The Airplane - Jay Spenser [126]
When triggered by the airplane descending to the surface below, the ground rising up to meet the airplane, or a combination of the two, a “ground prox” alert fills the flight deck with loud aural tones and strident words. Depending on the specific warning issued, flight crews might hear “Terrain, terrain,” “Pull up, pull up,” “Sink rate,” “Glide slope,” and so on.
Thanks to the introduction of GPWS, the number of CFIT accidents fell dramatically, but they still occasionally occurred. Particularly disheartening were those cases in which, as confirmed by the cockpit voice recording, the flight crews had willfully ignored what should have been a timely warning.
Part of the problem was that, like the proverbial boy who cried wolf, early GPWS systems were plagued with false alerts. But there was more than that going on, and experts from across the global commercial aviation community—airlines, pilots, manufacturers, government regulatory authorities, and other interested parties—convened to determine what it was.
This campaign against CFIT accidents was pursued on two fronts. One was GPWS itself as manufacturers fine-tuned the trigger-threshold algorithms (an algorithm is a series of defined mathematical steps) to reduce false alerts. While improvements were made, however, these systems could not achieve foolproof results because a radar altimeter only looks downward, not forward, and mountainous terrain can of course rise dramatically.
Even as those efforts progressed, multidisciplinary teams were probing why professional flight crews might choose to discount GPWS warnings. These human-factors studies examined the mental models that we human beings construct and maintain of the ambient situation as we understand it. They highlighted how persuasive these mental models can be and how, when they are at odds with reality, they can blind us to what would otherwise be evident. For example, a flight crew might be so convinced that the autopilot is holding altitude that they fail to realize it has been accidentally disengaged and the instruments now show a descent.
Based on these efforts, airline training was universally implemented with a revised procedure for responding to a GPWS alert. This was to climb out immediately in an act-first, ask-questions-later response. Thanks to these and other improvements, CFIT accidents in the world commercial jetliner fleet are largely a thing of the past.
But people weren’t done building on Espenscheid’s simple invention. This collective activity would transform air travel and further enhance safety.
Starting in the late 1970s, the United States launched into orbit a constellation of satellites known collectively as the NAVSTAR Global Positioning System. Maintained by the U.S. Air Force, this infrastructure is available to the entire world. A minimum of twenty-four operational GPS satellites (actually just over thirty at the time of this writing) girdle the earth in highly inclined orbits that ensure excellent coverage of the entire world.
GPS is freely available to aircraft, ground vehicles, ships, soldiers, scientists, hikers, and any other users worldwide. This system provides accurate positioning to within a meter (about a yard) for civil users. With atomic clocks aboard all the satellites, GPS also provides astonishingly accurate time information.
Other satellite navigation systems are also being fielded or planned, notably Russia’s GLONASS and the European Union’s Galileo systems. In addition to those global constellations, China and India have regional systems in the works. For simplicity’s sake, therefore, the pioneering GPS and those that follow are collectively referred to as the global navigation satellite system.
Aviation relies heavily on this GNSS infrastructure. Jetliners flying the world’s oceans can be certain of their position despite the lack of positive