During an IFR flight from Lisbon (LPPT) to Prague (LKPR), the flight crew of a TAP Air Portugal Airbus A320 200N descended below their cleared altitude of 4,000 ft AMSL during the initial phase of an ILS approach to Runway 06.
After passing the IAF BAROX, the aircraft continued descending below the cleared altitude. At approximately 3,500 ft, the Minimum Safe Altitude Warning (MSAW) activated on the Prague APP controller’s display. Air Traffic Control issued an immediate instruction to climb back to 4,000 ft, which was acknowledged by the crew; however, the aircraft continued to descend. A second urgent climb instruction was issued by ATC. Shortly thereafter, the crew reported a TAWS “TERRAIN” warning and initiated a steep climb, reaching approximately 5,800 ft before descending to the cleared altitude of 4,000 ft. The approach was discontinued, and the aircraft was later vectored for a subsequent ILS approach to Runway 06, which was completed safely.
Contributing Factors Identified Controlled rest on the flight deck had been conducted and was terminated only after descent had commenced. Absence of an arrival briefing. Ineffective crew communication and monitoring during a high workload phase. Perception error by the Pilot Flying, who believed the aircraft was high and attempted to intercept the glide slope from above. Failure of Threat and Error Management (TEM) barriers. Delayed response to warnings.