Re: India:747/Tu154 collision +TCAS

Date:         15 Nov 96 12:25:37 
From:         rjw@atc.dra.hmg.gb (Richard Weatherill)
Organization: DRA Malvern, England
References:   1 2 3
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"David G. Davidson" <gerhard@onramp.net> wrote:

>With a MODE C Xpndr on the other aircraft only a Traffic Advisory (TA)
>would be issued. To get a Resolution Advisory (RA) the other aircraft
>would have to have a MODE S Xpndr.

>So in this case, if the Saudia AC had an operating TCAS and the TU 154
>had an operating Mode C Xpndr, the Saudia AC should have received a TA
>from TCAS.

This is incorrect. A Traffic Alert can be issued even if the intruder
is squawking Mode A only. A Resolution Advisory depends on a Mode C
squawk (and on the situation becoming sufficiently critical). Mode S
opens up the possibility of co-ordinated RAs between the two aircraft
- if both are so equipped.

Richard Weatherill