Incident Overview
On 25 January 2006, at approximately 15:15 UTC, an Air France Airbus A320 (call sign AFR 1983) and a Deutsche Lufthansa Airbus A321 (call sign DLH 1072) were involved in a Traffic Alert and Collision Avoidance System (TCAS) Resolution Advisory (RA) in class C airspace within the Prague Flight Information Region (FIR), near the RAK NDB. The aircraft were operating scheduled passenger flights: AFR 1983 from Prague-Ruzyně to Paris-Charles de Gaulle, and DLH 1072 from Munich to Dresden. A third aircraft, a Lufthansa ATR72 (DLH 3272) also travelling from Munich to Prague, was involved in the communications mix-up that led to the incident.
Flight Paths and Communications
AFR 1983 was climbing from FL 150 to FL 230. At 15:14:41, the crew reported passing FL 196. The area controller (WL EC) instructed them to maintain FL 230. Shortly after, at 15:16:17, the controller issued a new instruction to stop the climb at FL 220. At that point, AFR 1983 had already reached FL 215. The controller then directed a 10-degree left turn for spacing.
Meanwhile, DLH 3272 was descending from FL 180 to FL 150 as instructed by WL EC. At 15:14:33, the controller told DLH 3272 to contact the approach controller (APP EC). The instruction was acknowledged, but the same frequency change was also heard and misinterpreted by the crew of DLH 1072.
DLH 1072 was cruising at FL 240. Upon hearing the instruction meant for DLH 3272, the DLH 1072 crew called APP EC and mistakenly reported descending to FL 120. Despite the controller having no radar contact with DLH 1072 at that time (due to a filter limiting the display below FL 150), the crew began descending from FL 240.
TCAS Resolution Advisory
At 15:16:11, the area controller's Short Term Conflict Alert (STCA) activated. He instructed DLH 1072 to stop descent at FL 230, but the crew did not acknowledge. He then ordered AFR 1983 to stop climb at FL 220 and turn left. AFR 1983 had reached FL 215.
At 15:16:06, APP EC, now aware of DLH 1072's erroneous descent, instructed them to maintain FL 220. However, by then DLH 1072 was passing FL 233. The crew immediately reported a TCAS RA, stating they were climbing. The controller told them to climb back to FL 220. DLH 1072 climbed to FL 237 and reported the conflict resolved.
AFR 1983 received a TCAS descent advisory and descended. The crew reported descending and then resuming climb to FL 230 after the conflict ended. The controller informed AFR 1983 that the conflicting traffic was DLH 1072, which had descended by mistake.
Aftermath
After the incident, both aircraft coordinated with ATC. DLH 1072 was instructed to descend to FL 170 and apologized for the error. AFR 1983 continued to FL 280 and then switched to Rhein ACC. The DLH 1072 crew later apologized and filed an airprox report. All flights continued to their destinations without further event.
Investigation
The Czech Accident Investigation Institute (ÚZPLN) investigated the incident. The report documented the sequence of events, including the miscommunication that led to DLH 1072's unsolicited descent. No probable cause was explicitly stated in the provided source.