Radio Altimeters and GPWS in Preventing CFIT Accidents

Introduction: Aviation Safety and the Role of Technology

Air travel is one of the safest means of traveling from one location to another in the world. Without air traffic, the business world would come to a screeching halt. Although businesses can transfer large amounts of digital communications data — eliminating much of the demand for mail and fax transmissions that characterized commerce just a decade ago — they still cannot transfer products, personnel, and other physical goods through electronic signals over the internet. Travel still relies on airliners and cargo aircraft that fill the skies around the world and around the clock. Experts agree that global commercial air traffic will grow at an average rate of 5% per year over the next 20 years, meaning that traffic will double within 15 years and nearly triple by the end of the following decade (Interavia, 1999).

The air travel industry is built on principles that have established and maintained its safety record. One of these principles is that of multiple redundant systems. Once an airliner is in the air, the forces of physics and gravity control its flight path. If a mistake occurs or a system fails, gravity and inertia cannot be paused while the problem is resolved. Each airplane is therefore built with multiple control redundancies to prepare for any possible failure.

A second principle, which can work in tension with the first, is that the pilot and crew are always in final control of the aircraft (Luccio, 2001). If systems malfunction, the pilot can override them. If conditions warrant, individual controls can be switched off to maintain aircraft control. Neither pilots nor airlines want an accident caused by a failed automated system that locked out the crew and produced a disaster attributed to a "technical malfunction."

During the past four decades, the aviation industry has recognized that the combination of equipment and personnel needs to be actively managed. The following overview tracks the growing body of knowledge that has accumulated regarding air safety.

1960s: Accidents were attributed to individual pilot error, mainly a lack of basic flying skills. Safety efforts focused on selecting candidates with appropriate psychomotor skills and proficiency-oriented handling training. Manufacturers focused on designing more reliable aircraft.

1970s: Accidents were attributed to individual pilot error, mainly a lack of technical proficiency. Safety efforts focused on psychomotor and cognitive skill selection and increased use of flight simulators. Manufacturers focused on designing more reliable, easier-to-fly aircraft, with built-in redundancy, fail-safe concepts, and greater automation assistance.

1980s: Accidents were attributed to cockpit crew errors, mainly team synergy failures and poor management of cockpit resources. Safety efforts introduced crew resource management (CRM) training. Manufacturers focused on reducing pilot workload, advancing fly-by-wire stability, expanding auto-flight capabilities, and providing error protections such as GPWS.

1990s: Every accident was reconceived as a failure of organization. Front-line operator behavior was understood to be strongly determined by systemic forces — selection, training, procedures, culture, working conditions, and organizational structures. Human error was not seen as failure per se, but as an intrinsic component of cognitive processes. Safety efforts evolved into fourth- and fifth-generation CRM training, emphasizing situation awareness, error management strategies, and metacognition. Manufacturers provided situation awareness augmentation tools, decision aids, and automatic protections such as EGPWS (adapted from Amalberti and Sarter, 2000).

For these reasons, any discussion of overcoming specific safety and technical problems in aviation must combine an analysis of technology and human interaction with it. While capable of operating aircraft in roughly 80% of situations and through 80% of a typical flight, technology cannot replace the human judgment required for the remaining 20%. Until artificial intelligence can fully replicate a pilot's capabilities, human judgment — and therefore human error — will always be part of the flight equation.

With these factors in mind, this paper investigates the use of radio altimeters as part of ground proximity warning systems (GPWS) and how these devices can help avoid controlled flight into terrain (CFIT) incidents and approach-and-landing accidents (ALA). The radio altimeter is an integral component of the GPWS. While barometric altimeters determine altitude above sea level and are affected by changing weather patterns, radio altimeters measure an aircraft's height above the ground and are particularly useful in determining actual flight paths during approach and landing events when visibility is diminished.

First-generation GPWS technology looks straight down, using the aircraft's radio altimeter to provide warning of threatening terrain. Against gently rising terrain, this provides better-than-nothing advance warning. However, as one pilot described its limitation, GPWS would remain silent in an aircraft flying across level ground and straight toward a vertical cliff. Fewer than 5% of the world's commercial aircraft fleet lacks GPWS; yet it is precisely these unequipped aircraft that are involved in nearly 50% of CFIT accidents (flightsafety.org.au).

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