What happened
On September 21, 2000, at 09:35 UTC, a commercial flight operated by Air Algérie, a Boeing 737-200 (DAH1170) traveling from Constantine to Basel, entered a near-collision course with an Air France BA46 (AFR1217) operating from Florence to Paris.
While the Air Algérie crew had been cleared to descend from flight level 310 to 270, the aircraft descended below the authorized altitude without permission. During this unauthorized descent, the aircraft passed through flight level 252. Upon being queried by the Geneva Area Control Centre (ACC) regarding its altitude, the pilot realized the error and began climbing back to the assigned flight level 270. During this recovery maneuver, the Boeing 737-200 crossed the path of the Air France aircraft with a separation of only 300 feet and a lateral distance of 1.3 nautical miles.
The investigation
The investigation focused on the sequence of events within the Geneva ACC jurisdiction. The radar controller observed that the altitude of DAH1170 was lower than permitted and subsequently prompted the crew to confirm their level. The investigation also noted that the Short Term Conflict Alert (STCA) system triggered a proximity warning during the event.
Investigators examined the communications between the aircraft and the controller, confirming that while the initial descent clearance to flight level 270 had been read back correctly, the subsequent unauthorized descent occurred after the aircraft was transferred to the INI EST sector frequency. It was also noted that the Air Algérie aircraft was not equipped with a TCAS/ACAS at the time, though the airline held a valid EUROCONTROL derogation for European airspace.
Findings
- The primary cause of the incident was the failure of the Air Algérie pilot to comply with the ATC clearance.
- The aircraft descended below the authorized flight level 270 without authorization.
- The loss of separation occurred because the pilot's attempt to climb back to the assigned altitude caused the aircraft to cross the flight path of AFR1217 at flight level 260.