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ACAS X MOPS
PURPOSE AND SCOPE
This document sets forth minimum operational performance standards for the Active
(Xa) and special Operations (Xo) system variants of Airborne Collision Avoidance
System X (ACAS X) equipment. 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. Non-cooperative aircraft are not
required to be detected with ACAS Xa/Xo equipment.
ACAS X will provide traffic advisories (TAs) and if warranted, resolution advisories
(RAs) in the vertical plane. RAs are indications given to the flight crew recommending
maneuvers intended to provide separation from all threats, or restrictions to 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 X RAs priority over ATC
clearances and instructions, i.e., flight crews are instructed to follow an RA even when it
conflicts with ATC guidance (Ref. ICAO ACAS Manual, Doc. 9863, §22.214.171.124).
Controllers typically have no knowledge of a RA 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 may not be presented to controllers. Some alerts (e.g. wind shear warnings,
stall warnings, and Ground Proximity Warning System warnings) have higher priority
than an ACAS X RA. 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
Incorporated within these standards are system characteristics that should be of value to
users, designers, manufacturers, and installers. These cha racteristics are intended to
accommodate the requirements of various users.
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 1.0.
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 the recommended bench test
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