Electromagnetic Induction

As our industry continues to evolve, electricity appears to be playing an ever more significant role in the operation of aircraft systems.

One example of this occurred in an aircraft with two independent AC systems powered by two operating solid-state inverters. In this case each inverter system would supply power independently and respectively to pilot and co-pilot instrument systems. It was observed during a routine flight that the captain's Magnetic Heading System (MHS) had deviated about 12 degrees from the co-pilot's system. Observation of other instruments confirmed the fault was on the left side.

During troubleshooting the technician noticed that when the captain's inverter was operating alone the compass system indicated properly. When the second inverter was activated an abrupt shift occurred on the left compass indicators. Reviewing wiring manuals provided several areas where wires from the No. 1 compass and No. 2 inverter ran in close proximity. But this was a 14-year-old aircraft, why would the problem only just begin?

Fortunately, the alternating current power system had a third inverter installed that was used as an in-flight replacement should one of the two primaries fail. It was then noted that when the standby inverter was selected in place of the left inverter the compass problem was gone. The No. 1 inverter was then checked with no abnormality detected and replaced as a precaution. The problem did reoccur. Next the technician monitored the outputs of both operating inverters using an oscilloscope with the ability to display two signals simultaneously. When the normal inverters were operating there was a noticeable phase split in their outputs. When the standby inverter was selected to the left side, the phase shift would disappear.

It turns out the discrepancy was a broken wire from the synchronization circuit on the No. 1 inverter into the aircraft common inverter synchronizer system.

Eliminating Induction Effects

In addition to ensuring proper spacing between conductors and operating AC power systems in phase, other precautions that are used to eliminate the effects of induction include a device known as a Faraday Cage.

A good example of this is the case grounded metal box surrounding circuit cards or, on a large scale, the properly bonded fuselage surrounding the internal contents of the aircraft. The intent here is to prevent stray charges from impacting the contained circuitry. A metal and grounded shielding encasing a wire has the same result. The key here is to ensure a leakproof system. If the shielding is present along the entire run of wire and terminates 6 inches from where the wire enters a bulkhead connector, a doorway has been formed that could allow stray signals to either enter or escape.

What Tools to Use

So what are the tools to use in the process to detect and resolve inductive anomalies? As was mentioned earlier the oscilloscope provides technicians the ability to view the actual operating condition of the circuit. If electrical noise is observed, actions can be taken to identify the source. The cost of a scope may not easily be justified by most technicians so alternate means of troubleshooting may involve wrapping suspect areas with aluminum foil and grounding it to test the impact of the infected system. This method is of course recommended for on ground troubleshooting.

Inductive testers such as a "Fox and Hound" are a lower cost and effective means to detect electrical noise as well as locate breakdown in shields. This device has a tone generator that can be connected to a suspect circuit and a signal tracer is then hand held while the technician moves the sensor along the conductor. If there is a break in the shield, an audible tone is produced by the signal tracer which corresponds to the injected signal.

Induction is something we can not see, electrical noise is something we cannot ordinarily hear, I guess the sense of touch will in many cases prove the existence of electrical phenomenon along with the sense of smell after the fact.

I recently heard that a think tank comprised of railroad executives just disproved the Theory of Flight. Oh well, I always wanted to work on trains anyway.

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