Product Focus: Aircraft Switches

Control of an in-flight aircraft is predicated on the pilot’s ability to alter critical aircraft components and/or conditions to safely carry out the flight plan. Switches are one way designers have traditionally provided for pilot input.

"Switching, or the changing of the ‘state’ of a device, is fundamental to control," says Charles Turner of Eaton Aerospace, "It is the basic man-machine interface."

Switches are the trigger for transmitting commands and their importance is critical. They must be reliable, easy to reach and easy to use.

By far the most common switch found in aircraft cockpits is the electro-mechanical switch, which consists of a mechanical element between conductive parts that are activated in response to an external force (the pilot pushes the button, turns the knob or flips the switch).

These switches are the mainstay of today’s cockpits, and most industry sources agree they will remain that way for some time.

The ability of electro-mechanical switches to carry heavy electrical loads ensures their acceptance. But sources agree the tactile feel they provide when they have been pushed, flipped or turned is a real plus as well.

Pilots, particularly those who grew up before computers were mass produced, like the positive feedback that an electromechanical switch provides.

But new frontiers are continually being explored as solid-state electronic switches and electronic instruments make their way into the glass cockpits of newer-generation aircraft. Glass cockpits, which utilize computers to display flight information and allow for pilot input, are simplifying pilot workload. They allow the pilot to focus on a few screens instead of a myriad instruments and gauges.

According to Honeywell’s Jamie Cohen, the difference between an electro-mechanical and a solid-state switch can be quite dramatic.

"Hard-wired electro-mechanical switches found in cockpits are generally not configurable into much more than a panel, while solid-state switches can take many forms. Electro-mechanical switches can be used in multiple types of applications, controlling everything from low level to 20 [amp] loads, while solid-state switches are more application specific," Cohen said.

Solid-state switches have numerous advantages over their electro-mechanical cousins, primarily because they have no moving contacts. They rely on electronic or magnetic sensing or some other non-mechanical method for changing states.

One example is the Hall-effect switch, which is a closed circuit based on a magnetic field. When activated, a Hall-effect sensor moves a magnet and the movement is picked up by a solid-state sensor.

The physics related to the Hall effect was discovered in the late 1800s, but it was only with the development of low-cost integrated circuits some 100 years later that such switches became suitable for mass production. They are favored for aircraft cockpits because of their high reliability.

"Solid-state technology has negligible wear, is less expensive to assemble, and is generally lighter in weight and smaller in size [than its electro-mechanical predecessors]," Turner said.

Additionally, Turner says, solid-state switches offer longer life expectancy and reduced maintenance costs. They are also more immune to contaminates.

However, solid-state technology is not for every application. Most importantly, for aircraft applications, the current-carrying capability of solid-state switches is not on par with electro-mechanical devices.

Additionally, Turner believes solid-state technology is "more likely to experience failures during environmental extremes, such as high or low temperature environments. Also, the failure mode of a solid-state device is more difficult to trace."

Terry Trumbull of Electro-Mech Components said while longer life and lower costs may be advantages of solid-state technology, when it comes to switches, this newer technology may not be able to offer the "heavy tactile feel often desired by the user."

Trumbull says these human interface issues can be resolved though. "Anytime the cockpit is rearranged from a previous version the pilots take some time getting used to [the changes]," he said.

Turner agrees human interface is important, but he believes the adjustment to solid-state switches is not that as big of a problem.

"To the degree that is possible," Turner said, "switch designers have provided mechanical feedback [such as a spring behind the push button] to give solid-state devices a similar feel to that of electro-mechanical switches. Secondly, pilots entering the business today grew up with touch screens and joysticks. They do not expect the mechanical feedback that their predecessors required."

Cohen adds in nearly all cases, pilots are involved in the design of cockpit interface equipment.

He said the idea that electro-mechanical switches are obsolete "is an obsolete concept itself. Electro-mechanical switches, properly designed for the specifics of an application, are still unchallenged relative to their versatility, cost and reliability.

"There will always be a market for electro-mechanical switches," Cohen said. "They are presently being actively designed into multiple applications and continue to have a strong presence in the industry."

Also, Cohen said, "switch products currently under development, but not presently adopted, combine some of the best elements of electro-mechanical products with some of the best elements of solid-state products."

In recent years original equipment manufacturers (OEMs) asked suppliers for help in achieving their weight and size goals.

Digitran’s Larry Wismer said, "The OEMs have become primarily integrators of systems and the design responsibility for reducing size and weight has been transferred to the first and second tier suppliers."

Suppliers are responding and are providing tools to help the OEMs and systems integrators.

Digitran, for instance, provides (via the Internet and free of charge) its EZ Rotary Configurator Design Tool that allows engineers and designers to quickly select the key parameters of a rotary switch and then download two- or three-dimensional drawings for direct insertion into their own system design model via AutoCad, Pro-E, SolidWorks or other software.

There are other switching technologies being researched, including the use of a photonic network that would provide an optical on-off switch to replace electric wiring on aircraft with fiber optic cabling.

Fiber-optic systems would be lightweight and compact, a real plus for aircraft. They would also be immune to electromagnetic interference and would not be susceptible to fire.

Turner says Eaton worked with McDonnell Douglas in the late 1980s and early 1990s on just such a system. However, despite the fact that the prototype was successful, the idea never reached production.

The idea of using fiber optic cabling is compelling, Turner says. Still with all the apparent advantages, the down side is cost and the relative complexity such a system demands.

"Who is going to be the first to spend the money to develop a complete system?" asks Turner.

Such new switching technology may come from shared R&D between industry and government.