Intermittent Avionics Fault Traced to a Bonding Jumper
A twin-turbine aircraft reports intermittent VHF communication noise and a sporadic heading error on the standby compass during climb. The faults appear only in IMC and clear on descent. The avionics technician finds no fault codes in the VHF or compass systems. A review of recent work shows that a static wick was replaced and a bonding jumper on the vertical stabilizer was removed for access and reattached. The jumper attachment hardware shows slight corrosion and the mating surface was painted over during a recent touch-up. Resistance measured across the jumper is 0.8 ohm. The aircraft is dispatched with the faults deferred as intermittent. The next crew reports the same symptoms.
1. Assess whether deferring the fault as intermittent was appropriate, and explain the mechanism linking the bonding jumper to the reported symptoms.
Read the worked answer
Deferring was not appropriate. The symptoms are repeatable under a defined condition (IMC climb) and a known recent maintenance action touched the bonding path. That is a diagnosable fault, not a random intermittent, and deferring it without diagnosis left a known defect in service.
The mechanism is straightforward. A bonding jumper provides a low-impedance return path for static and induced currents. A painted mating surface plus corroded hardware raises resistance to 0.8 ohm, far above the milliohm values expected for a good bond. Static charge that should bleed off accumulates and discharges as radio-frequency noise, which is heard on VHF. The same charge and current paths can create magnetic fields or voltage offsets that disturb a magnetic heading reference, producing the sporadic compass error. IMC climb changes moisture and electrostatic charging conditions, explaining why the fault appears there.
The correct action was to clean the mating surface to bare metal, replace corroded hardware, reattach the jumper, and verify bond resistance against the maintenance manual before release.
2. Describe the verification steps you would require before returning the aircraft to service, and the documentation that must accompany them.
Read the worked answer
Verification should be systematic. First, restore the bond: remove paint from the mating surfaces, install clean hardware to the specified torque, and measure resistance across the jumper with a bonded-meter method, comparing to the manual limit (typically a few milliohms for a structural bond). Record the value.
Second, confirm the static wick installation and resistance, since it was also disturbed. Third, perform an operational check of the VHF system and the standby compass, including a compass swing if the maintenance manual requires it after work affecting the vertical stabilizer. Fourth, if practicable, reproduce the condition or review system health data to confirm noise is gone.
Documentation must include the task reference, resistance readings, hardware replaced, any compass swing results, and a certification by the appropriately authorized person. The deferral entry must be closed out with the corrective action described. If the fault cannot be reproduced after the bond repair, the record should still show what was corrected and the basis for return to service.