Bonding Resistance After Repainting a Fuel Tank Access Panel
After repainting a metallic wing access panel that covers a fuel tank, you measure bonding resistance between the panel and the adjacent wing skin. The panel was removed for tank access, reinstalled with new screws, and the external surface repainted. The original bonding strap is fitted. Your calibrated 4-wire milliohm meter reads 11.2 mΩ; with the leads shorted it reads 0.3 mΩ. The maintenance manual limit is 2.5 mΩ, measured with a 4-wire method on clean bare metal. You notice primer on the panel mating surface and paint over the strap terminal. The tank has been defuelled and purged, and the aircraft is in a hangar. The previous inspection recorded 1.4 mΩ before painting. The aircraft is due for release after the paint work.
1. Evaluate the validity of the 11.2 mΩ reading and identify likely causes.
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The reading is not a valid bonding measurement as taken. The meter zero of 0.3 mΩ is small, but paint over the strap terminal and primer on the mating surface add contact resistance between the probe and the actual conductive path. A 4-wire meter measures the voltage drop between its probes, so probe contact on paint can inflate the result or make it unstable. The measured 11.2 mΩ is more than four times the 2.5 mΩ limit, but until the surfaces are prepared the number cannot be used to condemn the bond. Likely causes are paint or primer insulation, a loose screw, a damaged strap, or corrosion under the terminal. The previous 1.4 mΩ before painting supports the idea that the paint process introduced the high resistance. Clean the designated bonding points to bare metal and repeat the measurement before deciding.
2. Describe the correct rework and retest procedure, including safety precautions.
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Confirm the tank is defuelled and purged and that the hangar has adequate ventilation; treat the fuel tank as a confined and explosive-risk area and use appropriate bonding of test equipment. Remove the panel fasteners, clean paint and primer from the panel and structure bonding points to bare metal using approved methods, and inspect the strap for corrosion or damage. Refit the strap and fasteners, torque to the manual value, and apply approved corrosion protection to the completed joint without insulating the bonding path. Zero the 4-wire meter with the leads shorted, probe on bare metal, and measure. The result must be ≤2.5 mΩ. Repeat at each designated bond. Record the measurement, probe locations, meter serial and calibration, and any parts replaced.
3. Why does this bond matter beyond the ohmmeter reading, and what documentation would you expect?
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The panel bond is a current path for lightning strike and static discharge, and it helps keep the fuel tank structure at the same potential. A high-resistance joint can cause arcing or sparking at the panel interface, which is a fuel-vapour ignition risk, and it can upset avionics grounding or antenna return paths. The documentation should include the work order reference, panel location, original and final resistance, test method, meter calibration, surface preparation, torque values, and the approved data used. If the bond cannot be brought within limit, the panel should not be released; raise a defect and obtain engineering disposition. Sign-off should be by an authorized person after the retest, with a clear statement that the bonding limit was met.