A methodology to assess the safety of aircraft operations when aerodrome obstacle standards cannot be met

Abstract

When Aerodrome Obstacle Standards cannot be met as a result of urban or technical development, EASA in-line with ICAO allows proving an equivalent level of safety by carrying out an aeronautical study. However, detailed guidance in doing so is not provided. This paper aims at filling this gap with a proposed safety assessment methodology to value obstacle clearance violations around airports. It was applied for a safety case at Frankfurt Airport where a tower elevating 4 km out of threshold 25R violates severely obstacle limitation surfaces. The model refers to a takeoff and landing performance model (TLPM) computing precisely aircraft trajectories for both standard and engine out conditions at ground proximity forming the model's reference data: The generated tracks are used to estimate collision risk considering stepwise EASA/FAA, EU-OPS & ICAO clearance criteria. Normal operations are assessed with a probabilistic analysis of empirical takeoff / landing track data generating the local actual navigation performance (ANP) at site. The ANP leads through integration to collision risk for an aircraft with any obstacle. This step passed, the obstacle is tested for clearance within a "5-step-plan" against all performance requirements for landing climb, and takeoff climb. The methodology so delivers a comprehensive risk picture: The presented safety case for Frankfurt Airport showed an equivalent safety level despite the violation of standards: The collision risk during both normal and degraded performance operations was found to be still within ICAO Collision Risk Model (CRM) limits requiring only limited risk mitigation measures. The presented work should complement ICAO Doc. 9774 Appendix 3.