THE CHALLENGES FACING U.S. NAVY AIRCRAFT ELECTRICAL WIRING SYSTEMS

Transcription

1 THE CHALLENGES FACING U.S. NAVY AIRCRAFT ELECTRICAL WIRING SYSTEMS Jerome H. Collins Manager, Wiring Systems Branch (AIR ) Naval Air Systems Command Telephone: Page 1 of 11

2 Abstract The United States (U.S.) Navy flies 3,045 aircraft, approximately 921,658 flight hours per year in the harshest environments on the planet. This type of flying can take a toll on any system on the aircraft, but is particularly harsh on the electrical wiring system. Unlike an avionics system, which may be replaced several times during the life of the aircraft, the aircraft electrical wiring is treated as a fit and forget system on the aircraft. Some twenty-year-old aircraft have the same wiring systems they had when new. This has led to problems on U.S. Navy aircraft such as wire chafing damage, circuit breakers that do not function properly, and connectors that are corroded beyond repair, to name a few. The Naval Air Systems Command (NAVAIR) has identified three common causes for electrical wiring system failures on U.S. Navy aircraft: unqualified components being used on aircraft; improper installation of electrical wiring systems; and poor or non-existent maintenance and repair of electrical wiring systems. This paper will discuss the metrics used to identify that a problem exists with the electrical wiring systems on the aging U.S. Navy aircraft fleet, and will demonstrate the solutions that NAVAIR is pursuing in order to address the causes for wiring systems failures. This paper is based upon the research conducted over last three years by the author as well as countless years of lessons learned by others. Some of the topics covered in this paper will include a discussion of NAVAIR s involvement in the development of a High Performance Composite Wire Specification; the importance of the use the specification AS50881 Wiring Aerospace Vehicle during the design and installation of wiring systems; and the proper training of maintenance personnel in inspection, maintenance and repair of aircraft wiring systems. Page 2 of 11

3 Background To study the complexity of the challenge facing the U.S. Navy, with respect to aircraft wiring systems, one has to start with the aircraft itself. The 3,045 aircraft in the U.S. Navy inventory fly approximately 921,658 flight hours per year [1]. Of those aircraft, the average age is 20+years old. The average continues to increase as fewer new aircraft are acquired and the existing aircrafts service lives are extended. With that in mind, the wiring system has been, in general, a fit and forget system. Once installed, the wiring is generally not replaced throughout the life of the aircraft. In U.S. Navy aircraft, this approach has led to problems, since a typical aircraft can have upwards of 95,000 feet of wire. Exposure to severe elements such as wind, moisture, and temperature changes during a typical flight envelope can all shorten the life of the wiring system components. In order to combat this challenge, NAVAIR has assembled a team of skilled engineers, technicians, logisticians, and uniformed service members to research the problems occurring with electrical wiring systems within the U.S. Navy aircraft community and develop solutions to those problems. From a technical standpoint, the Wiring Systems Branch (AIR ) is tasked with the overall responsibility for all research, development, test, evaluation and in-service engineering support for U.S. Navy aircraft electrical wiring systems. This is referred to as cradle to grave responsibility for the electrical wiring system. Wire System Integrity The electrical wiring system on U.S. Navy aircraft is comprised of all the components used to distribute electrical signals and/or power on the aircraft including the wire, connectors, termini, switching devices (relays) and protective devices (circuit breakers). To ensure the integrity of all the components of the wiring system, NAVAIR has identified three focal areas: (1) Wiring Components, (2) Designs and Installations, and (3) Maintenance and Training. All three of these focal areas are critical for wire system integrity. This is depicted in Figure 1 below. NAVAIR Wiring Programs Wiring Components Wire System Integrity Maintenance & Training Designs & Installations Figure 1 NAVAIR Focal Areas for Electrical Wire System Improvement Page 3 of 11

4 Throughout the rest of this paper, these focal areas will be explored in more detail. In each focal area, the challenges facing NAVAIR will be discussed and the present state of NAVAIR s work in these areas will be detailed. However, before that can take place, the data that supports our efforts must be discussed. Metrics In order to understand the magnitude of the challenges facing the electrical wiring systems on U.S. Navy aircraft, NAVAIR collected three forms of information, or metrics. The first was related to safety of operations, the second was related to reliability/maintainability/availability of the aircraft and the last was visual inspections of the aircraft to validate the first two sets of information. Safety Data Safety Data for all U.S. Navy aircraft is maintained by the U.S. Navy Safety Center in Norfolk, Virginia. This data shows the mishaps for each aircraft Type/Model/Series in the U.S. Navy inventory. Mishaps are categorized into one of three classes as shown below [2] : Class A Mishap = The resulting total cost of damages to DoD or non-dod property in an amount of $1 million or more; a DoD aircraft is destroyed; or an injury and/or occupational illness result in a fatality or permanent total disability. Class B Mishap. The resulting total cost of damages to DoD or non-dod property is $200,000 or more, but less than $1 million. An injury and/or occupational illness result in permanent partial disability or when three or more personnel are hospitalized for inpatient care (beyond observation) as a result of a single mishap. Class C Mishap. The resulting total cost of damages to DoD or non-dod property is $20,000 or more, but less than $200,000; a nonfatal injury that causes any loss of time from work beyond the day or shift on which it occurred; or a nonfatal occupational illness that causes loss of time from work or disability at any time. During a 7-year period from 1995 to 2002, there were a total of 31 mishaps (6-Class A, 3-Class B, 22- Class C) [3] attributed to aircraft electrical wire system failures. In comparison to the other seven propulsion and power systems on the aircraft, electrical wiring systems ranked third highest overall (1 st system 130 mishaps, 2 nd system 60 mishaps, lowest system 4 mishaps). Readiness/Reliability/Maintainability The Propulsion and Power Department (AIR-4.4) of NAVAIR maintains readiness, reliability and maintainability metrics for U.S. Navy aircraft, with respect to aircraft wiring, on a quarterly basis (rolling 12-month average). Each of these metrics will be discussed now. Readiness The three types of readiness that will be discussed here are Mean Flight Hours Between Aborts, Non- Mission Capable aircraft and Partial Mission Capable aircraft. The definition for each of these is shown below [4] : Mean Flight Hours Between Aborts (MFHBA) Total flight hours divided by the sum of maintenance events initiated from pre-flight and in-flight aborts. Page 4 of 11

5 Non-Mission Capable (NMC) aircraft The amount of non-mission capable time is divided by the amount of time in a year, which shows the equivalent number of aircraft that were non-mission capable the entire year. Partial Mission Capable (PMC) aircraft - The amount of partial mission capable time is divided by the amount of time in a year, which shows the equivalent number of aircraft that were partial mission capable the entire year. During the most recent quarter of data (rolling 12-month average), 3 rd quarter of fiscal year 2005, the U.S. Navy had approximately 27,365 Flight Hours Between Aborts [1] due to wiring discrepancies. There were 78 Non-Mission Capable aircraft due to wiring deficiencies [1]. This is approximately 2.6% of the total 3,045 aircraft in the inventory. Furthermore, during this same time period, there were approximately 51 aircraft that were Partial Mission Capable, which is 1.7% of the inventory [1]. Reliability The type of reliability data discussed here will be Mean Flight Hours Between Maintenance Events (MFHBME) due to wiring system discrepancies. This is calculated by taking the total flight hours divided by the number of maintenance events [4]. During the 12-month rolling average period of 3 rd quarter of fiscal year 2005, the MFHBME for all aircraft in the inventory was 637 flight hours [1]. Considering that the total number of flight hours for all U.S. Navy aircraft is 921,658, this equates to 1,446 maintenance events per year due to wiring systems or 4 maintenance events per day. Maintainability The type of maintainability data discussed here will be Maintenance Man-hours per Flight Hours (MMH/FH) spent working on wiring system discrepancies [4]. During the 12-month rolling average period of 3 rd quarter of fiscal year 2005, the MMH/FH was 7.3 hours spent working on wiring system discrepancies for every flight hour. Visual Inspections The final form of information gathered to validate that a problem exists with the wiring systems on U.S. Navy aircraft is visual inspections. NAVAIR has a team of skilled assessors that perform Material Condition Assessments (MCA) on aircraft wiring systems. These assessors apply government and industry accepted practices [5][6][7][8] for installing and maintaining wiring systems when inspecting the condition of wiring systems on aircraft. The results of those inspections support the quantitative data found above. In general, the condition of the wire systems on operational aircraft is poor and the installation practices on new aircraft should be monitored for compliance with the government and industry accepted practices. Shown below are just samplings of some of the problems that have been found on U.S. Navy aircraft. Page 5 of 11

6 Material Condition Assessments Unused connectors need to be environmentally sealed and clamped/stowed. Preferably to structure. Bagging connectors promotes moisture. Connector highly corroded. Caps can become FOD, however the Connectors are properly stowed. Figure 2 Material Condition Assessment showing wire system condition Operational Aircraft Material Condition Assessments Window splices do not environmentally protect the splice from corrosion. When Troubleshooting a system and all else fails, it may be a window splice. Figure 3 Material Condition Assessment showing wire system condition Operational Aircraft Page 6 of 11

7 Material Condition Assessments Unconstrained Wire Laying on Hot Duct Figure 4 Material Condition Assessment showing wire system condition Production Aircraft Material Condition Assessments Chafing on Sharp Edge Figure 5 Material Condition Assessment showing wire system condition Production Aircraft Page 7 of 11

8 Focal Areas The challenge of improving the wiring systems on U.S. Navy aircraft requires a multifaceted approach. With this in mind, the Wiring Systems Branch (AIR ) within NAVAIR has grouped all of its efforts into three focal areas: Wiring Components, Designs/Installations, and Maintenance/Training. We will now explore each aspect of these focal areas. Wiring Components The beginning of every good wiring system begins with good components. However, before the components, such as a connector, can be developed, it must first have a specification written to define the requirements of that component. AIR is actively involved with the development of specifications for new wire system components as well as the upkeep of existing specifications. An instance where AIR is involved in the development of a new specification would be with the writing of the new High Performance composite wire specification; a follow-on to AS22759/80-92 [9]. Both the U.S. Air Force and the U.S. Navy requested that the Society of Automotive/Aerospace Engineers (SAE) develop a new High Performance composite wire with AS22759 as its basis [10][11]. The request stated that improvements to the specification should be made in the following areas: 1. Require a smooth and edge-free outer polytetrafluoroethyiene (PTFE) layer with complete bonding between homogenous layers and no visible tape ridges that can contribute to tearing of the outer insulation layer. 2. Increase the performance level required to Pass Wet Arc Propagation test. 3. Increase test duration on Forced Hydrolysis test to 5000 hours. 4. Add a lamination sealing requirement per AS4373 Method 809 at 260 C. 5. Add an outer layer durability test. A possible procedure for this test is EN3475 Test 503 or method outlined in Defense Supply Center-Columbus (DSCC) composite wire Drawing paragraph Add a Stripability Test, consider test method in DSCC composite wire Drawing 04Ml paragraph Add a State of Sinter Test, consider test method in DSCC composite wire Drawing paragraph SAE has taken on this work through a subcommittee, which both industry and government representatives are collaborating to develop the specification. AIR is hopeful that through this work, the restriction that currently exists to not use composite wire on naval air vehicles, can be removed. AIR is also involved in the upkeep of the existing wire systems component specifications. Since a majority of the wire system components specifications are SAE specifications, AIR works closely with the SAE AE-8 Aerospace Electrical/Electronic Distribution Systems Committee to ensure the specifications are kept current. Furthermore, AIR has an active qualification and retention program to qualify components to those specifications. This qualification and retention testing is done either by the manufacturer of the component or at the AIR test laboratories. Test procedures and results are reviewed by AIR and the component is either accepted onto the Qualified Products List (QPL) or rejected [12]. Page 8 of 11

9 Designs and Installations The next focal area is the design of wire systems for U.S. Navy aircraft and the installation of those wire systems in the aircraft. AIR is actively involved in the design of wiring systems used on U.S. Navy aircraft. Applying industry and government accepted principles [5][6][7][8] to the designs has proven successful in improving the longevity of the wiring system. However, being involved in the design aspect is not enough. It is also important to perform periodic assessments of the installation of the wiring system in the aircraft. AIR performs 30/60/90% assessments of the installation of wire systems into the aircraft to confirm build is to design. This has proven successful in locating problems before an operational failure occurs. Figure 6 shows an example of this. In this picture, the lugs of this fuel probe do not have captive fasteners, so the screws can fall off the lugs and become debris in the fuel cell. The design had these as captive fasteners, but this did not get into the final installation. Design vs. Build Fuel Probe harness connections do not have captive fasteners. Item #3 View Looking Down Figure 6 Example of Design versus Build Challenges Maintenance and Training From the Material Condition Assessments mentioned above, it has been determined that improvements need to be made in the maintenance practices for wiring systems on the U.S. Navy aircraft. There are several aspects to these improvements. First, the proper training must be provided to the individuals doing the maintenance. Second, proper maintenance procedures need to be documented for the maintainers. Lastly, the maintainers need the proper tools in order to perform their tasks properly. AIR observed a need for on-the-job training for maintainers. To that avail, a training program was developed and implemented whereby assessors from AIR go to the aircraft squadrons, assess the condition of the wiring on the aircraft, and then provide training to the maintainers on how to properly inspect and maintain the wiring on their aircraft. Page 9 of 11

10 AIR has also led the effort to update the Joint Service Installation and Repair Practices for Aircraft Electric and Electronic Wiring Manual [8]. This manual is used by maintainers to repair the wiring on the aircraft. It started as a manual with twenty-six volumes and is now only four volumes after all the redundancies and outdated information was removed. Lastly, AIR has performed the research, development, test and evaluation of new methods to troubleshoot wiring on the aircraft. To this avail, new tools such as handheld fault location tools have been evaluated and are in the process of being approved for use by the maintainers. These tools, when approved, will provide the wiring maintainer with another tool in his or her toolkit for troubleshooting failures and thereby reduce maintenance man-hours and increase readiness of the aircraft. Summary In summary, improving wiring systems on U.S. Navy aircraft is a threefold approach. The Naval Air Systems Command must ensure that qualified components are used in good designs and installations, and maintainers are trained properly, and have the right tools to keep the wiring systems functional. Page 10 of 11

11 References 1. Naval Air Systems Command Propulsion and Power Department. Aircraft Electrical Wiring Metrics dated June 30, 2005 (rolling 12-month average). Patuxent River, Maryland. 2. Navy Safety Center Aviation Directorate. OPNAV Instruction D and Marine Corps Order P5102.1B dated January 7, Norfolk, Virginia. 3. Naval Air Systems Command Propulsion and Power Department. PMB Safety Metric on Safety Mishaps dated July 31, Patuxent River, Maryland. 4. Naval Air Systems Command Propulsion and Power Department. PMB Metrics Operating Guide dated May 28, Patuxent River, Maryland. 5. Society of Automotive/Aerospace Engineers. AS50881B Wiring Aerospace Vehicle dated August Warrendale, Pennsylvania. 6. Society of Automotive/Aerospace Engineers. AS8700 General Specification for Installation and Test of Electronic Equipment in Aircraft dated July Warrendale, Pennsylvania. 7. Naval Air Systems Command. MIL-E-7080B Selection and Installation of Electric Equipment dated May 31, Patuxent River, Maryland. 8. Naval Air Systems Command. NAVAIR 01-1A Installation and Repair Practices for Aircraft Electric and Electronic Wiring. Patuxent River, Maryland. 9. Society of Automotive/Aerospace Engineers. AS22759 Wire, Electrical, Polytetrafluoroethylene/Polymide Insulated, Light Weight, Tin Coated, Copper Conductor, 150 C, 600 Volts. Warrendale, Pennsylvania. 10. Department of the Air Force Memorandum for SAE AE-8D Request for New SAE AE-8D Task Group Formation dated January 11, Wright-Patterson Air Force Base, Ohio. 11. Department of the Navy Official Letter Request for New Task Group Formation dated January 11, Patuxent River, Maryland. 12. Naval Air Systems Command-Propulsion and Power Department. Aircraft Wiring Qualified Products Laboratories: Description of Projects and Processes (Draft) dated January 3, Patuxent River, Maryland. Page 11 of 11