The Importance Of Being Grounded

Lightning strike during thunderstorm - being grounded
Image Source – Lightning strike during thunderstorm  

The importance of being grounded has recently been highlighted by Transport Canada and the FAA. The latest Feedback for Canadian Aviation Service Difficulty Reports, highlighted the importance of correct assembly of electrical terminals. An occurrence of a melted wire on the starter connector of aircraft APU resulted.

This last month the FAA also issued an AD ordering Boeing to fix some newly manufactured 737 electrical bonding issues.

The importance of proper grounding and bonding cannot be overstated. That said, one of the more common maintenance errors that are discovered when inspecting aircraft wiring is the improper assembly of bonding and grounding points.

Bonding and grounding requirements are; to ensure that an adequate low resistance return path for electric, avionic, communication and electronic equipment is achieved. This can withstand operating conditions and corrosion. This is essential for the reduction of coupling of electromagnetic fields into or out of the equipment. Additionally, for providing electrical stability to control the currents and/or voltages caused by static charges and discharges and for suppressing the hazardous effects thereof.

SAE document ARP1870 provides for detailed information on grounding, bonding and the application of related hardware, primarily for system design. AC43-13 Chapter 11 Section 15 provides similar general information for the AME, aircraft manufacturers standard practices provide airframe specific instructions.

Some of the more common mistakes are lack of preparation of the mating surfaces, terminals installed on top of lock nuts, missing lock washers, too many terminals on a single stud.

Correct assembly of ground stud

Correct assembly of ground stud
Image from SAE International AEROSPACE RECOMMENDED PRACTICE Manual ARP1870

These errors may not be immediately apparent as all systems can appear to be working correctly. The deleterious effects result in inflight fires and critical system failures. More commonly, intermittent glitches in electronic equipment. These can take tens of thousands on dollars and hundreds on manhours to track down.

The bottom line is to be aware of the airframes standard practices. For aftermarket approvals and equipment manufacturers installation manuals, refer to AC43-13 as directed. Do it right first time and prevent future problems.