Avionics & Electrostatic Discharge
The growth and change in helicopter aviation over the past 50 years has been nothing short of breathtaking. Helicopters can be found to play an important role in industries as diverse as off-shore oil support to emergency medical transport to airborne law enforcement to executive transport. The systems found on these helicopters have also gone through significant changes, and nowhere is this truer than in the realm of avionics. From vacuum tubes to transistors to integrated circuits, microelectronics and microprocessors, the transformation seems to be continuously moving forward towards smaller and more capable devices. Along the way, the size, weight and power consumption of these devices have decreased and their performance and reliability increased. From an aviation perspective, this is great!
Unfortunately, we never get something for nothing when dealing with these newer systems. When dealing with microelectronics that utilize semiconductor devices that measure a few millionths of an inch in thickness, new forces come into play that the helicopter maintenance professional has not had to deal with in the past. Static electricity, or electrostatic discharge (ESD) as it is more commonly referred to, is a major concern when handling many of the newer avionics systems. Estimates tell us that thousands of unnecessary failures and billions of dollars worth of microelectronics and circuit boards are scrapped annually due to damage from static electricity.
Components that are ESD sensitive are commonly referred to as ESDS parts. These parts are found on a wide variety of avionics systems and on many circuit card assemblies (such as the one shown in the lower left) and are field replaceable.
Electrostatic Discharge (ESD)
ESD is the sudden flow of electricity between two electrically-charged objects caused by contact, an electrical short or dielectric breakdown. A buildup of static electricity can be caused by tribocharging or by electrostatic induction. Examples of tribocharging include walking on a rug, rubbing a plastic comb against dry hair, rubbing a balloon against a sweater, ascending from a fabric car seat, or removing some types of plastic packaging. In all these cases, the friction between two materials results in tribocharging, thus creating a difference of electrical potential that can lead to an ESD event.
Another cause of ESD damage is through electrostatic induction. This occurs when an electrically-charged object is placed near a conductive object isolated from ground. The presence of the charged object creates an electrostatic field that causes electrical charges on the surface of the other object to redistribute. Even though the net electrostatic charge of the object has not changed, it now has regions of excess positive and negative charges. An ESD event may occur when the object comes into contact with a conductive path. For example, charged regions on the surfaces of styrofoam cups or bags can induce potential on nearby ESD-sensitive components via electrostatic induction, and an ESD event may occur if the component is touched with a metallic tool.
Just about all of us have walked across a nylon carpet and generated static electricity. If the relative humidity is low, a charge in excess of 25,000 volts can be generated. We don’t realize that we have done this until we touch something else, like a doorknob. When we do, we experience a static electricity discharge and it will probably generate a spark. Although this discharge is not harmful to us, it would critically damage ESDS parts.
Many ESD events occur without a visible or audible spark. A person carrying a relatively small electric charge may not feel a discharge that is sufficient to damage sensitive electronic components. Some devices may be damaged by discharges as small as 30 volts! These invisible forms of ESD can cause outright device failures, or less obvious forms of degradation that might affect the long-term reliability and performance of electronic devices. The degradation in some devices might not become evident until well into their service lives.
How to Identify ESDS Parts
In order to protect ESDS parts from static discharge, we have to identify those parts that are susceptible to ESD and take appropriate precautions. Personnel who handle and work with these avionic components need to be familiar with the labeling that identifies them as being ESDS parts. Some of the more common labels are shown below.
These labels will not only be found on the parts themselves, but on their shipping containers, storage units and where they are actually installed in the aircraft as well. Since ESDS parts come in many different shapes and sizes, they should not be handled without taking the appropriate precautions to minimize the risk of exposing the part to ESD. From receiving the part to installing it in the aircraft, the ESDS part should be handled with care at all times. ESDS protocol should be used.
Protecting ESDS Parts
Prevention of ESD centers around creating an electrostatic protective area (EPA). An EPA can be a container, a small working station or a large manufacturing area. The main principle of an EPA is that there are no highly charging materials in the vicinity of ESDS electronics, all conductive materials are grounded, workers are grounded, and charge build up on ESDS electronics is prevented. International standards are used to define typical EPA and can be found for example from the International Electrotechnical Commission (IEC) or American National Standards Institute (ANSI).
The name of the game in protecting ESDS parts is to prevent an electrostatic charge from entering the part. This is accomplished in two ways:
1. Electrically neutralizing anything or anyone that will come near or in actual contact with the part.
2. Electrically insulating the part from ESD.
Remember that a charged body does not need to come into actual physical contact with the ESDS part to cause damage. The ESDS part should be kept a minimum of at least one inch from any other part that might have a static charge. Static-generating sources should be kept a minimum of 12 inches from the ESDS part.
ESDS containers are the best way to store and move ESDS parts from one location to another, including out to the aircraft for installation. Antistatic bags are currently available in two colors, pink and dark gray. The pink antistatic bag is used to protect parts from generating a static charge while moving around inside the bag. The gray bag protects parts from electrostatic fields outside of the bag and from static buildup inside the bag from the part moving around. Once the part is safely in the approved antistatic bag/container, it can be handled by personnel who are not grounded.
Personnel who will be handling or installing the ESDS part need to be electrically neutralized also. This is typically accomplished by connecting all personnel and the ESDS part and the mounting unit for the ESDS part to the same electrical ground source. Before handling an ESDS part, the technician must connect a grounding strap to his or her bare skin and ensure that it is firmly in place. A test is then conducted to see that the resistance reading of the technician through the grounding strap is below a particular value. A wrist strap with a flexible wire is most commonly used for this purpose.
Prior to removing any ESDS part from the aircraft, ensure that the mounting unit that the ESDS part is in and the technician’s grounding strap are connected to the aircraft’s common ground point. The removed part should be placed in an approved antistatic bag or container immediately and the antistatic bag/container MUST be sealed/closed. An ESDS label should be affixed to the bag/container.
When installing an ESDS part in the aircraft, the same conditions apply. The ESDS part should be brought to the aircraft in a closed/sealed approved antistatic bag/container. Prior to removing the part from the antistatic bag/container, the technician MUST ensure that their grounding strap and the unit the ESDS part will be installed in are both connected to the aircraft’s common ground point. Upon removing the ESDS part from the antistatic bag/container, it should be seated in its mounting unit immediately. It should not be placed on the floor, aircraft seat or anywhere else.
If the ESDS part is going to be taken to a workstation or shipping area, then the area where it will be looked at should also be an EPA. All the same rules apply for handling the ESDS part as were applicable on the aircraft. Additionally, there should be signs easily seen and tape placed along the floor reminding people that they are about to enter an EPA and need to take appropriate precautions.
Types of Static Damage
Damage from static discharge is usually described as hard and soft failures. A hard failure caused by a static discharge event creates an immediate system failure. A soft failure caused by a static discharge event may only injure the ESDS part and cause unreliable operation and/or shorten the life of the overall system. Sometimes a part that has experienced a soft failure is said to be wounded. These parts will often pass bench and preflight tests and still fail in flight.
In closing, all aircraft maintenance professionals who work with and around ESDS parts need to be aware of what static electricity can do to these parts. Be proactive in helping take the precautions required to eliminate the risk that static electricity poses to them.