Flying - A-10 follow-up

[azur@netcom.com (Steve Schear)]

Date: Thu, 01 May 97 08:33:00 CDT
From: Corkum D Capt        2OSS/OSTC <corkumd@oss2.barksdale.af.mil>
Subject: Flying - A-10 follow-up

Corkum D Capt        2OSS/OSTC <corkumd@oss2.barksdale.af.mil>

"Both the A-10's transponder and the autopilot appeared to have been turned
off.  Perhaps there is a simple technical explanation why no one noticed
anything at first."

Let me try and fill in a few of the holes for non-flyers:

When aircraft fly in close formation typically only the lead aircraft has a
transponder turned on.  This is due to limitations with Air Traffic Control
(ATC) systems.  These systems are designed to prevent two aircraft from
getting too close together thereby preventing possible midair collisions.
Unfortunately the computers can't distinguish between aircraft that are on
a collision course and aircraft in formation that are intentionally close
together.  For this reason ATC operators often request wingmen to turn off
their transponders.

Once the A-10 dropped back he was not easily visible to the lead aircraft
who would be concentrating on entering the bombing range, working
coordination with ATC and the range controllers etc.  Inside the range ATC
no longer   maintains operational control of the aircraft and would not be
following any of the aircraft directly.

The first indication of the aircraft missing would be a failure of the
wingman to respond to a radio call.  When this happened it would first be
assumed that he was on the wrong frequency.  The lead aircraft would try to
find him on another frequency, then attempt to call him on the emergency
frequency "Guard".  When this failed the lead would likely assume that the
wingman had radio problems and would attempt to locate him visually. Only
after he failed to locate him visually would he begin to worry that
something happened.  His first   thought would be that the aircraft had a
major malfunction such as complete loss of electrical power, that forced
the pilot to abort the mission and return to base, but also prevented him
from calling to the lead aircraft.  The other
possibility would be that the aircraft was down due to pilot error or
aircraft malfunction.  The idea that the wingman simply took the aircraft
would likely never occur to the lead pilot.

The lead pilot at this point would abort his mission and attempt to
ascertain what happened to his wingman. He would contact ATC and ask if
they had  transponder track on the aircraft or reports of an aircraft down.
He would also perform a visual search of the area and attempt to call back
to the home base to see if they knew what happened.  By this time the rogue
pilot may have been gone for at least 30 minutes and was actually 200 miles
away.


The reason ATC could not track him is a technical issue.  Modern ATC radars
rely primarily on the aircraft transponder to provide location and altitude
information.  The actual radar returns "know as raw data" are only used as
a secondary means to track aircraft when their transponders have failed.
This must be performed manually since the computer has no way to
distinguish one radar return from another without the transponder.   This
method is only used when an ATC controller knows an aircraft transponder
has failed and must be
manually tracked.  Since the A-10 was not supposed to have his transponder
on, no-one would notice when he broke formation.  Only after the fact when
the FAA reviewed the taped radar returns were they able to piece together
the likely course of the A-10.

I hope this explained some of the more technical reasons for what happened.

David Corkum


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