What happened
On August 4, 2011, an Embraer 145 LU, registration LX-LGX, operated by Luxair, was performing a scheduled international passenger flight from Luxembourg International Airport to Madrid-Barajas Airport. During the approach phase, the aircraft was cleared by the RES sector controller to descend to 10,000 feet. However, the crew erroneously acknowledged a descent to 5,000 feet.
As the aircraft continued its descent, it breached the minimum altitudes specified by the Standard Terminal Arrival Route (STAR) and the minimum radar vectoring altitude. The aircraft reached a minimum altitude of 6,290 ft before the Enhanced Ground Proximity Warning System (EGPWS) issued "TERRAIN TERRAIN" and "TERRAIN PULL UP" alerts. The crew immediately disengaged the autopilot and initiated a climb to escape the terrain. The situation was only realized by air traffic controllers after the crew reported they were maintaining 7,000 feet due to nearby mountains.
The investigation
The investigation examined flight data from the Quick Access Recorder (QAR) and communications between the crew and two ATC sectors (RES and AIS). The investigation established that the aircraft had been flying below the required procedure altitude for approximately four minutes and below the minimum radar vectoring altitude for three minutes without detection by the crew or controllers.
Investigators also reviewed the ATC procedures and the capabilities of the SACTA surveillance system. It was noted that while the system includes a "Minimums Alert" function, this feature was not enabled at the time of the incident. Furthermore, the investigation looked into the specific roles of the planning controllers at the approach control station, finding that their exact tasks were not explicitly documented.