Look for the signs
Often, you won’t find any guidance in the Aircraft Operating Manuals or in Aircraft Maintenance Manuals. Instead, the information about handing the units is found in the Component Maintenance Manuals that you may not have available for reference. What if you saw a circle with a diagonal bar with a lightning strike through the bar? What does a triangle with a diagonal bar and a hand with grasping fingers mean? Lastly, what if you saw a segmented circle with three, inward pointing arrows? These are the JEDEC International Symbols for ESDS units. Sometimes, there is a placard that says "STATIC SENSITIVE."
Some avionics and electronic installations have placards that have "CAUTION" or "ATTENTION" placards. There may be specific instructions for handling Electro Static Sensitive Devices. In other cases, the placards may call for the use of a grounding wire or personnel wrist strap connected to a specific point in the airplane. In other avionics installations, it may not be readily apparent that the units are static-sensitive. If you remove and install old tube radios, be aware the rules have changed for newer radios. Only one static discharge can cause damage to components in a static-sensitive avionics/electronic unit or LRU plug-in circuit board. Over time, system functions may change with several static discharges. Of concern is the loss of system functions that may not be readily identified without some functional or diagnostic check.
The body electric
Let’s talk about electrostatic charges that are generated by your body, your hair, clothing, floor coverings, avionics/electronics units and the equipment racks. Any time there is a difference of the potential energy between two substances, there is a potential for an electrostatic discharge. An example would be electrostatic discharges between nylon fabric or human hair into steel or polyethylene. Never place these units in a plastic sack or wrapper. You must use conductive bags. When removing metal-encased units, care should be taken to ground them so that no potential energy difference exists between the airplane structure, the rack, or any associated control panel. Remember, the control panels are wired to the unit.
Never touch the electrical pin with anything. Unprotected electrical connectors can be a direct connection to sensitive internal components. You must use conductive electrical dust caps and connector covers. Conductive dust caps and connector covers are either black or gray in color. On ITT Cannon covers, look for the word "CONDUCTIVE." I recommend that if anti-static dust caps and connector covers are not available that you still use dust covers but spray them with anti-static solution and date them according to the instructions on the solution container. If you are replacing a unit, use the covers that come on the new serviceable unit. Never remove the covers until you have properly grounded the unit to the aircraft structure.
A precaution is to always completely remove electrical power when removing and installing ESDS units. Even some unrelated units may provide power through feedback circuits. Make sure that power is off for the whole system. Eliminate any contamination, including dirt or metal shavings, that might cause a connector pin to short and cause ESDS damage. Grounding the unit can now be effective. The grounding device or wrist strap may have to be checked with an ohmmeter. The wrist strap should have a resistance of less than 10 megohms. The grounding assembly that you use should have a minimum resistance of 250 kilohms and a maximum resistance of 1.5 megohms.
When You’re Hot You’re Hot, When You’re Not You’re Not, But You Still May Spark!
What did we find wrong with this airplane and what repaired it? I have probably asked more questions than I have answered. Those of us who have flown a Piper J-3 Cub know that the engine doesn’t need a starting or electrical system since it generates it own spark in the magnetos and we hand prop the engine to start it. If the wiring in the airplane has deteriorated, there are multiple problems that could present themselves to the pilot. As aircraft age, wiring becomes a more significant part of our inspections.
As we have seen, maintenance practices can also cause problems in the aircraft wiring and systems. Harsh detergents may cause corrosion in the fuselage and wiring and eventually system faults. Poor maintenance practices, like not protecting radios and line replaceable units from static discharge, may also lead to system faults.
We still have many unanswered questions. What should the maintenance record say after all this maintenance activity? Whose name should appear on the maintenance entries? Was the aircraft repaired or was it altered?
No matter what, our objective is to ’Keep ’em Flying."
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