Ali Crom
2nd Apr 2001, 22:33
Here's one I stumbled across for all you types who play with that electric string stuff .
By Michael A. Dornheim/Aviation Week & Space Technology
30-Mar-2001 5:41 PM U.S. EST
Recent reports on aging aircraft wiring are providing clues on what maintenance organizations can expect in new rules and procedures covering this issue.
Fleet inspection has shown that aircraft wire inevitably has breaches in the insulation even when it is relatively new. Small arcs can carbonize the insulation, particularly Kapton, leading to bigger arcs and the potential for fire.
The increased interest in the four years since the TWA Flight 800 accident has advanced the state of the art and led to calls for wiring to be considered a system in itself, instead of an afterthought used to connect other systems. New equipment is being devised, such as circuit breakers sensitive to brief arcs.
The broad-based group investigating wiring for the FAA, the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC), has recently finished reports on the five tasks it started in January 1999, said Kent V. Hollinger, the ATSRAC chairman. The reports are being submitted to Thomas E. McSweeny, FAA associate administrator for regulation and certification, and will help the agency formulate policy.
The ATSRAC comprises a wide range of manufacturers, airworthiness authorities, airlines, the U.S. Defense Dept., NASA and labor unions. The effort is voluntary; there is no contract from the FAA. Even though the committee's title says "systems" and not "wiring," in fact it has been primarily focused on wiring.
The five tasks are:
- Inspect the aging fleet. The committee chose to look at transports that were beyond the design lifetime. They inspected 81 in-service aircraft while they were in the shop, and examined six retired aircraft more intensely, including removing wiring samples for laboratory analysis. The lab work showed there were gaps in the insulation occurring at rates varying from 0.44-4.17 breaches per 1,000 ft. (AW&ST Oct. 23, 2000, p. 27). For a large transport with roughly 500,000 ft. of wiring, that suggests there are 220-2,085 breaches on board. Hollinger cautioned that "breach" was broadly defined; for example, 14% were unused wires with clipped ends and 9% couldn't be found in detailed inspection, but 73% seemed legitimate.
Power feeder cables had the worst rate, at 7.3 breaches per 1,000 ft. in pressurized areas and 6.0/1,000 ft. in unpressurized areas. The next worst type was complex harnesses in unpressurized areas at 3.4/1,000 ft. Aromatic polyimide (Kapton) insulation had the highest breach rate at 2.2/1,000 ft., followed by aliphatic polyimide (Poly-X) at 1.7/1,000 ft. and PVC/ glass/nylon at 1.5/1,000 ft.
Problems uncovered in the inspection have resulted in service bulletins and letters. For example, a power cable had signs of arcing due to routing that was not per the manufacturer's drawing. "Airlines might not realize the importance of routing," Hollinger said. Bundled parallel wires were found to be okay, even with different insulation types, but crossing wire bundles were not.
Based on the laboratory exam, the ATSRAC urged the FAA to research better nondestructive test techniques for wiring. The committee also recommended development of "smart" circuit breakers that detect brief arcing pulses, revising advisory circular AC43.13-1B for splicing techniques that can withstand the local environment, and having manufacturers review their designs for drip shields that keep liquids off wires.
- Review wiring service history, such as service bulletins, service letters and airworthiness directives. "We looked at service bulletins that were voluntary in the past to see if maybe some should be mandatory," Hollinger said. "We looked at repetitive-inspection airworthiness directives. The FAA has changed philosophy due to human factors work showing how inspections can be flawed, and prefers terminating action if available."
The ATSRAC reviewed 79 airworthiness directives and recommended that eight be changed to require terminating action. The committee also examined 714 service documents and passed 51 to manufacturers for further investigation. Boeing increased 27 to "alert" status and Airbus increased three.
- Improve inspection criteria and revise the process that develops them. The ATSRAC devised an "enhanced zonal analysis program" that provides the basis for an "enhanced zonal inspection program." A zonal inspection involves looking at everything in an area, such as a wheel well, but not in detail. What to look for regarding wiring has now been clarified and includes checking for contaminants and flammable materials on wires, separation of independent critical systems, and condition of high-voltage cables. The recommendations have been accepted by the Air Transport Assn.'s Maintenance Steering Group 3, which defines the interval and details of inspections.
- Review standard practices for repairing or modifying wiring. The panel has already recommended how to format wiring manuals to make them easier to use.
- Review training programs at airlines and repair stations to address aging wiring. The report to the FAA includes an eight-module lesson plan.
"It's up to the FAA what to do with the reports," Hollinger said. "They can ignore them, or action can either be internal or via the public comment process.
"We found that many recommendations aren't age-related, but affect wiring from when it is new." For example, minimum bend radius, adding drip shields, keeping metal shavings off wires and separating certain wires are not age-related.
On Mar. 13, the FAA presented a draft of a two-year plan for ATSRAC to implement its recommendations. Four new tasks have been proposed:
-Reformulate FAR Part 25 to place scattered references to wiring into a section of their own, emphasizing the view of wiring as a system. New requirements that consider the aging of wiring may be added.
-Develop a standard format and content for maintenance practices.
-Help the FAA develop regulations and advice for training programs. "There may be changes to FAR 121 [covering air carriers] and 145 [covering repair stations] to require wire training programs," Hollinger said. Advisory circulars would give a means of compliance.
-Devise enhanced maintenance practices. There may be rulemaking and advisory material to incorporate ATSRAC's recommendations, such as the enhanced zonal analysis program. Earlier, the FAA wrote a special FAR requiring all operators to have a fuel system safety program. "There may be a similar SFAR for wiring," Hollinger said.
THE U.S. NAVY is dealing with an increasingly old fleet of 4,000 aircraft that now has an average age of 17.2 years. The service estimates it removes about 5,000 power wires per year, and in 1996 there were 1,274 aircraft problems that required removal of power wire. About 5-9% of the 20 million man-hours spent on maintenance each year is finding and fixing wiring, and that's just at the field level. At least three aircraft types in the past two years have had rewiring programs.
The Navy started an Aging Aircraft Integrated Product Team in March 1999, and it has a wiring group based at the Naval Air Systems Command's Power Systems Div. at NAS Patuxent River, Md. The purpose is to serve as an information clearinghouse, provide solutions to aircraft programs, and develop risk and cost evaluation tools. The group works with the Aircraft Wiring and Inert Gas Generator Working Group (AWIGG), which shares information about wiring and fire suppression among the Defense Dept., NASA and other agencies. AWIGG is headed by Richard Healing, director of Navy safety and survivability.
THE WIRING GROUP has programs in three areas: wire diagnostics, "smart wire" and arc fault circuit breakers. The wire diagnostics effort is surveying existing test sets by applying them to a crashed F/A-18 airframe with inserted wiring faults. Seven sets have been evaluated and the results are being used to generate Navy requirements. Funding is now being sought for further evaluation by Navy depots starting by June, and to repackage and enhance one device so it doesn't require an expert operator, said Pall Arnason, project engineer for wire diagnostics and smart wire.
The Navy's $750,000 contract for smart wire was awarded last Aug. 30 to Management Sciences Inc. and the Center of Excellence for Smart Sensors at Utah State University. The idea is to have a self-monitoring wire system that can check itself before flight, and ultimately with live wires in flight, said Cynthia Furse, director of the Center of Excellence.
The Navy estimates it could save $65 million per year if it could detect 75% of open- and short-circuit faults and locate them to ±3 cm. (±1.2 in.), said Sean Field, Navy project engineer for power science and technology.
The concept uses "smart connectors," which have tiny reflectometers that detect and locate flaws in the wire between connectors. The reflectometer sends a signal down the wire, and changes in impedance cause reflection. In the extreme cases, an open circuit causes a full-strength in-phase reflection, and a short circuit causes a full-strength out-of-phase reflection. Subtler flaws cause less reflection and intermediate phase shift.
Furse is using a frequency domain technique that steps through frequencies and processes a mix of the received and transmitted signals to locate faults with 1.2-in. accuracy in cables that are 33-50 ft. long. The signal in the open/short cases is easily strong enough to be detected, but "frayed insulation has a smaller impedance change, it's harder to find the phase change and there are signal-to-noise issues," she said. "But test results look good to find lesser faults." The group is now looking at signatures of wires that have already arced.
The impedance plot of a healthy wire has a complex shape, and it would be difficult to make an assessment from one measurement, so the plan is to store a baseline plot and look for changes from that.
There is also a time domain technique, but the frequency domain reflectometer "is the smallest and cheapest circuit we could build," Furse said. Cost is expected to be well under $100 for a connector sensor, which has been shrunk to a 3Ž4 X 3Ž8 X 3Ž8-in. package for a first-generation unit that detects just opens and shorts. A second-generation unit that would detect less-than-complete faults and work on live wires is in research now. Many intermittent problems occur under flight conditions, and it is important to be able to test there.
Utah State will deliver a benchtop test system with specimen cables attached to realistic loads, leading to a laboratory demonstration around February 2002. Utah State will also study the architecture of how the smart connectors can be joined into a system tuned to work with five high-payoff Navy wire harnesses.
After the laboratory exercise, the Navy has a three-year budget to demonstrate the system in flight, Field said. If flight tests are successful, the technology could be available for implementation by 2005.
A PROBLEM WITH ARCING is that while it may have enough power to start a fire, often it is so intermittent that it won't trip the standard bimetallic thermal circuit breakers. For example, an arc that is five times the rated current of the breaker may last for 0.001-0.01 sec., but the breaker won't trip unless that level is sustained for several seconds.
The Navy's arc fault circuit breaker effort is run jointly with the FAA. Contracts worth about $1 million each were awarded to Eaton Aerospace and Hendry Telephone of Santa Barbara, Calif., in December 1999 to develop the devices.
Eaton's arc fault circuit interrupter (AFCI) adds arc sensor electronics that trip the standard bimetallic mechanism with a solenoid. The device works with alternating current; direct current arc sensing remains a research project. Potential advantages include an extra layer of safety, and "it's easier to add smart breakers than to rewire the aircraft," said Charles H. Singer, the Navy project engineer.
THE BIG WORRY is if AFCIs have an epidemic of tripping. An airplane that would be fine with a normal circuit breaker might become grounded with an AFCI. "It's tough to field something that shuts off due to an arc, and you can't find the arc," said James M. McCormick, aerospace arc fault program manager at Eaton. This creates pressure for advanced maintenance tools and for an AFCI design that doesn't have false trips. There is concern about false trips from electromagnetic interference that can be particularly strong on military aircraft.
The erratic current waveform of arcs should be identifiable, but some legitimate loads also are strange, such as strobe lights, pulsing valves and radars. Eaton has gathered load data on the ground on a Boeing 727, 737, 757, 777, an E-6 (Navy version of the 707), a Douglas C-9 (Navy version of the DC-9) and an Airbus A320. Trials of an AFCI on the C-9 in November 2000 had no false trips. A similar test with Hendry equipment is set for this summer.
The Eaton AFCI focuses on arc current that is greater than the circuit breaker rating. It looks at how current peaks vary from cycle to cycle--if they are consistent, the load is probably okay. It also integrates the energy, and looks for an accumulation --if there are just two strange peaks, it will forget about them after a while. "We focus on high-energy arcing faults; that gets 50-75% of them," McCormick said. "This is a plan to get a 75% solution as soon as possible. Any protection is better than no protection."
Eaton makes a residential AFCI that uses discrete components and has not decided whether to use this technology or a microprocessor for the aircraft application. The microprocessor may be more flexible, but "we would like to avoid the documentation and validation requirements of a microprocessor," McCormick said. "What can go wrong with resistors and capacitors?"
Eaton and Hendry will each deliver 20 units for flight test--10 on an FAA 727 and 10 on a C-9. The Eaton flights are set for June or July, and Hendry for March 2002. If results are good, one aircraft will be fitted with more ad data on the ground on a Boeing 727, 737, 757, 777, an E-6 (Navy version of the 707), a Douglas C-9 (Navy version of the DC-9) and an Airbus A320. Trials of an AFCI on the C-9 in November 2000 had no false trips. A similar test with Hendry equipment is set for this summer.
The Eaton AFCI focuses on arc current that is greater than the circuit breaker rating. It looks at how current peaks vary from cycle to cycle--if they are consistent, the load is probably okay. It also integrates the energy, and looks for an accumulation --if there are just two strange peaks, it will forget about them after a while. "We focus on high-energy arcing faults; that gets 50-75% of them," McCormick said. "This is a plan to get a 75% solution as soon as possible. Any protection is better than no protection."
Eaton makes a residential AFCI that uses discrete components and has not decided whether to use this technology or a microprocessor for the aircraft application. The microprocessor may be more flexible, but "we would like to avoid the documentation and validation requirements of a microprocessor," McCormick said. "What can go wrong with resistors and capacitors?"
Eaton and Hendry will each deliver 20 units for flight test--10 on an FAA 727 and 10 on a C-9. The Eaton flights are set for June or July, and Hendry for March 2002. If results are good, one aircraft will be fitted with more AFCIs. If those flight tests are good, the Navy may retrofit them into some aircraft. Boeing is interested in testing AFCIs on inflight entertainment circuits, McCormick said.
COST IS EXPECTED to be 4-5 times higher than a conventional circuit breaker, based on the residential AFCI experience. The main engineering difficulty has been shrinking the AFCI to fit in tight aircraft spaces. One with 20-amp capacity has been fit into a 25-35% larger size that normally takes up to 40 amps. The critical depth from the button to the terminal screws has been maintained, and they can be mounted on 0.78-in. centers. "If we can keep it below 0.8-in. centers, we can fit 90-95% of commercial aircraft sizes," McCormick said.
Arcing may play a large role in future aircraft as airframers consider 56-volt DC and 230-volt AC high-power systems to reduce wiring weight. "The military has 270 volts DC and there are big arcing concerns," McCormick said.
By Michael A. Dornheim/Aviation Week & Space Technology
30-Mar-2001 5:41 PM U.S. EST
Recent reports on aging aircraft wiring are providing clues on what maintenance organizations can expect in new rules and procedures covering this issue.
Fleet inspection has shown that aircraft wire inevitably has breaches in the insulation even when it is relatively new. Small arcs can carbonize the insulation, particularly Kapton, leading to bigger arcs and the potential for fire.
The increased interest in the four years since the TWA Flight 800 accident has advanced the state of the art and led to calls for wiring to be considered a system in itself, instead of an afterthought used to connect other systems. New equipment is being devised, such as circuit breakers sensitive to brief arcs.
The broad-based group investigating wiring for the FAA, the Aging Transport Systems Rulemaking Advisory Committee (ATSRAC), has recently finished reports on the five tasks it started in January 1999, said Kent V. Hollinger, the ATSRAC chairman. The reports are being submitted to Thomas E. McSweeny, FAA associate administrator for regulation and certification, and will help the agency formulate policy.
The ATSRAC comprises a wide range of manufacturers, airworthiness authorities, airlines, the U.S. Defense Dept., NASA and labor unions. The effort is voluntary; there is no contract from the FAA. Even though the committee's title says "systems" and not "wiring," in fact it has been primarily focused on wiring.
The five tasks are:
- Inspect the aging fleet. The committee chose to look at transports that were beyond the design lifetime. They inspected 81 in-service aircraft while they were in the shop, and examined six retired aircraft more intensely, including removing wiring samples for laboratory analysis. The lab work showed there were gaps in the insulation occurring at rates varying from 0.44-4.17 breaches per 1,000 ft. (AW&ST Oct. 23, 2000, p. 27). For a large transport with roughly 500,000 ft. of wiring, that suggests there are 220-2,085 breaches on board. Hollinger cautioned that "breach" was broadly defined; for example, 14% were unused wires with clipped ends and 9% couldn't be found in detailed inspection, but 73% seemed legitimate.
Power feeder cables had the worst rate, at 7.3 breaches per 1,000 ft. in pressurized areas and 6.0/1,000 ft. in unpressurized areas. The next worst type was complex harnesses in unpressurized areas at 3.4/1,000 ft. Aromatic polyimide (Kapton) insulation had the highest breach rate at 2.2/1,000 ft., followed by aliphatic polyimide (Poly-X) at 1.7/1,000 ft. and PVC/ glass/nylon at 1.5/1,000 ft.
Problems uncovered in the inspection have resulted in service bulletins and letters. For example, a power cable had signs of arcing due to routing that was not per the manufacturer's drawing. "Airlines might not realize the importance of routing," Hollinger said. Bundled parallel wires were found to be okay, even with different insulation types, but crossing wire bundles were not.
Based on the laboratory exam, the ATSRAC urged the FAA to research better nondestructive test techniques for wiring. The committee also recommended development of "smart" circuit breakers that detect brief arcing pulses, revising advisory circular AC43.13-1B for splicing techniques that can withstand the local environment, and having manufacturers review their designs for drip shields that keep liquids off wires.
- Review wiring service history, such as service bulletins, service letters and airworthiness directives. "We looked at service bulletins that were voluntary in the past to see if maybe some should be mandatory," Hollinger said. "We looked at repetitive-inspection airworthiness directives. The FAA has changed philosophy due to human factors work showing how inspections can be flawed, and prefers terminating action if available."
The ATSRAC reviewed 79 airworthiness directives and recommended that eight be changed to require terminating action. The committee also examined 714 service documents and passed 51 to manufacturers for further investigation. Boeing increased 27 to "alert" status and Airbus increased three.
- Improve inspection criteria and revise the process that develops them. The ATSRAC devised an "enhanced zonal analysis program" that provides the basis for an "enhanced zonal inspection program." A zonal inspection involves looking at everything in an area, such as a wheel well, but not in detail. What to look for regarding wiring has now been clarified and includes checking for contaminants and flammable materials on wires, separation of independent critical systems, and condition of high-voltage cables. The recommendations have been accepted by the Air Transport Assn.'s Maintenance Steering Group 3, which defines the interval and details of inspections.
- Review standard practices for repairing or modifying wiring. The panel has already recommended how to format wiring manuals to make them easier to use.
- Review training programs at airlines and repair stations to address aging wiring. The report to the FAA includes an eight-module lesson plan.
"It's up to the FAA what to do with the reports," Hollinger said. "They can ignore them, or action can either be internal or via the public comment process.
"We found that many recommendations aren't age-related, but affect wiring from when it is new." For example, minimum bend radius, adding drip shields, keeping metal shavings off wires and separating certain wires are not age-related.
On Mar. 13, the FAA presented a draft of a two-year plan for ATSRAC to implement its recommendations. Four new tasks have been proposed:
-Reformulate FAR Part 25 to place scattered references to wiring into a section of their own, emphasizing the view of wiring as a system. New requirements that consider the aging of wiring may be added.
-Develop a standard format and content for maintenance practices.
-Help the FAA develop regulations and advice for training programs. "There may be changes to FAR 121 [covering air carriers] and 145 [covering repair stations] to require wire training programs," Hollinger said. Advisory circulars would give a means of compliance.
-Devise enhanced maintenance practices. There may be rulemaking and advisory material to incorporate ATSRAC's recommendations, such as the enhanced zonal analysis program. Earlier, the FAA wrote a special FAR requiring all operators to have a fuel system safety program. "There may be a similar SFAR for wiring," Hollinger said.
THE U.S. NAVY is dealing with an increasingly old fleet of 4,000 aircraft that now has an average age of 17.2 years. The service estimates it removes about 5,000 power wires per year, and in 1996 there were 1,274 aircraft problems that required removal of power wire. About 5-9% of the 20 million man-hours spent on maintenance each year is finding and fixing wiring, and that's just at the field level. At least three aircraft types in the past two years have had rewiring programs.
The Navy started an Aging Aircraft Integrated Product Team in March 1999, and it has a wiring group based at the Naval Air Systems Command's Power Systems Div. at NAS Patuxent River, Md. The purpose is to serve as an information clearinghouse, provide solutions to aircraft programs, and develop risk and cost evaluation tools. The group works with the Aircraft Wiring and Inert Gas Generator Working Group (AWIGG), which shares information about wiring and fire suppression among the Defense Dept., NASA and other agencies. AWIGG is headed by Richard Healing, director of Navy safety and survivability.
THE WIRING GROUP has programs in three areas: wire diagnostics, "smart wire" and arc fault circuit breakers. The wire diagnostics effort is surveying existing test sets by applying them to a crashed F/A-18 airframe with inserted wiring faults. Seven sets have been evaluated and the results are being used to generate Navy requirements. Funding is now being sought for further evaluation by Navy depots starting by June, and to repackage and enhance one device so it doesn't require an expert operator, said Pall Arnason, project engineer for wire diagnostics and smart wire.
The Navy's $750,000 contract for smart wire was awarded last Aug. 30 to Management Sciences Inc. and the Center of Excellence for Smart Sensors at Utah State University. The idea is to have a self-monitoring wire system that can check itself before flight, and ultimately with live wires in flight, said Cynthia Furse, director of the Center of Excellence.
The Navy estimates it could save $65 million per year if it could detect 75% of open- and short-circuit faults and locate them to ±3 cm. (±1.2 in.), said Sean Field, Navy project engineer for power science and technology.
The concept uses "smart connectors," which have tiny reflectometers that detect and locate flaws in the wire between connectors. The reflectometer sends a signal down the wire, and changes in impedance cause reflection. In the extreme cases, an open circuit causes a full-strength in-phase reflection, and a short circuit causes a full-strength out-of-phase reflection. Subtler flaws cause less reflection and intermediate phase shift.
Furse is using a frequency domain technique that steps through frequencies and processes a mix of the received and transmitted signals to locate faults with 1.2-in. accuracy in cables that are 33-50 ft. long. The signal in the open/short cases is easily strong enough to be detected, but "frayed insulation has a smaller impedance change, it's harder to find the phase change and there are signal-to-noise issues," she said. "But test results look good to find lesser faults." The group is now looking at signatures of wires that have already arced.
The impedance plot of a healthy wire has a complex shape, and it would be difficult to make an assessment from one measurement, so the plan is to store a baseline plot and look for changes from that.
There is also a time domain technique, but the frequency domain reflectometer "is the smallest and cheapest circuit we could build," Furse said. Cost is expected to be well under $100 for a connector sensor, which has been shrunk to a 3Ž4 X 3Ž8 X 3Ž8-in. package for a first-generation unit that detects just opens and shorts. A second-generation unit that would detect less-than-complete faults and work on live wires is in research now. Many intermittent problems occur under flight conditions, and it is important to be able to test there.
Utah State will deliver a benchtop test system with specimen cables attached to realistic loads, leading to a laboratory demonstration around February 2002. Utah State will also study the architecture of how the smart connectors can be joined into a system tuned to work with five high-payoff Navy wire harnesses.
After the laboratory exercise, the Navy has a three-year budget to demonstrate the system in flight, Field said. If flight tests are successful, the technology could be available for implementation by 2005.
A PROBLEM WITH ARCING is that while it may have enough power to start a fire, often it is so intermittent that it won't trip the standard bimetallic thermal circuit breakers. For example, an arc that is five times the rated current of the breaker may last for 0.001-0.01 sec., but the breaker won't trip unless that level is sustained for several seconds.
The Navy's arc fault circuit breaker effort is run jointly with the FAA. Contracts worth about $1 million each were awarded to Eaton Aerospace and Hendry Telephone of Santa Barbara, Calif., in December 1999 to develop the devices.
Eaton's arc fault circuit interrupter (AFCI) adds arc sensor electronics that trip the standard bimetallic mechanism with a solenoid. The device works with alternating current; direct current arc sensing remains a research project. Potential advantages include an extra layer of safety, and "it's easier to add smart breakers than to rewire the aircraft," said Charles H. Singer, the Navy project engineer.
THE BIG WORRY is if AFCIs have an epidemic of tripping. An airplane that would be fine with a normal circuit breaker might become grounded with an AFCI. "It's tough to field something that shuts off due to an arc, and you can't find the arc," said James M. McCormick, aerospace arc fault program manager at Eaton. This creates pressure for advanced maintenance tools and for an AFCI design that doesn't have false trips. There is concern about false trips from electromagnetic interference that can be particularly strong on military aircraft.
The erratic current waveform of arcs should be identifiable, but some legitimate loads also are strange, such as strobe lights, pulsing valves and radars. Eaton has gathered load data on the ground on a Boeing 727, 737, 757, 777, an E-6 (Navy version of the 707), a Douglas C-9 (Navy version of the DC-9) and an Airbus A320. Trials of an AFCI on the C-9 in November 2000 had no false trips. A similar test with Hendry equipment is set for this summer.
The Eaton AFCI focuses on arc current that is greater than the circuit breaker rating. It looks at how current peaks vary from cycle to cycle--if they are consistent, the load is probably okay. It also integrates the energy, and looks for an accumulation --if there are just two strange peaks, it will forget about them after a while. "We focus on high-energy arcing faults; that gets 50-75% of them," McCormick said. "This is a plan to get a 75% solution as soon as possible. Any protection is better than no protection."
Eaton makes a residential AFCI that uses discrete components and has not decided whether to use this technology or a microprocessor for the aircraft application. The microprocessor may be more flexible, but "we would like to avoid the documentation and validation requirements of a microprocessor," McCormick said. "What can go wrong with resistors and capacitors?"
Eaton and Hendry will each deliver 20 units for flight test--10 on an FAA 727 and 10 on a C-9. The Eaton flights are set for June or July, and Hendry for March 2002. If results are good, one aircraft will be fitted with more ad data on the ground on a Boeing 727, 737, 757, 777, an E-6 (Navy version of the 707), a Douglas C-9 (Navy version of the DC-9) and an Airbus A320. Trials of an AFCI on the C-9 in November 2000 had no false trips. A similar test with Hendry equipment is set for this summer.
The Eaton AFCI focuses on arc current that is greater than the circuit breaker rating. It looks at how current peaks vary from cycle to cycle--if they are consistent, the load is probably okay. It also integrates the energy, and looks for an accumulation --if there are just two strange peaks, it will forget about them after a while. "We focus on high-energy arcing faults; that gets 50-75% of them," McCormick said. "This is a plan to get a 75% solution as soon as possible. Any protection is better than no protection."
Eaton makes a residential AFCI that uses discrete components and has not decided whether to use this technology or a microprocessor for the aircraft application. The microprocessor may be more flexible, but "we would like to avoid the documentation and validation requirements of a microprocessor," McCormick said. "What can go wrong with resistors and capacitors?"
Eaton and Hendry will each deliver 20 units for flight test--10 on an FAA 727 and 10 on a C-9. The Eaton flights are set for June or July, and Hendry for March 2002. If results are good, one aircraft will be fitted with more AFCIs. If those flight tests are good, the Navy may retrofit them into some aircraft. Boeing is interested in testing AFCIs on inflight entertainment circuits, McCormick said.
COST IS EXPECTED to be 4-5 times higher than a conventional circuit breaker, based on the residential AFCI experience. The main engineering difficulty has been shrinking the AFCI to fit in tight aircraft spaces. One with 20-amp capacity has been fit into a 25-35% larger size that normally takes up to 40 amps. The critical depth from the button to the terminal screws has been maintained, and they can be mounted on 0.78-in. centers. "If we can keep it below 0.8-in. centers, we can fit 90-95% of commercial aircraft sizes," McCormick said.
Arcing may play a large role in future aircraft as airframers consider 56-volt DC and 230-volt AC high-power systems to reduce wiring weight. "The military has 270 volts DC and there are big arcing concerns," McCormick said.