Today, most fixed-wing aircraft carry a device that dates back to the insights of one man: Leonard Greene.
Early on, Greene worked as a chemist but pursued a passion for flying. According to family accounts, one clear day in 1937, the 19-year-old was preparing to take a flying lesson at an airport on New York’s Long Island when an aircraft on approach suddenly plummeted to the ground. The pilot was killed. Having witnessed the tragedy, Greene set about learning what caused it.
It turned out that the pilot had entered a stall-spin. In the early days of aviation, stalls were a major cause of accidents. An aircraft wing is said to “stall” when it stops producing lift. When air meets the leading edge of an airfoil, part of the flow goes over the top and part goes underneath. (The place where the airflow divides is called the stagnation point.) As the aircraft’s nose pitches up, the angle between the airfoil and the wind increases, and eventually the orderly airflow above and below the wing breaks down. A turbulent storm of air forms over the top surface, and the wing can no longer produce lift.
Greene contrived a simple, wing-installed device that would warn the pilot that his wing was about to stall. He used rudimentary elements: a moveable vane that juts into the air, a small switch connected to the vane, and a battery-powered horn or light (or both). The theory is as simple as the technology: As the wing approaches stall, the stagnation point moves farther and farther aft of the leading edge, until air that would normally flow aft reverses its flow and goes forward; as it flows up and over the leading edge of the airfoil, it pushes the vane forward, closing the switch and in turn sounding the warning horn.
In 1946, Greene started a business, Safe Flight Instrument Corporation, to manufacture the device. The vane had to be placed so that it gave pilots enough warning to let them push the stick forward and recover before the wing actually stalled, so Greene did extensive testing to establish the best placement of the vane on each airplane type’s wing. During his career, Greene test flew and stalled so many aircraft that he lost count of the number. His son Randall, who today runs Safe Flight Instrument, says of his father: “He could see the air. I don’t mean that as a metaphor. I mean it literally. He could visualize the flow and pressure of air in his mind.” Leonard Greene died in 2006 at the age of 88.
Today, all U.S. aircraft must have some form of stall warning. In addition to Greene’s patented wing device, Safe Flight Instrument and other manufacturers — such as Rosemount and Teledyne — make airflow sensors for use on the fuselage.
Greene continued with his inventions, accumulating more than 200 patents and producing increasingly capable safety systems, such as automatic throttles. But the family business’ most ubiquitous product is its simplest.