Commercial aviation today is one of the safest modes of transport available to the public. The industry is centered around a safety culture and infrastructure built on years of learning and development, and few systems can claim to have improved aviation safety as much as the Ground Proximity Warning System, or GPWS. GPWS are vital safety systems in modern aircraft that are designed to prevent collisions with terrain or obstacles, and were designed to prevent Controlled Flight Into Terrain (CFIT) accidents.
What is Controlled Flight Into Terrain (CFIT)?
CFIT incidents happen when a fully functional aircraft, under control of the pilot, unintentionally collides with terrain, water, or obstacles (with pilots typically remaining unaware of the danger until it becomes too late to recover). The majority of CFIT accidents take place during the approach and landing stages of a flight and are more often linked with non-precision approaches and landings in low visibility.
Loss of situational awareness contributes significantly to many CFIT accidents, and crash sites frequently align with the approach path to an airfield. Lack of familiarity with the approach procedure or misinterpretation of approach charts are common causes, especially when the approach involves a ‘step down’ descent profile through different altitudes during the approach. A notable number of accidents would be partly or fully attributed to CFIT prior to the 1970s. So, what changed? Enter TAWS and GPWS.
Terrain Awareness and Reporting Systems (TAWS) and Ground Proximity Warning Systems
TAWS is a family of technologies that includes the Ground Proximity Warning System (GPWS) and the Enhanced Ground Proximity Warning System (EGPWS). These are systems specifically designed to improve a pilot’s situational awareness and reduce the likelihood of an accidental impact with terrain.
A strong example of a CFIT incident driven by a loss of situational awareness is the crash of Eastern Air Lines flight 401 in December 1972.
The core principle behind EGPWS involves a blend of multiple sensors, databases, and predictive algorithms. Radar altimeters gauge the aircraft’s height above ground, while GPS systems precisely track its location. These data sources are fused to create a detailed map of the surrounding terrain. EGPWS uses this map in conjunction with aircraft performance parameters to forecast potential conflicts.
The system employs a terrain database containing highly accurate topographical information. It compares the aircraft’s position with this database, forecasting the flight path and issuing alerts if it predicts a potential collision. EGPWS generates various warning alerts, such as “Terrain Ahead,” “Pull Up,” or “Obstacle Ahead,” providing pilots with timely information to take corrective action.
GPWS vs EGPWS
EGPWS technology emerged from earlier Ground Proximity Warning Systems (GPWS) that were primarily reactive, providing alerts when an aircraft approached terrain with insufficient clearance. However, these systems lacked the capability to anticipate and prevent potential collisions proactively. As a response, EGPWS was developed to offer advanced predictive functions.
EGPWS has evolved significantly over time. Early versions primarily focused on providing warnings based on an aircraft’s altitude and proximity to the ground. However, advancements in computing power and data accuracy have enabled more sophisticated features. Terrain clearance floor prediction, where the system calculates the minimum safe altitude for a given location, and the incorporation of real-time weather data are among the enhancements.
Moreover, the integration of Enhanced Vision Systems (EVS) and Synthetic Vision Systems (SVS) has further improved situational awareness. EVS uses sensors like infrared cameras to provide visuals even in low visibility conditions, while SVS generates computer-generated 3D terrain views to augment a pilot’s view of the external environment.
Safer skies – the true impact of GPWS
GPWS systems have had one of the most statistically significant impacts on aviation safety of any technology. Prior to the 1970s, an average of 3.5 commercial aircraft per year were lost to CFIT incidents. Following the introduction of GPWS and EGPWS. This quickly fell to 2 per-year in the 1970s and hull losses due to CFIT today are almost unheard of. It is clear that the mandating of GPWS systems on commercial aircraft has led to safer skies for the flying public.
Cover photo: Simon Prechtl, JetPhotos.
