Features Safety
Aviation Safety: Separation Anxieties

On July 1, two commercial jets crashed over southern Germany
resulting in the death of 71 people, including 52 children. The
accident occurred when a chartered Bashkirian Airlines Tupolev 154 and
a DHL Boeing 757 freighter collided over the northern shore of Lake
Constance near Überlingen. The Tu-154 was on a charter flight from
Moscow to Barcelona, and the B757 was on a routine cargo flight from
Bahrain to Brussels. The flights had just entered Swiss airspace, and
were communicating with Zürich Control.

According to information available, both jets received a TCAS
"TRAFFIC, TRAFFIC" alert a little less than one minute before the
collision. Approximately 15 seconds later, both TCAS systems issued a
resolution advisory (RA) warning the Tu-154 to climb and the 757 to
descend. One second after the RA, Zürich Control instructed the Tu-154
to "descend flight level 350, expedite, I have crossing traffic."
Fourteen seconds later, having received no response, the controller
again urged the Tu-154 crew to "descend level 350, expedite descent."
Both aircraft were on the same frequency, and no air traffic control
warning was issued to the DHL 757. The flights were cruising at 36,000
feet and initiated their descent roughly 30 seconds before impact; the
757 pilots obeying the TCAS "DESCEND" command, and seemingly the Tu-154
crew obeying ATC instead of the TCAS "CLIMB" command. Fourteen seconds
after the air traffic controller's first warning, and 30 seconds before
the crash, the Russian pilot hesitated briefly, and then began his
descent. Shortly thereafter, the two aircraft collided at 35,300 feet.

Although it is still too early to draw conclusions, one has to
wonder was this tragic accident the result of human error or was it a
failure of TCAS technology and RVSM procedures? It is expected that the
accident investigators will be analyzing potential ATC system
deficiencies, cockpit procedures, and close attention will be paid to
the role of TCAS and RVSM.