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Computer-Related Incidents with Commercial Aircraft

A340 and A330, airprox, North Atlantic Track E, 2 October 2000

2 October 2000

Synopsis An A340 and an A330 were both westbound on North Atlantic Track E, at Flight Level 360 and Flight Level 370 (1,000 ft above) respectively, under Reduced Vertical Separation Minima (RVSM). The A340 hit some clear-air turbulence, a "bump" which disconnected the autopilot, led to a change in control law and resulted in a "balloon climb". The aircraft were in sight of each other, with the A340 below and in front of the A330. The A330 captain felt the "bump" and noticed the wings flex on the A340. He observed the A340 suddenly climb, and received a TCAS Resolution Advisory (RA). Before he could react to the TCAS RA, the A340 passed through his altitude some 200 or so feet to his left. A close call. One could imagine that the A330 captain might have had time to react had the aircraft been separated by 2,000 ft (Conventional Vertical Separation Minima, or CVSM). The U.K. AAIB recommended in their Report that the overtaking procedures under RVSM be reviewed. Also that the incident be brought to the attention of Eurocontrol to consider its impact on the RVSM Safety Case. (I met with the Eurocontrol RVSM Managers concerning the Safety Case in late 2002. The Safety Case asserts that there are no interactions between RVSM and TCAS which needed to be considered. I do not recall that the Managers had anything to say about this incident.)

I recounted this incident in an article An automation-related AIRPROX incident in the Risks Digest, Volume 22 Number 19, 19 August, 2002.