Next time you are in an airplane next to the galley when the ground staff gently restocks the galley with pluggable containers, consider the interconnect environment found in aircraft and be impressed by the industry’s success in managing it.
Aircraft are heavily used, going through multiple thermal and humidity cycles every day. They must live in the worst of conditions, temperatures way below any we see on the surface, and landing in a blowing salty rain next to the ocean in Miami. We expect planes to do this year after year without failure. In addition, the airlines have no tolerance for unscheduled maintenance.
What does that mean for aircraft connectors?
They need to be durable, reliable, and fail-safe, even after all of the thermal, humidity, and air pressure cycles.
Civilian aircraft use several different types of connector families. Most of the flight-critical electronics are contained within rugged boxes, connected to each other with cabling. Both the boxes and cables need to be replaceable with minimum effort in the maintenance hangar or on the flight line. The airline industry and airplane manufacturers embrace the idea of a FRU, field replaceable unit, easily replaceable for maintenance or upgrades. The airframe may last 50 years, but electronics have a much shorter obsolescence cycle.
Cables need to be routed through complex paths in the airframe, around corners and through holes. To tolerate this well, special abrasion resistant cable jackets are used for aircraft cables and every component on the airplane must be non-flammable. Special dielectrics and flex circuit materials have been chosen for their ability to not contribute to a fire. The connectors for this cabling are quite similar to the cylindrical families used in military applications with one major exception. For every pound of weight in an aircraft, the plane will burn many gallons of additional fuel over the life of the aircraft. This puts a huge premium on reduced weight for cable connectors and strain relieving back shells. The newest planes are specifying composite connector components, highly engineered plastic materials that can meet the need for ruggedness, withstand the horrible environment, and still be substantially lighter than their metal predecessors. Composite connector shells have the additional advantage of not rusting in the presence of corrosive materials like salt spray, and overflow from the lavatory and galley dribbling through the cabin floor.
Speaking of the cabin floor, modern jets have some very sophisticated entertainment systems that can deliver the Internet and a library of stored content to every seat on the plane. Plane configurations constantly change as they add or subtract seats, depending on the needs of the airline. This means that the under-floor cabling systems need to be quite flexible and easy to reconfigure. Generally, a backbone of Ethernet cables runs down the plane to under floor boxes serving a block of seats. Cables connect each seat to this box. If you are fortunate enough to have an electric seat, the complexity of the harness is even greater. The Ethernet cables and connectors in these applications use a shielded Quadrax configuration of conductors to provide stable Ethernet connections through the entire system.
Today’s aircraft have an electronics bay to accommodate the many electronic functions in the aircraft. Racks frequently use blind mated rectangular connectors specified by ARINC standards, allowing boxes to be swapped by opening handles, inserting the box, and locking it in place. These FRUs can enable quick repair or maintenance on the flight line as well as allowing upgrades in total systems without changing the airplane.
The aircraft environment for EMI is horrible. In addition to all of the radios and radars needed to communicate and navigate the aircraft, uncontrolled sources of radiation from individual phones to lightning strikes and solar flares abound. The shielding systems for all of the electronic boxes and cables need to protect the electronics under all conditions.
Aircraft connectors need to be light and very reliable, tolerating lots of mechanical and environmental abuse while never failing. Applaud the next aircraft electronics design engineer you meet. They do an incredible job.