ACAS and the South German Midair

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From: Peter B. Ladkin (ladkin(at)
Date: Mon 12 Aug 2002 - 16:22:42 BST

I have put an extended version of the ACAS/Midair note on my WWW site,
accessible directly from 

For those who haven't had the chance to read it, but are nevertheless
interested, the paper points out some problems with ACAS; namely, that it
seems to have difficulty with some three-aircraft scenarios. In particular,
it argues that, given a three-aircraft scenario similar to that which 
obtained in the Southern German midair collision on July 1, the decision
to manoeuvre counter to an RA can be a least irrational strategy.

Andres Zellweger's scepticism concerning my brief comments on multiple-aircraft
conflict scenarios let me to argue these scenarios more carefully (to some
benefit - I had been mistaken about one scenario). There is one class of
conflicts which I do not see can be solved by the current TCAS II, namely
those in which an aircraft C is descending towards an aircraft B already
ascending due to an RA with an aircraft A, which got an RA to descend.
If the BC-conflict leads to a crossing RA, and aircraft C is already within
the alert threshold for a conflict with aircraft A, then I do not see that
the TCAS specification and algorithms can ensure a resolution between 
A and C.

The paper also benefitted from comments by Ira Rimson.


Peter B. Ladkin PhD FBCS CW(hon)
Professor of Computer Networks and Distributed Systems,
Faculty of Technology, University of Bielefeld, 33594 Bielefeld, Germany
Tel (Vx/msg/Fax) +49 (0)521 880 7319
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