Re: India:747/Tu154 collision +TCAS

Date:         13 Nov 96 02:41:21 
From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
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In article <airliners.1996.2386@ohare.Chicago.COM> writes:
>By the time this reaches the newsgroup this will be old news.
>But my question: shouldn't the Saudia 747 have had TCAS,
>sinc ethey do frequent US airspace?
>I -both- a/c need TCAS to make it effective?

In order to work, all airplanes in the airspace need to have Mode C-capable
transponders.  These transmit altitude information along with their data
packet.  Mode C transponders are cheap, and can be found in Cessna 152s on

TCAS is just a transponder-pinger.  It is not anything like tactical radar.
This is one reason that airspace representing the highest risk in the
United States requires that all airplanes participating have Mode C
transponder capability.  If that capability fails, the airplane would,
in effect, be invisible to airplanes with a TCAS capability.

TCAS, unfortunately, is a LOT more expensive and complex, compared to
transponders, so its use is restricted to operators flying complex,
expensive equipment.  That just means the big planes have the option of
detecting and avoiding smaller airplanes, and not vice-versa.

It is also important to keep in mind that TCAS is just a warning system.
ATC is still predicated on "see and avoid" when possible, and adherence
to standard procedures and flight plans by both pilots and ATC and the
competence of ATC when not.

Robert Dorsett                         Moderator, sci.aeronautics.simulation