Erroneous interpretation of a clearance, which went uncorrected, resulted in a Luxair Embraer ERJ-145's descending far below the minimum altitude on approach to Madrid, investigators have found.
The aircraft had been following the BAN 3B standard arrival which requires a minimum altitude of 10,000ft ahead of the initial approach fix, designated TAGOM.
But while a Madrid sector controller instructed the crew to "continue descent to 10,000ft" to be levelled at TAGOM, the crew instead replied that they were "descending 5,000ft". This readback error was not corrected.
Shortly afterwards, the flight was transferred to a different sector controller, by which time it had descended to below 7,700ft - beneath both the minimum chart and radar-vectoring altitudes.
Luxair's crew informed this controller that the flight was descending to 5,000ft to TAGOM, and was told to maintain heading after the waypoint.
The descent was not challenged and the aircraft continued to lose height before a terrain-proximity warning ordered the crew to "pull up". It reached a minimum altitude of 6,290ft.
Only after being given a turn instruction did the pilots tell the controller that they were maintaining 7,000ft owing to the presence of mountains.
Spanish investigation authority CIAIAC says that the pilots recognised that they had acknowledged the original descent instruction incorrectly. But they pointed out that a clearance to 10,000ft was, in their experience, more commonly given as "one zero thousand" rather than the controller's improper phraseology.
The ERJ-145 was below the minimum procedural altitude for 4min and below vectoring altitude for 3min, says CIAIAC.
No-one was injured in the 4 August 2011 incident. CIAIAC is recommending that the Spanish air traffic control system SACTA be modified to activate its altitude-alert function.