scope:

Introduction

This document sets forth minimum operational performance standards for Traffic Alert and Collision Avoidance System II (TCAS II) equipment. TCAS II is intended to improve air safety by acting as a last-resort method of preventing mid-air collisions or near collisions between aircraft. By utilizing Secondary Surveillance Radar (SSR) technology, TCAS II equipment operates independently of ground-based aids and air traffic control (ATC). Aircraft equipped with TCAS II have the ability to interrogate airborne transponders to monitor range, bearing, and reported altitude of other aircraft in the vicinity and assess the risk of collision. Non-transponding aircraft are not detected. TCAS II Version 6.0 was first introduced in the United States in 1990 with collision avoidance logic Version 6.0 and later 6.04 and 6.04a. TCAS II Version 7.0 was introduced in the United States and Europe in 2000 and was subsequently mandated by the International Civil Aviation Organization (ICAO) for all commercial turbine-powered transport aircraft worldwide having more than 19 passenger seats or having a maximum take-off weight above 5700 kg. TCAS II is referred to as the Airborne Collision Avoidance System II (ACAS II) in ICAO terminology.

TCAS II provides traffic advisories (TAs) and if warranted, resolution advisories (RAs) in the vertical plane. RAs are indications given to the flight crew recommending manoeuvres intended to provide separation from all threats, or restrictions to manoeuvres to maintain existing separation. (In this document the term separation means physical separation, i.e., absence of collision, and should not be confused with the provision of ATC minimum separation.) TCAS II operating procedures give RAs priority over ATC clearances and instructions, i.e., flight crews are instructed to follow an RA even when it conflicts with ATC guidance. Controllers have no knowledge of a TCAS RA unless notified by the flight crew via the radio. (RA information is provided by TCAS II to Mode S SSRs, but it is not normally presented to controllers.) Only wind shear warnings and Ground Proximity Warning System warnings have higher priority than a TCAS 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 TCAS II, specifically, the risk of mid-air collision is reduced by at least a factor of three. Globally, operational experience has shown that TCAS II has significantly improved the safety of flight.

Note: There are three members of the TCAS family. TCAS I is a pilot warning indicator which displays proximate traffic and alerts the crew to other aircraft which may become potential near midair collision threats. TCAS II in addition provides recommended vertical escape manoeuvres to the crew to avert potential near midair collisions. The term TCAS IV is reserved for future TCAS equipment that has additional capabilities beyond those of TCAS II equipment to generate resolution advisories in the horizontal plane (e.g., "TURN RIGHT," "TURN LEFT"). The equipment described by this document does not have horizontal resolution capability. In this document, TCAS I is denoted "TCAS I"; the use of the unqualified term "TCAS" refers exclusively to TCAS II; and TCAS IV is explicitly denoted "TCAS IV".

TCAS provides an effective collision avoidance function in traffic densities as high as 0.3 aircraft per square nautical mile (24 transponder-equipped aircraft within five nautical miles of the TCAS-equipped aircraft).

When TCAS operates in traffic densities of 0.3 transponder-equipped aircraft per square nautical mile or higher, it provides protection against collisions with other aircraft that are closing at speeds of less than 500 kt. TCAS is also capable of providing protection against aircraft closing at relative speeds of up to 1200 kt in airspace characterized by densities of less than 0.06 transponder-equipped aircraft per square nautical mile, which is equivalent to eighteen transponder-equipped aircraft within ten nautical miles of the TCAS aircraft.

In addition to the minimum standards contained in this document, several options are described herein which will allow TCAS to request and/or receive information from other MODE S-equipped aircraft. These optional features provide for the exchange of information via the proposed MODE S extended squitter or an air-to-air data link. The purpose for including these options in this standard is to permit a uniform minimum capability to upgrade or enhance TCAS in the future. The specific applications for these optional features and the applications' corresponding minimum operational performance characteristics are not part of these standards.

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.