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 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 by the flight crew. The airplane itself was fully airworthy and under control at the time of the accident; had the same circumstances arisen in conditions where the ground was visible, a crash would not have ensued.
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 also 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 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.