View Latest Blog Entries
Close
Categories
Testing & Assessment Certification Aging Wires & Systems Management Standard & Regulation Conference & Report Maintenance & Sustainment Research Protection & Prevention Arcing Miscellaneous
Popular Tags
Visual Inspection MIL-HDBK MIL-HDBK-525 FAR AS50881 FAR 25.1707 Electromagnetic Interference (EMI) Wire System High Voltage FAR 25.1709 Degradation Arcing Damage
All Tags in Alphabetical Order
25.1701 25.1703 Accelerated Aging ADMT Aging Systems Aircraft Power System Aircraft Service Life Extension Program (SLEP) arc damage Arc Fault (AF) Arc Fault Circuit Breaker (AFCB) Arc Track Resistance Arcing Arcing Damage AS22759/87 AS4373 AS4373 Method 704 AS50881 AS5692 AS6019 AS85485 AS85485 Wire Standard ASTM F2799 ATSRAC Attenuation Automated Wire Testing System (AWTS) Bent Pin Analysis Best of Lectromec Best Practice Cable cable testing Carbon Nanotube (CNT) Certification Chafing Chemical Testing Circuit Breaker Circuit Protection Coaxial cable cold bend comparative analysis Compliance Component Selection Condition Based Maintenance Conductor conduit Connector connectors contacts Corona Corrosion Corrosion Preventing Compound (CPC) Cracking D-sub data analysis data cables degradat Degradation Delamination Derating dielectric constant Distributed Power System DO-160 Electrical Aircraft Electrical Component Electrical Testing Electromagnetic Interference (EMI) Electromagnetic Vulnerability (EMV) EMC EMF EN3197 EN3475 EN6059 End of Service Life End of Year Energy Storage engines Environmental Environmental Cycling ethernet EWIS Component EWIS Design EWIS Failure EWIS Thermal Management EZAP FAA AC 25.27 FAA AC 25.981-1C Failure Database Failure Modes and Effects Analysis (FMEA) FAQs FAR FAR 25.1703 FAR 25.1707 FAR 25.1709 fault tree Fixturing Flammability fleet reliability Flex Testing Forced Hydrolysis fuel system fuel tank ignition functional testing Fundamental Articles Future Tech Green Taxiing Grounding Harness Design Hazard Analysis health monitoring heat shrink tubing high current high Frequency high speed data cable High Voltage History Hot Stamping Humidity Variation ICAs IEC60172 Instructions for Continued Airworthiness Insulation insulation resistance IPC-D-620 ISO 17025 Certified Lab Kapton Laser Marking life limited parts life projection Maintenance Maintenance costs Mandrel Mechanical Testing MECSIP MIL-C-38999 MIL-C-85485 MIL-DTL-17 MIL-DTL-3885G MIL-DTL-38999 MIL-E-25499 MIL-HDBK MIL-HDBK-1646 MIL-HDBK-217 MIL-HDBK-454 MIL-HDBK-516 MIL-HDBK-522 MIL-HDBK-525 MIL-HDBK-683 MIL-STD-1560 MIL-STD-1798 MIL-STD-464 MIL-T-7928 MIL-T-81490 MIL-W-22759/87 MIL-W-5088 Military 5088 modeling MS3320 NASA NEMA27500 No Fault Found off gassing Outgassing Overheating of Wire Harness Parallel Arcing part selection Performance Physical Testing polyimdie Polyimide-PTFE Power over Ethernet Power systems predictive maintenance Probability of Failure Product Quality Radiation Red Plague Corrosion Reduction of Hazardous Substances (RoHS) relays Reliability Research Rewiring Project Risk Assessment SAE Secondary Harness Protection Separation Requirements Series Arcing Service Life Extension Severe Wind and Moisture-Prone (SWAMP) Severity of Failure Shield Shielding signal cable silver plated wire smoke Solid State Circuit Breaker Space Certified Wires stored energy supportability Sustainment Temperature Rating Temperature Variation Test methods Test Pricing Testing Thermal Circuit Breaker Thermal Endurance Thermal Index Thermal Shock Thermal Testing Tin plated conductors Troubleshooting TWA800 UAVs verification Visual Inspection white paper whitelisting Wire Ampacity Wire Certification Wire Comparison wire failure wire properties Wire System wire testing Wire Verification work unit code

Mechanical Performance of Cables at Low Temperatures

Testing & Assessment

Key Takeaways
  • Mechanical performance at low temperatures cannot be an assumed capability of aircraft wires/cables.
  • A common means to determine a wire/cable’s performance in low-temperature conditions is the cold bend test.
  • Although a single mechanical stressing of a cable at low temperatures may not cause damage, continued stressing beyond its design specifications likely will.
  • Listen to the podcast here.

The environmental extremes under which wiring exists in aircraft can rapidly degrade materials that are not prepared or designed for those conditions. Without a doubt, high-temperature ranges that are typically considered for aircraft wiring eliminate most wire insulation types that are suitable for ground-based home applications. Lectromec has covered several types of high-temperature tests in past articles.

But at low temperatures, what tests exist to help identify which wires are ideally or adequately suited for aerospace applications? One of the tests that investigates the performance of wires in cold conditions is known as the cold bend test.

Applications

From an application perspective, it is entirely likely that a wire in a non-environmentally controlled zone of an aircraft will be at -55°C while the aircraft is at cruising altitude. Any wiring attached to a surface or panel that may move during flight must be able to move without creating wire/cable insulation cracks.

For those wires installed in the non-environmentally controlled zones, it is entirely probable that the wire will be at the low-temperature extreme during flight then rapidly increase to 30°C during the descent. During that transition, the wiring and wiring harnesses are very cold and moisture in the air will condense on the wiring. This moisture can work its way into the insulation cracks and start to cause signal degradation or lead to an electrical arcing event. To be in line with regulation 25.1703, each EWIS component must be installed according to the limitations of its components. To determine that a wire/cable will not degrade during performance, testing must be done.

Assessment

While there are several test methods that are industry accepted for cold bend testing, they all follow a very similar process. In this test, the wire is placed in a low-temperature freezer until it is thermally stable at the low-temperature. Typically, this temperature is between -65°C and -55°C and is dependent upon the particular test procedure. After the set duration, the wire is then slowly wrapped around a large mandrel. The intention of this slow wrap around a mandrel is to prevent undue stresses on the insulation that may lead to cracking.

When considering the test method for the evaluation of wires at cold temperatures, it is important to understand that the stress and mechanical bending is done very slowly. Lectromec has seen guidance to maintainers suggesting freezing cold compresses air should be sprayed on wires and connectors to help find intermittent faults. Rapid flexing of wire insulation at the cold temperatures can cause rapid degradation of the insulation and quickly lead to stress cracks. It is not recommended for use of cold compressed air to freeze wire harnesses as part of a troubleshooting processes.

Attenuation of coaxial cable after low temperature exposure
Low temperature exposure and mechanical stress can impact signal cable performance. In the case of this coaxial cable, a single cold bend test was not able to significantly impact performance.

While this test and its consequences are obvious for power cables, there are also consequences for signal cables. For coaxial cables such as MIL-DTL-17, the cold bend test is also a requirement. Within these coaxial/impedance-controlled cables, if there is a crack or damage to the dielectric, then there can be signal attenuation and reflections of the transmitted signal which may cause the connector system to stop functioning due to too high of a signal loss.

In Lab Evaluation

In the test data shown here, two attenuation traces are displayed. The first shows the attenuation of a new 10-foot coaxial cable. The second figure shows the attenuation after the cable has undergone extreme cold exposure and cold bending at -75°C (this low temperature was selected to push the material construction beyond its limits). While at the extreme temperature, the cable was wrapped around a mandrel, the sample was then returned to room temperature then the cable attenuation was evaluated. The comparison between the two data sets pre- and post-exposure show almost no change of material performance. It is likely that a connected system would not see a performance impact.

Conclusion

Low-temperature stressing of wire is a fact of life for many wires on aircraft. Thankfully, there are wire specific tests in place to evaluate their performance and prevent poor quality insulations from being installed on aircraft. Tests such as the AS4373 method 702 help to determine wire performance in these situations.

Michael Traskos

Michael Traskos

President, Lectromec

Michael has been involved in wire degradation and failure assessments for more than a decade. He has worked on dozens of projects assessing the reliability and qualification of EWIS components. Michael is an FAA DER with a delegated authority covering EWIS certification and the chairman of the SAE AE-8A EWIS installation committee.