Safety analysis of upgrading to TCAS Version 7.1 using the 2008 U.S. Correlated Encounter Model
May 11, 2009
As a result of monitoring and modeling efforts by Eurocontrol and the FAA, two change proposals have been created to change the TCAS II V9.0 logic. The first, CP-112E, addresses the safety issues referred to as SA01. SA01 events have to do with the reversal logic contained in the TCAS algorithm, e.g., when TCAS reverses the sense of an RA from climb to descend. Typically, reversals occur to resolve deteriorating conditions during and encounter. V7.0 contained reversal logic based on certain assumptions and engineering judgment, but operational experience obtained since deployment has compelled a re-evaluation in areas of that logic, specifically having to do with late reversals. The second change proposal, CP-115, rectifies observed confusion surrounding the aural annunication AVSA during an RA by replacing it with the annunciation LOLO, and changing the TCAS V7.0 display and logic to appropriately support the change. Collectively, the changes to teh TCAS logic in both CP-112E and CP115 are referred to as TCAS II V7.1. Included in this document is a safety study that consideres V7.1 as a whole, and also the first safety study that uses teh U.S. correlated encounter model developed by Lincoln Laboratory for testing TCAS. Also included is a discussion of simulation capabilites developed at Lincoln Laboratory for evaluating CP-115 and for future analysis of TCAS in high density areas. Our study indicates that mroe risk lies in remaining with the current version of TCAS over upgrading to V7.1, and that no negative impact on safety in high density airspace occurs as a result of CP-115.