Intermittent Radio and a Bonding Resistance Reading
A Category A mechanic reports that the VHF com radio on a light twin intermittently drops reception when the aircraft is on the ground and the avionics bay access panel is open. With the panel closed, the fault is not reproducible. A visual inspection shows the antenna coaxial connector is tight, but the braid screen appears corroded where it enters the connector backshell. A bonding check between the antenna base and the adjacent fuselage skin reads 0.9 Ω; the maintenance manual requires bonding resistance not greater than 2.5 mΩ. The aircraft's structural repair manual does not cover antenna bonding. The mechanic proposes cleaning the connector, re-terminating the braid, and returning the aircraft to service without further measurement.
1. Interpret the bonding reading and explain why it is significant for the reported symptom.
Read the worked answer
0.9 Ω is 900 mΩ, which is roughly 360 times the 2.5 mΩ limit. This is a failed bonding check, not a marginal one.
Significance:
- A high-resistance bond between antenna and skin degrades the RF ground plane, raising the antenna's impedance and reducing radiated/received signal - consistent with poor reception.
- The symptom changing with the access panel open/closed suggests the panel itself is part of the ground path or is masking a poor bond by providing an alternative path when closed.
- A poor bond is also a lightning-protection and static-discharge concern, so it is an airworthiness issue beyond radio performance.
The reading must be treated as a defect to be rectified, and the cause of the high resistance (corrosion at the braid/backshell) is the likely culprit.
2. The mechanic wants to clean and re-terminate, then release the aircraft. What is wrong with this plan and what should happen instead?
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Releasing to service without re-measurement is not acceptable. The bond resistance is the objective evidence that the repair worked; without a post-repair measurement there is no basis for a release.
Correct sequence:
- Rectify the corrosion and re-terminate the braid per approved data (the SRM does not cover it, so the correct approved data - e.g. the avionics/electrical standard practices manual or an approved repair scheme - must be identified).
- Re-measure bonding resistance and confirm it is ≤ 2.5 mΩ.
- Perform an operational check of the VHF com with the panel both open and closed to confirm the symptom is gone.
- Record the defect, the rectification and the measurement in the technical log.
Because the SRM is silent, the mechanic cannot invent a repair; approved data must be sourced. This is a judgement about release-to-service discipline, not a memorised figure.
3. The fault is not reproducible with the panel closed. How should this affect your diagnostic approach before you sign anything off?
Read the worked answer
A fault that disappears when the panel is closed is evidence that the panel is providing an unintended return path or that the open-panel condition changes the antenna environment. Signing off on the basis of 'not reproduced' would be premature.
Approach:
- Treat the open/closed difference as diagnostic information: measure bonding with the panel open and again with it closed, and compare.
- Inspect the panel-to-structure bonding straps and fasteners - a panel that improves the bond when closed is itself suspect.
- After repair, repeat the operational check in both configurations.
- If the symptom cannot be eliminated, record it as an unresolved defect and escalate rather than clearing it.
The judgement point is that intermittent, configuration-dependent faults require the fault to be positively eliminated, not merely to fail to appear.