scope:

PURPOSE AND SCOPE

Introduction

This document sets forth minimum operational performance standards for the Airborne Collision Avoidance System X (ACAS X) equipment, including both Active surveillance (Xa) and special Operations (Xo) functions. Unless otherwise noted, ACAS X references in this document refer to ACAS Xa/Xo.

ACAS X is intended to improve air safety by acting as a last-resort method of preventing mid-air collisions or near mid-air collisions (NMAC) between aircraft. By utilizing surveillance information from Secondary Surveillance Radar (SSR) and ADS-B technology, ACAS X equipment operates independently of ground-based aids and air traffic control (ATC). Aircraft equipped with ACAS X have the ability to interrogate airborne transponders and receive ADS-B Messages to determine the location of other aircraft in the vicinity and assess the risk of collision. ACAS Xa/Xo equipment is not required to detect non-cooperative aircraft.

ACAS X provides Traffic Advisories (TAs) and Resolution Advisories (RAs) in the vertical plane. RAs are indications given to the flight crew recommending maneuvers intended to avoid collisions with all threats, or restrict maneuvers to maintain existing separation. (In this document the term separation means physical separation, i.e., absence of NMAC, and should not be confused with the provision of ATC minimum separation.) ICAO standards give ACAS RAs priority over ATC clearances and instructions, i.e., flight crews are instructed to follow RAs even when it conflicts with ATC guidance (Ref. ICAO Doc. 8168 - PANS-OPS, §3.2.c.2). Controllers typically have no knowledge of RAs unless notified by the flight crew via the radio. RA information is provided by ACAS X to Mode S SSRs and ADS-B ground radios, but typically is not presented to controllers. Some alerts (e.g. wind shear warnings, stall warnings, and Ground Proximity Warning System warnings) have higher priority than ACAS RAs. Results of United States and European safety studies show that there is a significant safety benefit to be gained from the widespread carriage and operation of ACAS, specifically, the risk of mid-air collision is reduced by at least a factor of three (Ref. ACASA/WP1.8/210D/V1.1 27-12-01). Globally, operational experience has shown that ACAS systems have significantly improved the safety of flight.

Incorporated within these standards are system characteristics that should be of value to users, designers, manufacturers, and installers. These characteristics are intended to accommodate the requirements of various users.

Document Structure

This document is published in two volumes.

Section 1 of Volume I is intended to provide information needed to understand the rationale for equipment characteristics and requirements stated in the remaining sections. It describes typical equipment applications and operational goals and is the basis for the standards stated in the document. Definitions essential to proper understanding of this document are also provided in Section 1.

Section 2 of Volume I contains the minimum performance standards for the equipment. These standards define the required performance under standard operating conditions and stressed physical environmental conditions. It also details bench test procedures that demonstrate compliance, including specific bench tests for the collision avoidance logic performance.

Section 3 of Volume I describes the performance required of the installed equipment. Tests for the installed equipment are included when performance cannot be adequately determined through bench testing.

Volume II contains the Algorithm Description Document (ADD), which includes required and suggested collision avoidance algorithms for ACAS X. The algorithms are presented in the Julia programming language with informational commentary text.

Both Volumes I and II contain certain standards and performance requirements that ensure that ACAS X is fully interoperable (see §1.5.5) with other airspace elements and equipment. It is mandatory that these interoperability provisions be met without exception or deviation. Other technical and performance requirements stated herein that were derived from the particular implementation approach that formed the basis for the development of this MOPS may be waived if the manufacturer or installer provides evidence of equivalent system performance for an alternative method of implementation and if the alternative implementation does not violate interoperability provisions.

Since the measured values of equipment performance characteristics may be functions of the methods of measurement, standard test conditions and methods of test are recommended in this document.