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Design Philosophy

The wake turbulence timers the WTT4 and WTT5 Wake Turbulence Timers were intended to replace had been in use since the mid-70s in the FAA's Western Mountain Region.  Much of the electronics supporting air traffic safety is from a similar time frame.  The requirement for long-term reliability and maintainability (the ability to repair and service the equipment in the future) is obviously critical to the wake turbulence timers.

This requirement dictated proven technologies likely to be around long into the future.  LCDs and oLED displays can provide all sorts of flashy animated displays, but there are issues trying to obtain compatible components even over a time period of months or years — display size and mounting arrangements change and new drivers chips may require software changes to work properly.  7-Segment LED displays have been around many years and remain a mainstay to this day.  Using a common standard size, supplied by many different manufacturers, ensures a supply well into the future.  The lifetime of LEDs is well understood.  In most cases, they don't burn out; instead they get dimmer with operating time.  50,000 hours of operating time is considered the useful life of an LED, at which point its brightness will have decreased by 50%.  Even then, they continue to operate for many thousands of additional hours.  7-Segment displays can't show all sorts of cool animations, but they will display, in a bright visible way, what the Air Traffic Controllers need to know.  Boring?  Perhaps.  Reliable and maintainable well into the future?  Absolutely.

 The switches are a key element of the wake turbulence timers.  They get pressed many times a day in service — in fact, it's estimated that in 15 years at the busiest airports, the switches may have been pressed a million times!  The switches used in the Wake Turbulence Timers are manufactured by a large German industrial switch supplier.  These switches are rated for over a million operation cycles, so they will provide a long service life.  The switches are from a popular line used in a lot of industrial machinery.  Changes are good that replacement switches will be available far into the future.

Switches don't always survive to see their rated life; sometimes Air Traffic Controllers can be less than gentle when pushing the button in the stress of the moment.  If worse comes to worse and a switch does fail for some reason, a technician can have a new one installed and the timer back in operation in less than 15 minutes with no soldering required.

 Other components of the timer have been selected with equal care.  There are only two components that aren't stock standard parts available from numerous sources.  The first is the microcontroller brain of the timer — it's from Microchip Technologies, who pride themselves on never having obsoleted a microcontroller.  Failure is highly unlikely, but if it ever does occur, replacements will be available.  The other key part is the LED display driver chip.  It's an extremely common part used in a wide variety of electronics so it's likely to be around for many years.  Additionally, there is a pin-for-pin alternative from another manufacturer.  
Not only was long term reliability of the hardware critical - the timers had to be simple to operate with minimum interaction and little chance for confusion.  A single touch selects any of the four available intervals and starts  the countdown.  No extra steps required to set the interval which might be overlooked in a stressful moment.

The two timer sections are clearly separated and they are color-coded to help the Controller differentiate which section is supporting what function when both sections are in use.