Diagnosing mysterious avionics challenges
By Jim Sparks
Did you ever encounter one of those problems that surface at random and have the ability to hide when the aircraft is in the shop?
During my years working as a tech rep for an aircraft manufacturer, I would periodically hear from an operator that their machine has a "Gremlin." I can honestly say that I think I saw these little varmints on several occasions in my earlier years of turning wrenches (usually after 24 to 36 hours of continuous troubleshooting).
Those who have seen the movie Gremlins may recall in Hollywood’s rendition, a Gremlin is a cute, furry little critter that thrives on creating mischief and disrupting the lives of all those in the immediate area. Also, there are both good gremlins and bad gremlins. The good gremlins attempt to make temporary repairs of the systems that the bad gremlins disrupt. This is one reason the job of diagnosis and ultimate problem resolution is made more challenging.
Unfortunately for us in the aviation business, this mischief may result in a potential life-threatening situation. Gremlins are creatures of habit, that is, they only come out under certain conditions. Exorcising these little varmints in a timely fashion is a feat that generally requires a complex plan of attack and in some cases, voodoo rituals.
Frequently, atmospheric conditions such as rain have an effect on gremlin activity. Sometimes temperature, or possibly pressurization, will play a role in when a gremlin appears.
The first step in gremlin eviction is to determine lifestyle characteristics. The first action is to determine when they come to life. Are they active on the ground, or only in flight? It is essential to identify when the problem will occur. Many aircraft utilize extensive ground/flight sensing systems that can have a dramatic effect on the operation of numerous aircraft systems. This is, however, only part of the consideration of ground flight. One example is a pilot-reported discrepancy where the crew could not use the #2 communication radio at certain locations on their home airport. However, as soon as the aircraft was in the air, the radio operated as it should. After numerous man-hours were invested troubleshooting and most all the components had been replaced, it was determined that with the location of the #2 communication antenna as being under the belly of the aircraft, in certain positions on the airfield, the airframe structure would effectively block the radio transmission.
Another situation that can create system disturbances is the build up of electro-static charge, and this is not just limited to accumulation on the fuselage exterior. Often, glass instruments in the flight compartment are excellent storage devices for static charge. Other anomalies may be the result of operating the aircraft with the landing gear retracted instead of extended. Frequently, advanced aircraft will employ the use of electronic proximity switches to sense the position of various landing gear components. The proper operation of these devices require them to produce an internal high frequency alternating current. Sometimes this can result in a feedback through the aircraft electrical system. De-energizing a landing gear circuit at various positions in its cycle while monitoring the operation of a suspect faulty system may provide direction to a faulty component.
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