An Airbus A321 operated by Lufthansa went into a deep dive after iced sensors were feeding the aircraft's systems false data. According to a
report by German news magazine "Der Spiegel", the plane rapidly descended for minutes, dropping 1000 meters per minute. The crew reportedly only regained control of the aircraft after "switching off onboard computers".
The incident happened to LH1829 enroute from Bilbao to Munich on November 5th, 2014 with 109 peeps on board. Germany's aviation security authority BFU is investigating a "severe incident", the incident will be featured in their latest bulletin due shortly.
AV Herald has a little more detail (apparently, French authorities are a little quicker and more frequent with their bulletins): About 15 minutes after takeoff, LH1829 was climbing through FL310 "when the aircraft on autopilot unexpectedly lowered the nose and entered a descent reaching 4000 fpm rate of descent. The flight crew was able to stop the descent at FL270."
According to AV Herald, the loss of altitude had been caused by two angle of attack sensors having frozen in their positions during climb at an angle. That caused the software to assume a stall. Alpha Protection activated forcing the aircraft to pitch down, which could not be corrected by stick input. The crew disconnected the related Air Data Units and was able to recover the aircraft.
What the German press is labelling a "near crash" scenario apparently isn't an isolated incident: According to "Der Spiegel", Lufthansa records show more than a dozen cases of iced or otherwise blocked sensors. Airbus reportedly offered to replace the sensors in question and updated the software.
Also, the manufacturer issued an emergency airworthiness directive stating that "when Alpha Prot is activated due to blocked AOA probes, the flight control laws order a continuous nose down pitch rate that, in a worst case scenario, cannot be stopped with backward sidestick inputs, even in the full backward position. If the Mach number increases during a nose down order, the AOA value of the Alpha Prot will continue to decrease. As a result, the flight control laws will continue to order a nose down pitch rate, even if the speed is above minimum selectable speed, known as VLS.This condition, if not corrected, could result in loss of control of the aeroplane." Airbus advised airlines to implement procedures for crews to switch off two ADRUs in this case.