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XL Airways Germany Flight 888T
Airbus A329
27 November 2008

The investigators’ interest focused on the Air Data Inertial Reference Unit (ADIRU) following recent similar incidents involving Airbus A330s operated by Qantas, exhibiting sudden uncommanded manoeuvring.

Analysis of the data led to an interim finding that the crew lost control of the aircraft. While conducting a planned test of low-speed flight at low altitude, the aircraft was descending through 3000 feet on full autopilot for a go-around. Landing gear was just extended when at 15:44:30 UTC the speed dropped from 136 to 99 knots in 35 seconds. The stall warning sounded four times during violent manoeuvring to regain control. By 15:46:00 the warning had silenced as the aircraft regained speed in a rapid descent, but six seconds later, at 263 knots, the aircraft had only 340 feet elevation and was 14 degrees nose down. A second later it was in the water.

In September 2010, the BEA published its final report into the accident. One of the contributing causes was incorrect maintenance procedures which allowed water to enter the angle of attack (AOA) sensors. The water then froze in flight, rendering the sensors inoperative and thus removing the protection they provided from the aircraft’s flight management system. The primary cause of the accident was that when the crew attempted an improvised test of the AOA warning system (which was not functioning due to the blocked sensors) they lost control of the aircraft. The aircraft’s computers received conflicting information, and completely disconnected. This led to all automation built into the aircraft’s systems to deactivate. Since the pilots were likely relying on the computer to optimize the aircraft to climb, and it didn’t, the crew was unable to recover from the dive. The crew was unaware that the AOA sensors were blocked, but they also disregarded the proper speed limits for the tests they were performing, resulting in a stall. Five safety recommendations were made.

There has been a LOT of issues with the pitot tubes on any Airbus airliner equipped with fly-by-wire systems where if the pitot tubes are not properly kept heated at altitude, the resulting freezing of the pitot tube can cause abnormal behavior of the fly-by-wire system. This is why once the black boxes from this flight is analyzed, they need to compare against what happened on Air France Flight 447 and possibly Indonesia AirAsia Flight 8501, where fly-by-wire abnormalities caused by possibly frozen pitot tubes caused flight crew reactions that may be crashed both planes.