Modeling terrain awareness and warning systems for airspace and procedure design

Terrain Awareness and Warning Systems (TAWS) are on-board systems that alert crews to imminent Controlled Flight into Terrain (CFIT). Originally required by the FAA in 1974 for all Part 121 aircraft, this technology has advanced to include GPS data, terrain and obstacle databases, and sophisticated flight deck displays to provide increasingly accurate and timely alerts. TAWS equipment is capable of generating a set of audible and visual alerts based on the most likely situations which lead to a high risk of CFIT. These include excessive descent rates, excessive vertical deviation from an Instrument Landing System (ILS) glideslope, proximity to terrain with landing gears up, and descent immediately following a takeoff. Although the incident rate of CFIT has been greatly reduced over the past few decades, areas of risk still exist, typically near small airports surrounded by mountainous terrain. Risk also remains due to economic factors. The high cost of hardware and software upgrades delays the universal adoption of the most advanced TAWS equipment. Older equipment is much more likely to generate nuisance alerts - warnings to the crew during periods of low CFIT risk. These nuisance alerts desensitize crews to TAWS, leading to situations where crews may ignore or not fully respond to alerts that indicate real danger. Separately, FAA NextGen efforts are driving major redesigns of airspaces and procedures across the country. The primary factors in redesign focus on minimizing the costs associated with delay and fuel burn; however, these redesigns must also ensure that changes do not introduce new CFIT risks. Current practices depend on incomplete evidence to determine that risk has not increased. The continued elimination of remaining CFIT risk, along with the possibility of introducing new risk during redesign, necessitate a comprehensive TAWS model to understand current and potential hazards. Without a TAWS model to provide rapid feedback during simulated design changes, several months may pass before observing that a design change has increased CFIT risk. In this paper, we describe the construction and validation of a standards-based TAWS model to capture the behavior of a variety of equipment. This paper also describes a process for measuring the effect of procedure redesign on TAWS alert rates. Finally, we include a case study of analyzing approaches into the airport at Eagle, CO - the airport with the highest TAWS alert rates in the country. In this case study, we find that equipment upgrades would eliminate nuisance alerts for aircraft using currently published approach procedures.