The TCAS II credibility window and AF447


TCAS Indicator (Image Source: Public Domain)

In an earlier post, I discussed the possibility that the AF 447 ECAM reported TCAS fault was due to the TCAS becoming inoperative when the TCAS System Performance Monitor had detected that the own aircraft track has coasted for 5 seconds. The latest revision of the BEA interim report provides an analysis of the effects of changes in air speed upon calculated altitude for the A330-200 air data system (BEA 2009). Armed with this data and using the TCAS II MOPS specification we can gain further insights into the magnitude of initial unreliable air data parameters in the AF 447 disaster.

The BEA calculated that for an A330-200 at FL 350 flying at Mach 0.8 in standard atmosphere a decrease in the Mach from 0.8 to 0.3 would cause an increase in temperature of 23 degrees C and a decrease in altitude of 300 ft (BEA 2009). RTCA Change Proposal CP 98 (RTCA 99) states that when an aircraft’s vertical rate is constant or a mild vertical acceleration is taking place (0.25g or less), the credibility window will reject all isolated and all pairs of successive bad measurements with a positive or negative error of 300 ft or more. Based on the TCAS standards fault criteria it therefore appears that the initial icing induced pitot air speed error resulted in a reduction in airspeed of at least 0.5 mach and subsequently causing a reduction in calculated altitude of at least 300 ft, assuming that the pitot sensor failure mode is consistent with other recorded incidents (such as the Air Caraibes incident) involving icing of the pitot sensor.


BEA, Interim report no. 2, on the accident on 1st June 2009, to the Airbus A330-203, registered F-GZCP, operated by Air France flight AF 447 Rio de Janeiro – Paris, Report Number f-cp090601ae2, November 2009.

RTCA SC-186, TCAS Change Proposal CP 98, 23 Sept 1999.

RTCA SC-186, Draft TCAS II Minimum Operational Performance Standards (MOPS) Version 7 (DO-185B), 27 April 1997, accessed from