The document provides an overview of ACAS X from multiple perspectives: - Technically, ACAS X is an aircraft collision avoidance system using surveillance systems and software to decide when to provide alerts to pilots. - From a human factors perspective, it displays nearby traffic and generates advisories like "Traffic Traffic" or manuever guidance like "Descend Descend". Pilots are required to follow Resolution Advisories. - Safety analyses show ACAS reduces collision risk by about 10% with one equipped aircraft and 1% with two, assuming pilots follow advisories, though about half of remaining risk is due to ACAS-caused collisions.

1. ACAS X Overview
Prepared by Garfield Dean
ACAS X intro

2. Overview
 Technical view
 Human factors / Procedural view
 Safety view
 Standards view
 Certification view
 Cost benefit view
 Legal view
ACAS X intro Page 2

3. Technical View
ACAS X is a collision avoidance system
• A communication and surveillance system (radar, ADS-B)
• With software that decides when to alert
• And human interfaces (display and aural alerts)
ACAS X intro Page 3

4. Human Factors view
 Display nearby traffic
 Generate warning alert (“Traffic Traffic”, highlight
intruder) known as TA (traffic advisory)
 Generate manoeuvre advice (“Descend Descend”,
highlight intruder) known as RA (Resolution Advisory)
ACAS X intro Page 4

5. Procedural view
 ICAO PANS ops, PANS ATM define procedures
 Pilot uses the TA to prepare for a possible RA
 Turn off autopilot
 Look for the intruder
 Pilot shall follow the RA
 Unless this will jeopardize the safety of the aircraft
 Pilot shall inform ATC about RA
 Start of RA and clear of conflict
 Controller shall not issue clearances or instructions if
told about an RA by pilot
 Beware of variations
 (by countries, service providers, airlines etc.)
ACAS X intro Page 5

6. Safety view
 Safety performance (system works as designed)
 Risk ratio
 Risk of collision with ACAS / Risk of collision without ACAS
 About 10% with 1 aircraft equipped, 1% with 2 aircraft equipped
 Assumes pilots follow their RAs
 Approximately half of remaining risk is due to collisions caused
by ACAS
 Tested with billions of simulations
 Safety risks (system fails)
 Very small component of overall risk
 Aircraft are allowed to fly without
ACAS for up to 10 days
ACAS X intro Page 6

7. Standards view
 ICAO Standards and Recommended Practices (SARPS)
 Worldwide standard (Annex 10 vol. 4)
 In practice follow technical standards
 RTCA / EUROCAE standards
 Minimum Operating Performance Standards (MOPS) DO-
185B / ED143
 Based on requirements analysis, extensive study, consensus
among experts.
ACAS X intro Page 7

8. Certification view
 FAA Technical Standard Order (TSO – C119)
 Describes one way (not the only way) to have parts and
appliances approved.
 Relies heavily on MOPS and references to other
(environmental, software, hardware) standards.
 Europe (EASA) has an equivalent document E-TSO – C119
 FAA/ EASA certification officers follow the MOPS
development to ensure that the material is suitable to
develop a TSO.
 European legal mandate for ACAS makes reference to
the E-TSO.
ACAS X intro Page 8

9. Cost benefit
 $100,000 equipment
 $50,000 to install
 $5,000 to upgrade
 Software only
ACAS X intro Page 9
 Reduce collisions by
factor of about 4
 Justifies a mandate for
all aircraft with more
than 19 passengers or
5700kg
 Justifies an upgrade if it
saves 1 mid air collision
in 15 years in Europe.

10. Legal view
 ICAO mandate for ACAS
 Local implementations of the mandate
 Refer to TSO / ETSO
 Uberlingen Accident
 Barcelona court of appeal Judgment
ACAS X intro Page 10