The Airplane - Jay Spenser [125]
Over the next four decades, Lloyd Espenscheid performed ground-breaking research as a scientist in AT&T’s engineering department. One result was a 1924 patent for a railroad collision-avoidance system. Its purpose was to provide timely warning of another train on the tracks ahead by emitting radio waves and sensing their bounced return. Another patent arose from a more sophisticated version of this invention turned vertically to serve pilots.
Espenscheid had come up with aviation’s version of the sounding line, a weighted cord lowered into water to ascertain its depth. That maritime practice, which dates back to ancient times, had in the latter nineteenth century provided American journalist, humorist, and satirist Samuel Clemens—one of America’s greatest writers—with the pen name Mark Twain. It was a term recalled from Clemens’ days as a Mississippi riverboat pilot on the eve of the U.S. Civil War. The riverboat’s leadsman would lower a plumb line to take soundings of the river’s muddy depths. Knots tied in this rope marked off the depth in fathoms. A cry of “mark twain” denoted two fathoms (12 feet or 3.7 meters) of water beneath the keel, and that meant safety.
Espenscheid’s electronic counterpart for aviation was very different from Paul Kollsman’s barometric altimeter. Whereas the latter device showed the airplane’s height above sea level, Espenscheid’s radio altimeter bounced a signal off the surface below instead to tell pilots how high they were above the ground. This was potentially valuable information at night or in bad weather.
Astonishingly precise time measurements would have been required to determine height by actually timing the echo. However, an easier alternative existed that worked just as well. This was to transmit a continuous downward signal that oscillated up and down in frequency at a set rate, creating the radio-wave equivalent of a siren’s ululation. The greater the distance from which this wavering signal bounced back, the more the frequency of the airplane’s transmission would have shifted in the interim. Because comparing two frequencies is a simple matter, Espenscheid employed this frequency modulation approach.
The Western Electric Company—AT&T’s manufacturing affiliate—marketed the first commercial radio altimeter in 1937. The more accurate term radar altimeter came into use following World War II (radar is an acronym for radio detection and ranging).
In modern flight decks, the radar-altimeter readout appears on the primary flight display screens in front of the pilot and copilot. These displays show at a glance the airplane’s attitude, speed, altitude, autopilot mode, and a wealth of other flight-instrument information.
During the approach and landing phases, the radar altimeter also provides inputs for a computer-generated voice that calls out the decreasing altitude in increments as the airplane nears the ground. Flight crews use these automated announcements to know when to flare, which means to pull back on the control wheel and moderate the contact of the wheels with the runway. While pilots can also gauge this visually, the precise readouts help them land more consistently.
People soon got the idea of leveraging radar altimetry to serve other purposes. The first additional use came in the late 1960s with the introduction of the ground proximity warning system (GPWS), which alerts crews to decreasing separation between the airplane and the ground.
Analysis of commercial airliner accidents showed many to be the result of controlled flight into terrain. These CFIT (pronounced “SEE-fit”) accidents were often the result of a loss of situational awareness, fatigue, or fixation on minor systems failures and other distractions