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A Summary of Airborne Collision Avoidance Systems (ACAS)

The Airborne Collision Avoidance Systems (ACAS) was introduced to minimize the risk of mid-air and near mid-air collisions between aircraft. It serves as a last-resort safety net regardless of any separation standards. ACAS II, the most modern version, is a system based on Secondary Surveillance Radar (SSR) transponder signals. ACAS II interrogates the Mode C and Mode S transponders of surrounding aircraft and uses their replies to track their altitude and range, issuing alerts to the pilots as needed. Unfortunately, ACAS II will not detect aircraft that do not have transponders equipped and will not issue resolution advice for traffic without an altitude reporting transponder.

ACAS II works separately from the aircraft navigation, flight management systems, and Air Traffic Control (ATC) ground systems. Additionally, while assessing threats, it does not take into account ATC clearance, pilot intentions, or Flight Management System inputs. In general, ACAS II is not connected to autopilot, except in the case of the Airbus AP/FD (autopilot/flight detector) TCAS capability, which provides automated responses to resolution advisories.

Presently, the only commercially available versions of ICAO (International Civil Aviation Organization) standard ACAS II technology is the TCAS II version 7.1 (Traffic Alert and Collision Avoidance System). ICAO mandates state that all ACAS II units must be compliant with TCAS version 7.1. In the United States, where some mandates are different, there are still many aircraft operating with versions 6.04a and 7.0.

There are two types of alerts that can be issued by ACAS II: Traffic Advisory (TA), or Resolution Advisory (RA). TAs are intended to help the pilot acquire a visual of the conflicting aircraft and prepare for a potential RA. If the ACAS II determines there is a risk of collision, an RA is generated. RAs serve to tell the pilot the range of vertical speed at which they should fly their aircraft to avoid the threatening aircraft. The visual indication of this is shown on the flight instruments and accompanied by an audible message denoting the intention of the RA. when the two aircraft diverge, a “Clear of Conflict” message will be generated.

Upon issuance of an RA, the vertical direction of the RA is coordinated with other ACAS II aircraft via a mode S link, so the two aircraft can choose compatible maneuvers. Instead of restoring a prescribed ATC separation, the RAs goal is to achieve collision avoidance by establishing a safe vertical separation of 300-700 feet. ACAS II operates on relatively brief timescales: the maximum generation time for a TA is 48 seconds prior to the Closest Point of Approach (CPA) while that of an RA is 35 seconds. These time scales are shortened at lower altitudes as aircraft typically fly slower when closer to the ground. Unexpected or sudden aircraft maneuvers can result in a TA/RA with a much shorter lead time than the standard 48 and 35 seconds, and it is possible that an RA will not be preceded by a TA if the threat is imminent.

The effectiveness of an RA is evaluated by the ACAS II on a second-by-second basis and the RA can be strengthened, weakened, reversed, or terminated as necessary. Each ACAS-equipped aircraft is surrounded by a protected volume of airspace. The size of this volume depends on multiple factors, such as the altitude, speed, and heading of the aircraft involved. The safety benefits of ACAS are usually expressed in the terms of risk ratio - whether the system makes safety better or worse. Various studies have determined that ACAS improves flight safety by more than 21%.

ACAS equipment is available from four major vendors: ACSS, Garmin, Honeywell, and Rockwell Collins. Though the implementation is different for each vendor, they all provide the same core functions. For all types of ACAS equipment and their associated parts, look no further than NSN Gamut. Owned and operated by ASAP Semiconductor, we can help you find all types of parts for the aerospace, civil aviation, defense, electronics, industrial, and IT hardware markets. Our account managers are always available and ready to help you find all the parts and equipment you need, 24/7-365. For a quick and competitive quote, email us at or call us at 1-714-705-4780. Let us show you why we consider ourselves the future of purchasing


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