Saturday, December 3, 2011

Case Study: Ueberlingen Midair Collision

Before we move any further, lets do a case study to drive home the point of Errors and Violations.

On the night of July 1, 2002, a Boeing 757 collided with a Tupolev-154 at 35,000 feet, resulting in 71 fatalities. Initially, this accident was immediately blamed on two individuals. First, the pilot of the Tupolev aircraft and second the controller on duty. Let us re-examine the event, highlighting fundamental human and system errors that occurred that night: errors that contributed to one of the worst midair collisions in recent history.

Kindly visit the following links to view video re-construction  of the events. The first link is a 10 minute video that those of you who may be short of time can watch to get the essentials of the accident. Those with more time at hand may prefer the second video that discusses the case in much more detail over a 45 minute period.

The following narrative draws heavily from the research paper presented by Dr. Ashley Nunes & Dr. Tom Laursen of the University of Illinois, Aviation Human Factors Division Savoy, IL in coordination with Sky guide, Air Traffic Control Operations, Zurich Area Control Center, Switzerland at the 48th Annual Chapter Meeting of the Human Factors and Ergonomics Society, September 20 - 24, 2004, New Orleans, LA, USA.

Known Sequence of Events

The Boeing 757 (registered to DHL) was en route from Bergamo (Italy) to Brussels on a heading of 004 degrees at FL 260. The Tupolev-154 (registered to Bashkirian Airlines) was flying from Munich to Barcelona on a heading of 254 degrees at FL 360, correcting its heading twice within the last minute to end up on heading of 274 degrees. Both aircraft were equipped with the Traffic Collision and Avoidance System (TCAS) and their trajectories put them on a converging course at a 90° angle in airspace above Lake Constance, Germany.

Under a contractual agreement between the German and Swiss government, this airspace was under the authority of the Zurich Area Control Center (ACC). After making contact with the B757, the Swiss controller issued two clearances to the B757. First he cleared the B757 to climb to FL 320 and at time 21.26.36 to climb to FL 360. At time 21.30.11 the T-154 called in. After that, the Swiss controller did not initiate any contact with either aircraft until just seconds before the TCAS system aboard gave both pilots a traffic advisory. Following this, the controller instructed the T-154 to descend from FL 360 to FL 350 to avoid collision with the B757. However, the TCAS on board the T-154 and B757 instructed the pilots to climb and descend respectively. After receiving contradictory instructions, the T-154 pilot opted to obey controller orders and began a descent to FL 350 where it collided with the B757, which had followed its own TCAS advisory to descend. All 71 people were killed.