In the U.S. Navy, where operational readiness is directly tied to the mission capability of highly complex aircraft, preventive maintenance is the difference between mission success or mission failure.
For more than seven years, the Naval Air Systems Command (NAVAIR) Wiring Systems Branch of the Propulsion and Power Engineering Department has been working to address wiring system problem areas. The training has been compiled from in-service aircraft evaluations to address relative issues that lead to wiring discrepancies.
The Wiring Awareness Inspection Techniques (WAIT) training team travels around the globe to train sailors and marines. The training exposes them to the correct processes for proper inspection techniques.
"After attending the WAIT class and observing the fleet maintainer’s reaction to the experience, I believe that this an excellent tool to maintain or even increase all in-service Seahawk RFT and safety numbers," says Christopher Rowe of the H-60 Helicopter Program (PMA-299).
"The information being passed between the fleet maintainers and the wiring assessors will allow for better and more consistent wiring inspection techniques and standards from squadron to squadron. The information will also allow PMA-299 to identify areas to improve both maintenance plan and design."
Identifying Wiring Issues
Wiring-related problems in the Navy’s aircraft attribute to an average of 1.1 Class A, 1 Class B, and 1.6 Class C mishaps per year. There have been a total of 37 mishaps in the last 10 years related to aircraft wiring, according to propulsion system safety metrics monitored from Oct. 1, 1998 to March 30, 2008.
The training class is designed to teach how to better identify and correct wiring discrepancies before they have an impact on safety of flight and/or aircraft wiring systems. The problem most often is due to chafed wire conditions. The cause of the chafing can be attributed to any number of things, not excluding maintenance itself. Awareness of the causes of chafed wiring is a key in preventing those conditions. The hands-on class is designed for all maintenance ratings.
"Everyone is an inspector," says Dave Quinzani, WAIT training team lead. "This training attempts to instill a common-sense approach to wiring inspection. For example, it’s no surprise that the best tools are a flashlight, mirror and awareness of system problem areas."
"Most training is done on the job," adds Quinzani, a former aviation electrician’s mate chief. "These skills are typically mentored from one maintainer to the next, making this on-the-job training very useful throughout the Navy and Marine Corps."
The WAIT class is effective, he continues, because of a hands-on approach. The team wants to see the maintainers interact with the aircraft. The training heightens their awareness to wiring problem areas.
"You can only get so much from reading a book, or looking at slides," Quinzani says. "The hands-on experience brings the text and lessons to life."
Curriculum
The WAIT training is a three-day evolution. On the first day, instructors and the command’s collateral duty inspectors/quality assurance representatives (CDIs/QARs) assess the installation and condition of the wiring on squadron aircraft. They look for common trends such as chafing and corrosion, photograph problem areas and incorporate this information into a brief given on the next day.
Day two is a half-day classroom briefing where instructors teach the problems, causes and remedies of common wiring issues. Part of that time is spent focusing on the previous day’s assessment of the squadron’s own aircraft. For the rest of the day, the class heads out to the hangar where the assessors and maintainers get their hands dirty practicing the new inspection techniques.
The third day combines the training of the first two, to address issues identified during the assessment. Some discrepancies take just a handful of hours to correct; the others will require an investment of several weeks. Either way, the NAVAIR team is confident that passing along more than a few tried-and-true techniques will pay dividends down the line for the unit/platform and the Navy.
Perspective
"The class gave me a pretty good perspective on how to respect wire in aircraft," said AT3 Lawrence Callier of HSC-28 in NAS Norfolk. "The most important lesson I learned was that ‘problem’ wire could be anywhere. Something could look fine, like nothing is wrong with it, and it could be one of the biggest problems you could have." Chafed wire is predominantly caused by improper routing and clamping. When these conditions exist, the insulation on the wire wears away and exposes the raw wire. The results can be a simple short which causes an instrument to no longer function or in most severe cases a fire which can lead to a class A mishap.
"This [WAIT training] class has opened my eyes to things, that even as a QAR, I’ve overlooked just because I didn’t see the importance of it, and nobody had trained me the right way to say ‘it should be this way vice that,’" says AT1 Christopher Chambers, of HSC-28 at NAS Norfolk.
"For the junior guys, this is great for them to see how to inspect aircraft wire properly."
Failure Modes in Aircraft Wiring
The following information is taken from Lectromec’s www.wirefacts.com.
For those larger aircraft flying today, more than 200 miles of wire flies inside, controlling everything from engine speed to the intensity of the lights in the bathrooms. With as little as 0.002 inches of insulating material between the conductor and the environment, wire failure can be very important. Knowing failure modes and how to properly maintain wire is important. The following lists a selection of failure modes, both frequent and infrequent, with wires.
Aging and Stress Failure: Two forms of aging influence insulation failure: repeated accumulation of damage from excessive intermittent stress and progressive deterioration of the strength property of the insulation.
Bending: Most accepted best practice manuals suggest that all attempts should be made to ensure that the bend radius of a wire is at least 10 times greater than its diameter. Tighter bend radii will place additional strain on the wire, which may present itself as cracks in the surface insulation.
Chafing: Common chafing can result from: loose supports and restraints; unrestrained cross over bundles; loosely bundled conductors with different insulation surfaces; P clamp with excessive pressure or guard missing; loose cable rubbing over a stanchion, frame, edge of lightening hole, etc.; peculiarities of installation; bundles installed in fore and aft direction dragged during acceleration/deceleration and from elongation/contraction of airframe due to pressure changes; and cross-hull direction of vibration resonance with wire; cable touching relatively vibrating or stationary stanchions, etc.; and from mixing wiring types.
Clamps, Ties and Lacing: Wire can be so tightly compressed that an indentation of the insulation can form at the entrance that leads to failure of the insulation. Similar failures can occur when plastic ties that have too sharp an edge or are excessively tightened. Lacing on bundles can develop a similar mode of insulation failure if too much tension is applied in the lacing process.
Delamination: When the bonding material fails to hold together the successive wraps of insulation, this is known as delamination. Wrapping of high performance materials is a means to significantly increase the thermal and mechanical performance of wire insulation.... Failure of the bonding material can lead to reduced abrasion resistance or the ingress of contaminates, such as fluids, which may reach the conductor, leading to electrical shorting or corrosion.
Insulation Cracks & Splicing: Much of the wire in an aircraft is trained from support point-to-support point with moderate curvature. This mode of insulation failure can take several forms, including use of non-environmental splices with water, dirt, and other fluids forming a conducting path into the splice, incorrect crimping, and using incorrect size splice.
Open Wire Failure: This is when there is a break of the conductor of the wire and electrical energy is no longer able to be transferred from one end of a wire to the other. Some insulations will actually maintain their integrity longer than the conductor from repeated stresses (such as bending).
Parallel Arcing: In a parallel arc, the current goes from one wire directly to the structure (ground) or a second wire that is at a different voltage. The current in the arc does not go through the ‘complete’ circuit and therefore is not limited by the load; the arc is in "parallel" with the load. Only the resistance in the arc, the resistance of wire and the internal impedance of the power source limit the current in the arc.
Series Arcing: Series arcing starts with a poor connection of the conductors due to a loose terminal or crimp, corrosion, or dirt in a connection. This leads to the connection between the current-carrying wire and terminal being made and broken repeatedly, sometimes many times a second.
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