Instrumentscollision avoidance
Airborne Collision Avoidance System
An Airborne Collision Avoidance System (ACAS) is an aircraft system that operates independently of ground-based equipment to provide advice to the pilot on potential conflicting aircraft using transponder signals. Pilots receive resolution advisories (RAs) to alter their flight path to avoid collisions. Under CASR Part 91, the specific operational requirements for ACAS II use are currently reserved (MOS 26.27–26.29), though the requirement to follow RAs is referenced in MOS 11.06.
Key Points
- •Operates independently of air traffic control ground infrastructure.
- •Resolution advisories (RAs) must be followed immediately, even if they conflict with an ATC instruction (MOS 11.06).
- •The pilot must promptly return to the last assigned level when the conflict is resolved and notify ATS.
- •A pilot who complies with an RA does not breach the requirement to comply with an ATC clearance or instruction (regulation 91.257).
- •ACAS II carriage requirements under MOS 26.27–26.29 are currently reserved with no requirements presently prescribed.
Exam Relevance
Knowing that a system Resolution Advisory takes legal precedence over an ATC instruction is a highly testable rule of the air.
Related Terms
Learn More
Dive deeper into instruments with our comprehensive study guide.
Instruments Study Guide →Practice Instruments Questions
Test your understanding with exam-style questions.