UAV / Drone (Aerospace) Cable Assembly Considerations
UAV, RPV & Drone (Aerospace) Cables
Aerospace cables are used in a variety of applications including data, fiber, Ethernet, power, video and more. Cables for aerospace applications must be prepared in a unique manner in order to comply with environmental, weight, aging, and safety requirements. Weight is of particular importance since large aircraft can require over 100 miles of wire weighing more than 3000 pounds. The lower the cable weight, the more economical the aircraft fuel consumption is, along with greater speed potential.
Aerospace Cable Considerations
- Weight: The weight of an aircraft cable can be reduced by utilizing wires with a higher temperature rating. These wires require less copper and hence weigh less. Another method of minimizing weight is to incorporate improved insulation.
- Environment: There are a variety of environmental considerations to account for including temperature, exposure to fluids, moisture, vibration, and pressure.
- Flammability: It is important that aircraft cables emit minimal smoke and dangerous gases if they are exposed to fire.
- Ease of installation: Aerospace cables must be easy to cut, strip, mark and terminate. They must also be flexible to allow for conduit routing.
- Overload conditions: The cables must be able to maintain circuit integrity in case of a current overload situation.
- Wireframe abrasion: Aerospace cables must be resistant to abrasion from surrounding conduits. Abrasion can be magnified with aircraft vibration.
- Surface properties: Cables must be able to be clearly marked, must be smooth, and receptive to label adhesion.
Aerospace Cables and EMI
Aerospace cables must adhere to specific Electro Magnetic Interference (EMI) requirements. Aerospace cables should follow these guidelines:
- Provide sufficient grounding at both ends of the cable to chassis ground.
- Shield the cable signals and minimize the transfer impedance (The voltage that develops on the other side of the cable surface due to the cable signal current).
- Minimize the use of braided shielding and instead use foil.
- Twist data signal wires which will help to cancel out emission fields.
- Use filtered connectors
Aerospace Cable Failure Modes
Care should be taken in the cable design to avoid failure modes which include the following:
- Aging: The wiring insulation can degrade with time. This happens when the electrical insulation dries out and becomes brittle. The insulation can then crack with aircraft vibration.
- Arc tracking: Arc tracking occurs when there is an electrical current along the insulation coating instead of just the wire conductor. Heat generated by the current flow causes a breakdown of the insulation material.
- Flash over: Once initiated, arc tracking can grow with time resulting in an arc across adjacent wires and the insulation material starts to burn. This occurs especially with Kapton wire which is now not permitted in aircraft wiring.
- Chafing: Chafing occurs as wires vibrate and rub against each other or against the aircraft structure.
Aerospace Cable Specifications
There are a variety of specifications that can be imposed on the design of an aircraft cable. In some applications the cable manufacturer must subject sample cables to various stress tests in order to prove that the specifications can be met. Some of the specification standards for aerospace include:
- BMS: BMS stands for Boeing Material Specification. These cover a variety of cable types including data bus and fiber.
- M27500 and M22759: These are military grade specifications used for aerospace cables. Many more MIL spec variations can be required depending on the application.
