As aircraft of today become increasingly more dependent on computerized or electronically controlled systems, the typically low power inputs and in some cases outputs of these systems are easily corrupted when unwanted induced electrical signals find their way into otherwise isolated circuits. One such occurrence was reported on a business jet aircraft utilizing an electrical starter along with a computer that would receive all engine data from various analog or discrete sensors, digitize all the data and then transmit it to the electronic displays in the flight deck.
On certain occasions during engine start the crew would report illumination of a "Fuel Filter Clogged" message. In each case the fuel filter was inspected with no contamination detected. The computer that received the input from the bypass sensor was also replaced. In addition the wiring from the sensor to the computer was rigorously examined for electrical shorts to ground with no problem detected. A discrete signal (specific typically low voltage) was put out on the wire going to the fuel filter by-pass sensor. When an actual by-pass occurred, the sensor switch would close supplying an electrical ground to the computer, dropping the discrete voltage to close to zero volts which in turn, triggered the warning message. It was noticed that the problem would disappear when new main batteries were installed and would tend to reoccur when the batteries had been in service for several months.
Eventually when the aircraft operator got tired of replacing parts, an oscilloscope was connected to the bypass sensor and an engine start was initiated. Even though no by-pass message was present the technicians did observe a voltage change on the discrete signal. Thinking perhaps the voltage drop on the bus accompanying the start was corrupting the computer, the technicians then shut down the problem engine and started another. During that start they continued to monitor the signal at the problem engine sensor. This time no change occurred in the discrete voltage. They then decided to shut down the operating engine and restart the one producing the problem. It should be noted that all the starts had been done using the main aircraft batteries.
During the start of the troublesome engine the Fuel Filter Clogged message did appear and again the technicians did notice a significant voltage spike at the sensor connector. At that time a visual inspection revealed the sensor wire was routed in close proximity to the main power feed to the starter. The decision was made to separate the wires and another start was made, this time no appreciable voltage change was observed on the scope and no message was broadcast to the flight deck.
Why did new batteries remedy the problem? With age batteries tend to loose some capacity which means under a significant load such as an engine start, a bigger voltage drop can be anticipated which in turn results in a higher current flow to the starter. So as the batteries degraded over time in the aircraft, the induced voltage spike would increase in amplitude and eventually reach the critical point where the sensing computer would detect a threshold voltage where the alarm circuit was activated. It also just so happened that the problematic engine was the last one started by the checklist procedures, so on a day-to-day basis the main batteries were already in a somewhat discharged condition when it came time to start this engine.
In this case relocating the wiring creating adequate separation between the Feeder for the starter and the smaller discrete sense wire for the Fuel Filter by-pass sensor provided the solution.
AC System Signals
Alternating current (AC) systems are often instigators of unwanted induced electrical signals. Many aircraft using multiple constant frequency electrical systems will include a means of synchronization. This will keep both AC power supplies, such as inverters, operating in phase. Electrical conductors co-located are less likely to experience the effects of mutual induction when the potential on both are rising and falling at the same time.
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