Because safety comes first at Delta Air Lines, any indication of abnormal system performance in the cockpit is investigated. The anomaly can be as subtle as a momentary error message on a monitor or as obvious as a landing five feet left of the runway center stripe. Whatever the indication, maintenance personnel remove the suspect avionics units and return them to the central repair facility in Atlanta.
In a typical week, Delta runs 2,000 avionics units through its repair facility. In the majority of cases, technicians identify the problem and the unit is repaired, returned to the manufacturer or replaced. Sometimes, however, a unit passes all the tests of the manufacturer’s specifications, in which case it is labeled No Trouble Found (NTF). An NTF can mean the trouble lies elsewhere, but it also can be a symptom of system degradation due to static electricity.
The circuit-card assemblies used in avionics units are highly susceptible to static damage. Some of the more sophisticated assemblies are sensitive to levels of static electricity as low as 30 V to 50 V. On older aircraft such as the Boeing 727 and 737, components in the avionics units are vulnerable to static damage, although not at such sensitive levels.
Electrostatic damage can range from catastrophic to slight. Catastrophic failure is the literal meltdown of sensitive components that renders a unit inoperable. The middle range is latent damage that causes a unit to break down, perhaps not immediately but often prematurely.
At the opposite extreme is the degradation that causes parametric changes in component operation manifested as temporary or intermittent failures. The occasional hiccups caused by this slight weakness may sometimes explain why a unit indicates a fault in the cockpit but tests fine in the repair shop.
Avoiding static damage is essential for avionics units, each valued from $500 to more than $100,000. To ensure that passenger safety is never compromised, all critical systems have built-in redundancy, some three and four deep. No flight is dispatched unless all required systems operate flawlessly.
When a unit must be removed for repair, the overhead alone averages $650 to $750 per incident. That includes shipping the unit to and from Atlanta (and perhaps back to the manufacturer) and handling the paperwork. Removal of the units also means that spares must be on hand, which necessitates a substantial inventory.
When testing of the removed unit results in an NTF, technicians must investigate potential explanations for the erratic behavior. Because some NTFs are inevitable, a unit with a first-time occurrence—after passing rigorous inspection and testing—may be placed back in inventory and on aircraft, often to live out its expected usefulness. But when the same unit comes in a second or third time and still tests to specifications, technicians must dig deeper to identify the problem.
By reducing removals and NTFs, Delta not only could enjoy savings in shipping and inventory, but also reduce line-maintenance activities and repair-shop workloads. In an operation like Delta’s, which has 200 personnel in six avionics shops, these reductions could significantly lower the total cost of parts ownership.
Operating in a Gray Area
In early 1995, static control in Delta’s avionics repair operation was much the same as it had been since the late 1970s and early 1980s. Floor surfaces were covered with waxed, static-protective flooring in some places and mats in others.
Technicians wore wrist straps and worked at benches covered with static-protective table mats. They understood they must be grounded while working on static-sensitive units. But internal practices were limited in procedures and requirements designed to reduce equipment exposure to electrostatic discharge.
This approach created confusion. For example, mechanics who removed avionics units from the aircraft knew they were static-sensitive but were not thoroughly aware of the ESD phenomenon and the science behind it.
Stores inspectors, aware they were not supposed to remove static-sensitive units from their special packaging, sometimes could not read the hand-written paperwork inside so they could not do their jobs properly. Technicians performing the repair often received conflicting messages from OEMs and other vendors about how to guard against static damage while working on different units. For example, one said to wear a static-control smock while another said to use ionized air.
It was a question about ionized air that led to Delta’s investigation into improved static control. Does Delta need an ionized air gun to blow dust off a circuit card? The search for an answer led to more questions:
Is Delta doing enough to control static?
What are other companies doing?
What standards govern static-control programs?
A check with an engineering resource center at a major university provided no definitive answers. Then Delta contacted Steven Moreno, a sales representative with Thomco Specialty Products in Atlanta. He arranged for presentations by Cal Warriner, a 3M static analyst and static-control engineer certified by the National Association of Radio and Telecommunications Engineers. Delta invited two local FAA inspectors to participate in these exchanges.
Delta was aware of the limitations of current technology to prove a direct cause and effect between ESD and equipment malfunction. With this in mind, the avionics departments looked hypercritically at the prospect of buying new equipment.
Certainly, technological advances in static control had occurred. But which of those, if any, were absolutely necessary and which were simply nice to have?
Continuous Monitoring
An analysis of Delta’s operations revealed different static-control needs at different stages; that is, the first stage of removing units from the aircraft to the last stage of actually repairing the units. In particular, Delta learned that at the last, most intensive stage, something must be done to ensure that technicians were grounded at all times.
Some companies do periodic wrist strap testing. At appointed times during the day, workers test their wrist straps to ensure they are functioning. But this method has serious drawbacks. A wrist strap might fail five minutes after testing and the worker would not discover it until the next check hours later. Or a wrist strap might fail intermittently because of overstretching or dryness of the skin which periodic testing cannot detect. Moreover, the regimen of stopping work and conducting a test three or four times a day lessens productivity.
The alternative is continuous monitoring. With this method, the worker’s wrist-strap ground cord is plugged into a jack wired to a monitoring device. If ground is lost, the monitor immediately flashes a red light and sounds an alarm. The worker knows the instant it happens and can take steps to remedy it. Immediate detection helps prevent static damage, avoiding unit failure and rework time. In addition, workers are freed from periodic testing.
For Delta, the choice was continuous monitoring. The avionics department selected the 3Mä 720 Workstation Monitor. The purchasing department included it with other product requests in 3M’s Integrated Solutions Program.
Thomco developed a kit containing a monitor, a wrist strap and mat. Delta shop managers and supervisors were informed of the availability of the kit and given the discretion of ordering as many as they needed. To date, not only has Delta ordered these kits for the Atlanta repair shops but also for other areas, including inspection and stores facilities in Dallas and Tampa.
Training Program
By enrolling in 3M’s Preferred Customer Program, Delta was able to take advantage of various services including static-control audits, general consultation and employee training. As shops began ordering kits, Delta set up three levels of training:
Level A—Mechanics who remove avionics units from aircraft and personnel with minor involvement watched a Boeing-produced video on static-control basics. The video stressed the importance of covering all connectors with static-protective caps.
Level B—Stores inspectors, engineers and engine-shop personnel attended a one-hour class taught by Delta personnel and the Thomco representative. This class focused on safeguards in handling static-sensitive units, such as using static-shielding bags for transporting circuit boards.
Level C—Repair technicians attended a three-hour class taught by 3M that covered everything from the proper use of wrist straps to what to do if the monitor sounded an alarm. Technicians participated in this class after using the newly installed monitors for two or three months, which gave them an opportunity to bring technical questions based on actual experience.
The training reviewed static-control principles so workers would understand not only what to do but why. One popular presentation was the use of a charge plate analyzer to measure the voltage generated by various body movements such as handing an object from one person to another. After seeing that this simple transfer can cause static charges as high as 5,000 V to 20,000 V and more, workers were enthusiastic about static control.
As a final step, Delta worked with 3M to develop 30 pages of static-control guidelines for the company operating-procedures manual. The procedures included in-house auditing to ensure compliance by employees. As part of the development process, Delta sent review copies to the local FAA inspection team to keep them informed.
Reaping the Benefits
After implementing the updated static-control program, Delta observed some significant results. The number of NTFs dropped by 44% over a 15-month period. NTFs, line-maintenance activities and repair-shop workloads have declined, producing savings in labor, overhead and inventory.
Second, the updated static-control program has resolved much of the confusion related to compliance with manufacturers’ specifications for maintenance of avionics units. As a result of the training and written procedures, Delta consolidated all vendors’ requirements into one coherent, manageable program. If a question arises with a manufacturer, Delta refers it to 3M’s static analyst for resolution.
Third, workers in nonelectronics areas such as shipping and stores have improved their job performance through better understanding of static control. In particular, stores inspectors now know when to reject avionics units that are improperly packaged. They also know how to safely remove a unit from packaging when necessary to read a hand-written inventory label.
Overall, the improved static-control program has minimized the occurrence of NTF, enhanced the quality of the repaired product and contributed to fleet reliability.
About the Authors
Martin Story, a component group leader, manages three of Delta’s avionics shops. He joined Delta in 1978 as an aircraft avionics mechanic, gained experience as a technical analyst in avionics and was promoted to his current position in 1995. (404) 714-4203.
Paul Sipe is a technical analyst who has worked for Delta Air Lines for eight years, starting in line maintenance and the avionics shop. He held similar positions in the U.S. Air Force, where he earned an associates degree in avionics technology and later worked with the Air National Guard. (404) 714-3360.
Delta Air Lines, Technical Operations Center, Dept. 334ATL, Hartsfield International Airport, Atlanta, GA 30320.
Copyright 1997 Nelson Publishing Inc.
May 1997
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