Ken Shirriff Reverse Engineers a 1977 Aircraft Airspeed Indicator, Finds Surprising Complexity
"For all that complexity, " Shirriff notes, "the unit is essentially a voltmeter."
Noted reverse engineer Ken Shirriff has turned his attention to vintage aircraft instrumentation, looking at how a 1970s airspeed indicator works — and succeeding in bringing it to life outside the cockpit for which it was designed.
"This indicator was used in the cockpit panel for business jets such as the Gulfstream G-III, Cessna Citation, and Bombardier Challenger CL600. It was probably manufactured in 1977 based on the dates on its transistors," Shirriff explains. "You might expect that the indicators on an aircraft control panel are simple dials. But behind this dial is a large, 2.8-pound box with a complex system of motors, gears, and feedback potentiometers, controlled by two boards of electronics."
The instrument in question was designed to read off the aircraft's current airspeed along with the maximum airspeed possible given environmental conditions including altitude; in addition, a double-digital rotating dial provides an instantaneous look at the speed in Mach numbers — reading a maximum of 0.99, given the dial was not designed for use in supersonic aircraft. Behind the dial, meanwhile, are no fewer than three DC motors and two circuit boards hosting a surprising array of electronic components.
"This unit is much more complex than I expected for a simple display, with servoed motors controlled by two boards of electronics," Shirriff explains. "Air safety regulations probably account for much of the complexity, ensuring that the display provides the pilot with accurate information. For all that complexity, the unit is essentially a voltmeter, indicating three voltages on its display. The indicator doesn't have any smarts: the pointers just indicate voltages fed into it from an air data computer."
Shirriff isn't the only one looking at re-using vintage aircraft instrumentation: maker Glen Akins has been working on doing the same for a range of Selsyn-based indicators from World War II-era fighting craft, recently building a Raspberry Pi Pico-powered adapter board which brings the analog technology firmly into the digital domain.
Shirriff's full write-up is available on his blog, along with a schematic of the servo circuits driving the dial.