Interview Questions for Avionics Wiring and Harness Assembly

Crimping an aircraft wire is a critical process that ensures a secure and reliable electrical connection. It involves using a specialized crimping tool to deform the terminal’s barrel around the wire, creating a strong mechanical and electrical bond. Think of it like squeezing a metal sleeve tightly around the wire to hold it in place.

The process typically involves these steps:

1. Wire Preparation: Strip the wire insulation to the correct length specified by the terminal manufacturer. Ensure a clean, even strip to prevent poor conductivity and potential short circuits. Any nicked or damaged wire must be discarded.
2. Terminal Selection: Choose the correct terminal type and size, matching the wire gauge and application. Incorrect terminal selection can lead to failure.
3. Wire Insertion: Insert the stripped wire into the terminal barrel, ensuring the wire extends to the appropriate depth marked on the terminal. A fully inserted wire will create a proper crimp.
4. Crimping: Place the terminal and wire into the crimping tool jaws, ensuring proper alignment. Squeeze the tool firmly and completely, allowing the tool to crimp the terminal securely around the wire. A properly crimped terminal will show a solid, uniform crimp without any gaps or distortion.
5. Inspection: Inspect the crimped terminal visually, checking for proper deformation and absence of damage to the wire. Pull gently on the wire to ensure the connection is secure. A visual inspection is important, sometimes aided by a magnifying glass.

Improper crimping can result in loose connections, potential short circuits, or even complete wire failure in flight, which are all serious safety hazards. Therefore, it’s crucial to use the correct tools, follow the manufacturer’s instructions, and always inspect the completed crimp.

Avionics utilizes a variety of wire terminations, each designed for specific applications and wire gauges. The choice of termination depends on factors such as current carrying capacity, vibration resistance, environmental protection, and ease of installation.

• Crimp Terminals: These are the most common type, using a crimping tool to create a secure connection. Sub-types include insulated and non-insulated versions and various shapes (e.g., ring terminals, spade terminals, bullet terminals) to match different connectors.
• Solder Terminals: These involve soldering the wire to the terminal, offering a very reliable connection but requiring more skill and careful execution to prevent cold solder joints or damage to the wire. This type of termination needs to be inspected very closely for proper adhesion and integrity.
• Screw Terminals: These use a screw to clamp the wire securely within the terminal, offering a simple and reliable connection, often preferred for higher currents. These are generally used with larger wires and are commonly used for power distribution.
• Quick Disconnect Terminals: These are specialized connectors for quick and easy mating and unmating, commonly found for removable components or testing points.
• Printed Circuit Board (PCB) Terminals: These are designed for connecting wires directly to PCBs, typically using surface mount technology or through-hole components.

Each termination type has advantages and disadvantages depending on the specific application. Selecting the incorrect termination can lead to performance issues or even safety hazards.

Safety is paramount when working with aircraft wiring. A single mistake can lead to catastrophic failures. Key precautions include:

• Lockout/Tagout Procedures: Always follow proper lockout/tagout procedures to prevent accidental energization of circuits.
• Personal Protective Equipment (PPE): Wear appropriate PPE, including safety glasses, gloves, and insulated tools. Always use tools rated for the voltage levels present.
• Grounding and Bonding: Ground yourself and the aircraft to prevent static electricity buildup and potential sparking.
• Proper Tool Usage: Use only the correct tools and follow the manufacturer’s instructions carefully. Never use damaged or improperly maintained tools.
• Fire Safety: Have a fire extinguisher readily available and know how to use it. Aircraft interiors are often flammable.
• Work Area Safety: Maintain a clean and organized workspace to avoid tripping hazards or accidental damage to wiring.
• Following FAA Regulations: Strictly adhere to all relevant FAA regulations and procedures.

Failure to adhere to these precautions can result in serious injury, damage to the aircraft, or even a fatal accident.

Wire gauges are designated by American Wire Gauge (AWG) numbers. Lower AWG numbers represent thicker wires with higher current-carrying capacity, while higher numbers indicate thinner wires with lower capacity. The selection of wire gauge is critical because it directly influences the ability of the wire to carry current without overheating.

For example:

• AWG 22: Commonly used for low-current circuits, such as signal wiring.
• AWG 16: Used for higher-current applications, such as power wiring for accessories.
• AWG 8: Used for high-current applications, such as battery power distribution.

Selecting the wrong gauge can cause overheating, leading to insulation failure, fire hazards, and ultimately, system malfunction. Aircraft wiring diagrams specify the required gauge for each circuit, and these specifications must be strictly adhered to.

Proper wire routing and bundling are essential for maintaining the integrity and functionality of the aircraft wiring harness. They are equally important for safety and maintainability. It ensures that wires are protected from damage, prevent interference, and help in organized maintenance.

Routing ensures that wires are routed in a way that minimizes stress and avoids sharp bends or kinks which can lead to breaks. It is also important to ensure the wires are routed away from heat sources or moving parts.

Bundling involves grouping wires together using appropriate clamps, ties, and sleeves. This keeps the harness organized, protects wires from abrasion, and reduces weight. Proper bundling prevents short circuits and keeps wires easily traceable for repair or troubleshooting.

Improper routing or bundling can lead to chafing, short circuits, and open circuits, potentially resulting in system failures.

Installing a wire harness into an aircraft is a complex procedure requiring precision and meticulous attention to detail. It typically involves:

1. Preparation: Inspect the aircraft and harness for any damage. Verify all components are correct and aligned with the aircraft’s wiring diagrams.
2. Routing: Route the harness according to the aircraft’s wiring diagrams, carefully adhering to the specified paths and avoiding sharp bends or kinks. Use appropriate clamps, ties, and sleeves to secure the harness.
3. Connection: Connect the harness to the aircraft’s electrical system, ensuring all connections are secure and correctly polarized. Double-check all connections.
4. Testing: Thoroughly test the installed harness to ensure proper functionality and to verify that there are no short circuits or open circuits. Testing usually involves using a multimeter and following predefined tests defined by the manufacturer and or the FAA.
5. Documentation: Document all installations and tests to comply with the aircraft’s maintenance records.

Improper installation can lead to short circuits, system malfunctions, and safety hazards.

Wire harness failures can stem from various causes, many preventable through proper maintenance and installation procedures:

• Corrosion: Moisture and salt can corrode wire terminals and connectors, leading to poor conductivity or complete failure. This is a major issue in high-humidity environments.
• Abrasion: Wires rubbing against other components can cause insulation damage, exposing the wires and potentially creating short circuits.
• Vibration: Continuous vibration can cause wire fatigue and breakage, especially at connection points. This is a key concern for aircraft wiring that regularly experiences high vibration.
• Overheating: Using the incorrect wire gauge, poor connections, or excessive current can cause overheating and melt insulation, leading to short circuits or fires.
• Poor Crimping: Inadequate crimping can lead to loose connections which can result in intermittent or complete signal or power failures.
• Improper Installation: Incorrect routing, bending, or bundling can cause stress on wires, leading to premature failure.

Regular inspection and preventative maintenance are critical to minimizing the risk of wire harness failures.

Troubleshooting a faulty wire harness involves a systematic approach, combining visual inspection with electrical testing. Think of it like detective work – you need to gather clues to pinpoint the problem.

First, I’d start with a thorough visual inspection. Look for obvious damage like cuts, abrasions, or corrosion on the wires, connectors, and terminals. Check for chafing against other components or signs of overheating (discoloration, melting). This initial step often reveals the culprit.

Next, I’d consult the aircraft’s wiring schematic. This diagram acts as a roadmap, showing the path of each wire and its connections. Tracing the circuit from the suspected faulty component is crucial. I’d use a multimeter to check for continuity (if a wire is broken) and voltage (to see if power is reaching its destination). A continuity test ensures the electrical path is complete, while voltage checks confirm the proper flow of electricity. For example, if a light isn’t working, I’d check the voltage at the light’s terminals; if absent, I’d trace back along the circuit until I find the break in power.

Advanced techniques might involve using a wire harness tester, which can automatically check multiple circuits simultaneously. This speeds up the process, especially in complex systems. Ultimately, careful documentation of each step is crucial – documenting my findings and the repair process ensures the problem is solved efficiently and prevents recurrence.

Avionics connectors need to withstand extreme conditions, so the types used are carefully chosen for their robustness and reliability. Some common types include:

• Circular Connectors: These are frequently used because they can handle many pins and are durable. Examples include MIL-DTL-38999 (often referred to as ‘Mil-Spec’ connectors) and their variants, known for their environmental sealing and high contact density. They’re like the workhorses of the industry, found across a variety of aircraft systems.
• Rectangular Connectors: Used when space is limited or for specific signal types. These often incorporate specific shielding and grounding features to minimize interference.
• Coaxial Connectors: Essential for high-frequency signals such as those in radar and communication systems. These connectors are designed to maintain signal integrity, often incorporating features that minimize signal loss and reflection.
• Fiber Optic Connectors: Increasingly prevalent for high-bandwidth data transmission. Their ability to transmit vast amounts of data over long distances is a significant benefit in modern aircraft. Examples include SC, ST, and LC connectors.

The choice of connector depends on factors like the number of pins required, environmental conditions (temperature, pressure, humidity), signal type, and required shielding.

Proper torque when installing connectors is absolutely critical for reliable electrical connections and to prevent damage. Think of it as Goldilocks and the Three Bears – too little torque, and the connection might be loose and unreliable, potentially leading to intermittent failures or even signal loss. Too much torque, and you risk damaging the connector, causing it to malfunction or fail completely.

Each connector type and wire gauge has a specified torque value. These values are usually found in the manufacturer’s documentation and the aircraft’s maintenance manual. Using a torque wrench is essential to ensure the correct tightening force is applied, preventing both under-tightening and over-tightening. Over-tightening might strip the threads or damage the connector body, potentially creating a dangerous situation due to loose connections or circuit interruptions, while under-tightening may lead to poor contact. A consistent torque value across all connectors helps ensure uniform contact pressure, improves signal integrity and extends the lifespan of the entire wiring system.

Interpreting wiring schematics requires a methodical approach. Think of them as maps for the electrical system. They use symbols and conventions to represent various components and their connections.

I usually start by identifying the key elements: power sources, loads (lights, motors, sensors), and the wiring interconnecting them. The schematic shows the circuit paths, indicating how the components are connected. It uses standardized symbols, like those in the SAE AS5470 series, to represent components and connectors. Color codes on the diagram correspond to the wire colors in the actual harness, enabling me to trace specific circuits. For example, a schematic might depict a circuit that shows the power flow from the battery, through a fuse, through a switch, and finally to a landing light. By carefully following the lines and symbols on the diagram, I can identify where a fault might be occurring and quickly troubleshoot.

Understanding these symbols, along with the wire color codes, is crucial. Practice and experience are key to becoming proficient in interpreting these often complex diagrams.

My experience with wire harness testing encompasses various procedures, from basic continuity checks to advanced automated testing. For basic checks, I use a multimeter to verify circuit continuity and measure voltage drops. This is fundamental for detecting broken wires or poor connections. I’ve also used automated wire harness testers that significantly improve efficiency by automatically testing multiple circuits simultaneously and checking the wiring integrity against a pre-programmed database.

More complex testing might involve specialized equipment, for example, to simulate real-world operating conditions, ensuring that the harness can withstand the rigors of flight. This can include thermal cycling and vibration testing to ensure the harness’s durability. Throughout my work, I’ve always meticulously documented test results and followed stringent quality control processes to guarantee the harness’s airworthiness. This documentation is essential for complying with industry standards and regulations, and is a crucial part of ensuring aircraft safety and reliability.

Wire insulation in avionics needs to meet stringent requirements for fire resistance, environmental protection, and durability. Different types of insulation are selected based on the specific application and environmental conditions. Some common types include:

• Teflon (PTFE): Excellent high-temperature resistance and chemical resistance. Often used in high-temperature areas of the aircraft.
• Silicone Rubber: Good flexibility and high-temperature resistance, making it suitable for various applications.
• Ethylene Propylene Rubber (EPR): A common choice for its good flexibility, chemical resistance, and relatively low cost.
• Cross-linked Polyethylene (XLPE): Offers excellent resistance to abrasion and chemicals.
• Polyvinyl Chloride (PVC): A versatile and cost-effective material, but may not have the same high-temperature performance as some others.

Each type has its own set of properties, such as its temperature range, chemical resistance, and flexibility. Choosing the right insulation is crucial for the reliability and longevity of the wire harness, as improper insulation can lead to short circuits, open circuits, or other failures that compromise safety.

Proper grounding in aircraft wiring is paramount for safety and operational reliability. It ensures that stray electrical currents are safely channeled away, preventing interference with sensitive electronics and protecting personnel from electrical shocks. Grounding provides a return path for current, and without it, you risk voltage spikes and potential damage to components. It’s like the plumbing system of an electrical circuit, ensuring everything flows properly.

Imagine a situation where an electrical fault occurs; a properly grounded system provides a safe path for the fault current to flow to earth, preventing build-up of dangerous voltages. Insufficient grounding can lead to interference between systems and malfunctions. Think of it as preventing electrical noise. Grounding also protects against static electricity build-up, a significant concern in the aviation industry. In essence, proper grounding is a critical safety measure that significantly reduces the risk of equipment damage, fire, and electric shock within the aircraft.

Ensuring compliance with AS9100, the internationally recognized standard for the quality management systems in the aerospace industry, is paramount in avionics wiring and harness assembly. It’s not just about ticking boxes; it’s about building a culture of quality.

We achieve this through a multi-faceted approach. First, we meticulously document all processes, from initial design and material selection to final testing and delivery. This documentation serves as a roadmap, guiding every step and allowing for continuous improvement and traceability. Regular internal audits, performed by trained personnel, assess our adherence to AS9100 requirements and identify areas needing attention. We also undergo rigorous external audits by accredited certification bodies to verify our compliance. Non-conformance reports are thoroughly investigated, root causes are identified, and corrective and preventative actions are implemented to prevent recurrence. This proactive approach ensures that our processes consistently meet the highest quality standards, crucial for the safety and reliability of aircraft systems.

For example, our traceability system allows us to track each wire, connector, and component throughout the entire manufacturing process, from raw material receipt to final installation. This capability is essential for identifying and rectifying any issues that may arise, minimizing potential downtime and ensuring that only certified and approved materials are used.

My experience encompasses a wide range of wire harness manufacturing techniques, each chosen based on factors like complexity, volume, and cost. I’ve worked extensively with manual assembly, where skilled technicians carefully route, terminate, and assemble wires, ideal for intricate, low-volume harnesses.

I’m also proficient in semi-automated and fully automated assembly methods. Semi-automated methods utilize specialized equipment like wire cutting and stripping machines to enhance efficiency and consistency. Fully automated systems, often employing robotic arms and programmable logic controllers (PLCs), are particularly suitable for high-volume production runs, significantly reducing lead times and ensuring high repeatability. These systems often integrate quality control checks at various stages to minimize errors.

One project involved a high-volume production run for a commercial aircraft’s entertainment system. The sheer number of harnesses necessitated a fully automated approach, incorporating vision systems for accurate wire identification and automated crimping and soldering. This reduced production time by approximately 40% compared to manual assembly, while also maintaining high quality standards.

Wire harness documentation is the backbone of our operations – it’s crucial for traceability, maintenance, and future modifications. We use a comprehensive system incorporating both physical and digital documentation.

Physical documentation includes detailed schematics, wire lists, and assembly drawings, carefully maintained and readily accessible. Digital documentation utilizes Enterprise Resource Planning (ERP) systems and specialized wire harness design software. These systems store all relevant information, from design specifications to manufacturing records, enabling easy retrieval and analysis. Each harness is assigned a unique identification number, linking all related documents throughout its lifecycle. This allows for efficient tracking of materials, labor, and testing results.

For example, a change in a component requires updating all related documents in our system. The system then flags all affected harnesses, facilitating efficient and accurate modifications without compromising safety or compliance.

Wire identification methods are critical for ease of maintenance and troubleshooting. Several techniques are employed, often in combination.

• Color Coding: This is a common and cost-effective method, using different colored wires to denote different circuits. However, it’s limited in its ability to identify a large number of circuits.
• Heat Shrink Tubing with Labels: Heat shrink tubing with printed labels provides more durable and easily readable identification than simple color codes. The labels can include circuit numbers, wire gauge, and other relevant information.
• Printed Wire Markers: These are small, printed labels applied directly to the wires. They offer good legibility but may not be as durable as heat shrink tubing.
• Wire Tags: These are small tags attached to wires, often with more space for information.
• Barcoding and RFID Tags: These advanced technologies offer enhanced traceability and data management capabilities. They allow for automated identification and tracking of wires throughout the assembly and lifecycle of the aircraft.

The choice of method often depends on the complexity of the harness, the environmental conditions, and the required level of traceability.

Strain relief is absolutely critical in aircraft wiring to prevent wire breakage and connector damage from vibration, stress, and movement during flight. Without adequate strain relief, wires can become loose, potentially leading to short circuits, signal loss, or even complete system failure – posing significant safety risks.

Several methods are used to provide strain relief. These include using clamping devices, strain relief boots, and properly routing wires to avoid sharp bends or excessive tension. Connectors themselves often incorporate strain relief features, such as locking mechanisms and cable clamps. The proper selection and application of strain relief methods are essential to ensure the long-term reliability and safety of the aircraft’s electrical systems.

Imagine a scenario where a wire connecting a critical flight control system becomes loose due to lack of strain relief. The resulting signal interruption could have catastrophic consequences. Proper strain relief prevents this by providing a secure and stable connection, even under the extreme conditions encountered during flight.

Soldering is a crucial skill in aircraft wiring assembly, requiring precision and adherence to stringent quality standards. We use various soldering techniques, depending on the application. Common methods include hand soldering, using a controlled temperature soldering iron and high-quality solder with a suitable flux to ensure a clean and strong connection.

For larger-scale production, we may employ automated soldering systems, which ensure consistency and speed. These systems often incorporate automated inspection processes to verify the quality of the solder joints. In all cases, strict adherence to procedures is followed, including cleaning and preparation of the wire ends to remove any contaminants before soldering.

Beyond technique, the selection of solder and flux is critical. We use only aerospace-grade materials that meet the relevant specifications to ensure the reliability and durability of the connections in harsh environments. Inconsistent soldering can lead to weak or unreliable connections, impacting the aircraft’s safety and operability.

Working with high-voltage wiring in aircraft presents unique challenges due to the increased risks of electrical shock, arcing, and fire. Special precautions must be taken to ensure the safety of personnel and the reliability of the system.

These challenges include:

• Increased risk of electrical shock: Strict safety protocols, including lockout/tagout procedures, are mandatory to prevent accidental contact with energized high-voltage circuits.
• Arc Flash Hazards: Arcing can occur if high-voltage wires become damaged or improperly insulated. This can cause fires and injuries. Specialized personal protective equipment (PPE), including arc-flash suits, is essential when working with high-voltage systems.
• Insulation Requirements: High-voltage wiring requires specialized high-quality insulation materials to prevent short circuits and electrical breakdown. Regular inspection and testing of insulation is crucial.
• Specialized tooling and techniques: Specialized tools and techniques are required to handle high-voltage components safely, and training is essential for personnel involved in their installation and maintenance.

Thorough training, stringent safety protocols, and the use of appropriate PPE are essential to mitigate these risks and ensure a safe working environment.

Modifying a wire harness is a critical process demanding meticulous attention to detail and adherence to strict safety regulations. It’s never a simple ‘cut and splice’ operation. We begin by thoroughly reviewing the engineering change order (ECO) to understand the modifications required. This includes examining schematics, wire lists, and any relevant documentation. Then, we carefully trace the existing harness to pinpoint the exact location for modification. This step is crucial to avoid damaging other circuits.

The modification itself might involve adding new wires, rerouting existing ones, or removing obsolete components. Each step involves the use of proper stripping tools, crimping tools, and heat-shrink tubing to ensure robust, reliable connections. After the modification, we perform thorough testing to validate circuit integrity and functionality using both continuity checks and voltage measurements.

For instance, I once had to modify a harness for a flight control system. A new sensor needed to be integrated. The entire process, from review of the ECO to final testing, took several days and involved rigorous documentation of every step, which was later reviewed by quality assurance.

My experience encompasses a wide range of connectors used in avionics, including both circular and rectangular types. Circular connectors, such as MIL-DTL-38999 and AMP-SEAL, are commonly used for their robust and reliable performance in harsh environments. I’m proficient in their various shell sizes, contact arrangements, and termination techniques (crimping, soldering). Rectangular connectors, like those from TE Connectivity or Amphenol, offer higher contact density and are often used for data transmission or in situations where space is limited. I’m familiar with their specific locking mechanisms and pin assignments.

Understanding the specific characteristics of each connector type, such as their environmental sealing capabilities, voltage ratings, and current carrying capacity, is crucial. This knowledge allows me to select the appropriate connector for a given application and ensure its proper installation to meet the stringent requirements of aircraft systems. For instance, on a recent project, we used MIL-DTL-38999 series II connectors for their excellent environmental sealing to protect against the extreme temperature variations and moisture experienced at high altitudes.

I’m highly proficient in using wire harness routing software, primarily Capital Harness XC and similar CAD tools. These tools allow us to design and document wire harnesses digitally, eliminating the need for cumbersome manual drawings. The software helps in creating 3D models of the harness, allowing us to virtually route wires around components and obstructions, optimizing the harness layout for weight and space efficiency. Furthermore, these systems can automatically generate wire lists, connector pinouts, and other essential documentation. This improves accuracy and reduces errors compared to manual methods.

For example, on a recent project involving a complex engine control system, the software allowed us to simulate the harness installation virtually, identifying potential interference issues early in the design phase. This saved considerable time and resources that otherwise would have been spent on rework. This digital approach also facilitates design collaboration and change management, which are critical in complex projects.

Ensuring the quality of a completed wire harness involves a multi-step process. We start with meticulous attention to detail during the manufacturing process, using only high-quality materials and adhering to strict procedures. Then, a series of inspections and tests are carried out at each stage of the process. These include visual inspections to check for correct wire routing, proper connector termination, and adherence to labeling standards.

Next, we perform electrical tests using specialized equipment to verify continuity, insulation resistance, and high-pot testing. This ensures that each wire is properly connected and that the entire harness meets electrical specifications. Finally, we conduct environmental testing, simulating harsh conditions such as extreme temperatures and humidity, to evaluate the harness’s durability and reliability. Any discrepancies found during the testing phase lead to immediate corrective actions and thorough documentation of any defects and repairs.

Think of it like building a complex puzzle – every piece must be correctly positioned and securely fastened to achieve the desired result. In our case, the result is a reliable and safe system, and any shortcuts can have catastrophic consequences.

Aircraft wiring faces extreme environmental conditions, so the selection of materials and design are critical. Temperature extremes, from freezing conditions at high altitudes to the intense heat generated by engines, are a major factor. We must use materials that can withstand these fluctuations without degrading or becoming brittle. Humidity and moisture are also significant concerns; this can lead to corrosion and insulation breakdown. Furthermore, aircraft wiring must be able to withstand vibrations and shocks encountered during flight. It also must resist the effects of chemicals and UV radiation. Therefore, materials must have appropriate temperature ranges, insulation resistance, and durability.

For instance, the use of specialized coatings, such as Teflon or silicone, is often necessary to protect wires from moisture and chemicals. Similarly, the choice of connector sealing and harness routing is also vital in protecting wiring from harsh conditions. Meeting these standards is essential for the safety and reliability of the aircraft.

Copper is the most commonly used wire material in avionics due to its excellent conductivity and relatively low weight. However, aluminum is sometimes used in high-voltage applications because of its lower weight. I have experience working with both materials and understand their properties and limitations. Copper offers superior conductivity and is generally easier to work with, while aluminum is advantageous when weight saving is crucial, especially in larger aircraft. Aluminum, however, requires specialized crimping and termination techniques to ensure reliable connections.

The choice of wire material often depends on the specific application and the trade-off between weight, conductivity, and cost. For example, in high-current applications where weight is less of a concern, copper is preferred due to its superior conductivity. In high-voltage systems, aluminum’s weight advantage might outweigh its slightly lower conductivity. Regardless of the material, the wire must meet specific aerospace standards for conductivity, insulation, and tensile strength.

Proper labeling and traceability are paramount for maintenance and repair. We use a combination of methods to ensure this. Each wire is individually labeled with its unique identification number, often printed directly onto the wire or using heat-shrink sleeves with printed markings. These labels typically adhere to industry standards such as SAE AS50881 or similar specifications, ensuring clarity and consistency. In addition, detailed schematics and wire lists are maintained for each harness. These documents link the wire’s identification number to its specific function within the aircraft system.

This rigorous system ensures that if a repair is necessary, the appropriate wire can be quickly identified and replaced without causing damage to other circuits or components. It’s crucial for safety and also reduces maintenance time. For example, if a wire is damaged, its label will indicate its precise location in the aircraft and its function. This makes repairs much more efficient and reduces the risk of incorrect repairs.