Citing last summer’s midair collision between a DHL International Airways Boeing 757 and a Bashkirian Airlines Tupolev Tu-154 over southern Germany, the NTSB has recommended that the FAA modify its ATC data-processing backup systems to provide conflict alerts to the greatest extent possible.

The collision occurred in a corner of the en route portion of the Zurich, Switzerland  area control center’s airspace (near the northeastern limit of the controller’s area of responsibility) while both airplanes were operating under IFR and in communication with ATC personnel at the facility. A single controller in Zurich was responsible for en route airspace in the eastern half of Switzerland, portions of southern Germany and an approach control sector in the vicinity of Lake Constance, Germany, encompassing the Friedrichshafen Airport.

Approximately five minutes before the collision, the controller was working three airplanes–the two accident aircraft and an Airbus A320 approaching Friedrichshafen. The controller had just handed off two other aircraft to other controllers and had just received the Tu-154. The A320 pilot was communicating with the controller on a different frequency from the accident airplanes and, due to a communication line outage, the controller was relaying messages to the Friedrichshafen tower through the Airbus pilot. The controller was using two different displays–one that covered a large range for en route aircraft and another focused on the Friedrichshafen Airport area.

At 2334:49, when the 757 and the Tu-154 were about 6.5 nm apart, the controller instructed the Tu-154 pilot to descend to FL350. Seven seconds later, the controller repeated the instruction and told the pilot to “expedite” because of “crossing traffic.”

At 2335:07, the Tu-154 pilot responded that he was descending. At the same time, radar data indicated the 757 began a descent. Twelve seconds later the 757 pilot said he was descending as a result of a TCAS advisory. The controller did not respond. At 2335:34, the airplanes collided, killing the two crewmembers aboard the cargo 757 and the 71 passengers and crew on the Tu-154.

The NTSB said that, on the night of the accident, the Zurich area control center was operating with reduced equipment capability. Due to scheduled maintenance, the primary ATC radar-processing system was not available to the controllers, and a backup system similar to the direct access radar channel (DARC) backup system in the U.S. was in use. As in U.S. airspace, the primary ATC radar-processing system provides automated conflict alerts by monitoring aircraft course, speed and altitude by providing a visual and audio warning to controllers when aircraft are projected to come closer than allowed by prescribed separations. The Zurich backup system is not equipped to provide such automated conflict-alert warnings to controllers.

In an ATC simulation of the circumstances leading to the Lake Constance accident, the FAA Technical Center demonstrated that–assuming the primary operating system was functioning–the FAA’s en route ATC radar processing equipment would have detected the conflict and provided conflict-alert warnings sufficiently in advance. Conflict-alert functionality does not extend to all ATC backup systems, however.

The availability of conflict alerts in the backup system for Tracon facilities also depends on what equipment and software is in use and the cause of the primary system failure. Although the newer automated radar terminal systems (ARTS) are somewhat more sophisticated than the older ARTS 3A, the Safety Board noted that all versions of ARTS are susceptible to failure modes that leave conflict alert unavailable. Even the new Stars has no conflict-alert function in its backup.

The Safety Board said there does not appear to be any obstacle in the computer software or hardware that would prevent ATC backup radar-processing systems from providing conflict-alert capabilities.