
The fact that the coupler tunes on some frequencies but not at 5.6 MHz is consistent with an installation whose impedance changes with frequency. It does not prove the coupler itself is faulty. The SWR of 3.5:1 is also a symptom of a mismatch, not a reason to simply increase transmitter power or accept lower performance. The paint over the straps is significant because the bonding meter may show a reading through the paint or through a small contact point, while the RF path is still poor.
2. What corrective actions and tests would you carry out before considering release?
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I would first restore the bonding paths. Remove paint and primer from the bonding contact surfaces, clean them to bare metal, and refit or replace the straps so that each connection is metal-to-metal and protected against corrosion after assembly. Pay particular attention to the antenna coupler base, the antenna ground plane, and the access panels that form part of the HF return path. Re-measure bonding resistance and require it to be below the manual limit, not merely lower than before.
I would then inspect the coupler connectors, coax shield termination, and antenna feed for corrosion, looseness, or paint contamination. With the HF test set, I would check tuning and SWR across the operating bands, including 5.6 MHz, and confirm that the coupler can find a match without excessive tuning time or arcing. If the coupler still fails after the bonding and connections are correct, I would troubleshoot or replace it as required.
I would not release the aircraft on the basis that HF 'works on some frequencies'. The maritime patrol role makes HF a required system, and the manual bonding limit is the acceptance criterion.
3. The operator says the aircraft can depart because HF is only a secondary radio. How would you handle the release decision?
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I would check the actual operational requirement and the approved documentation rather than accept the label 'secondary'. If the aircraft is used for maritime patrol over water and HF is required for that operation, a known HF installation defect is not automatically acceptable. If there is an approved minimum equipment list or deferral provision, I would confirm whether this specific defect is eligible, what limitations apply, and whether the aircraft can legally perform the intended task. I would not create a deferral for a bonding resistance that is far outside the maintenance manual limit.
If the system cannot be brought within limits before the departure time, I would record the aircraft as not released for the HF-required task or advise that the task cannot be conducted as planned. The record should show the bonding readings before and after, the paint removal and strap replacement, the tuning and SWR results, and the manual reference. The four-hour deadline is an operational pressure, not an airworthiness justification.