On 5 November 2014, an Airbus A321 [D-AIDP] being operated by Lufthansa on a passenger flight from Bilbao to Munich was just reaching the top of climb to cruise at FL310 in day VMC when abnormalities in the airspeed display prompted the crew to disengage the Autopilot. A steep descent which could not initially be stopped by crew input to the flight controls then followed and 4000 feet of altitude was lost before flight path control could be regained, albeit using abnormal flight control inputs. Advice by ACARS from Company Maintenance on simplifying control facilitated the completion of the flight, uneventfully, to destination.
After delegation of authority from the State of Occurrence on 11 November, an Investigation was commenced by the German Bundesstelle für Flugunfalluntersuchung (Germany) (BFU) and is ongoing. Data from the SSFDR and the 2-hour SSCVR has been successfully downloaded and has been central to the Investigation, along with aircraft and maintenance documentation and "various witness' statements".
It was noted that both the 52 year-old Commander (16,438 flying hours) and the 35 year old First Officer (6473 flying hours) had achieved most of this time on the A320 series aircraft type.
The aircraft had night-stopped in Bilbao and the incident flight was the first of the day. The First Officer was acting as PF. The departure was made in light rain with a thunderstorm in the area of Bilbao and the climb, with the AP engaged, made in forecast moderate icing conditions. From 8 minutes after take off, the FDR recorded a constant 4.2° angle for AOA1 and, from a minute after that, a constant 4.6° angle for AOA2. The commander advised that he had noticed the indications of the Alpha Protection Band in the Primary Flight Display (PFD) increasing unusually rapidly. As the cleared level of FL310 was neared, the First Officer advised that he had reduced the rate of climb from about 800 fpm to 500 fpm to allow the aircraft to accelerate.
Shortly after this, the First Officer disengaged the AP and made a brief sidestick input to reduce the pitch. The nose of the aircraft then dropped much further and when he attempted to counteract this movement with the sidestick, there was no response. Approximately 45 seconds after the AP disengagement, the First Officer advised the Captain of his inability to arrest the descent and the Commander took over as PF. At that time the pitch angle -3.5° and the rate of descent had reached around 4000 fpm. On applying maximum backward sidestick input, the pitch down began to reduce, the rate of descent decreased and it was possible to regain level flight at FL270. However, this required a continuous backward sidestick input of "more than 50% of the possible input" and the AP could not be re-engaged. No relevant QRH drills could be found.
The two Flight Augmentation Computers (FAC) were then reset by the crew in turn but this had no effect. The crew then sent an ACARS message to Company maintenance control with a short description of the situation and asking whether an in-flight simultaneous reset of the FACs was possible. A reply was received that it was and that doing so would put the flight control system into Alternate Law. The crew sent a further message advising that a constant pull on the sidestick was necessary to maintain level flight, that trim was not available, that the Alpha Protection Band moved very rapidly on the speed display and that the Centralised Fault Display System (CFDS) was showing the failure message "PH6 AOA3". The reply suggested retrieving the AOA values via the Multifunctional Control and Display Unit (MCDU) and, if necessary, turning off ADR 3. The latter was done but made no difference so it was restored. AOA values had already been automatically sent by ACARS and, in a further manual message, the crew were advised that a review of the AOA values indicated that AOA1 and AOA2 may be frozen and therefore registering an incorrect (too high) AOA. The turning off of ADR 1 and 2 was suggested, noting that such action would lead to Alternate Law. A minute later, a follow up message suggested just turning off ADR 2. This was then done and Alternate Law took effect as stated. The continuous sidestick input was no longer necessary in order to keep the aircraft level and the AP was available again.
The crew stated that they had then decided to continue to destination "and use the remaining flight time of about one hour to become familiar with the systems' functions and prepare the landing". The rest of the flight was uneventful and, fifty minutes later, a normal landing at Munich followed.
The three AOA sensors were examined by the Original Equipment Manufacturer (OEM) under supervision and found serviceable. Airbus examined the event data and confirmed the AOA 1 and 2 had frozen at 4.5º and remained so until part way through the descent into Munich. When the AP was disconnected at the top of climb, the Alpha Protection value for the prevailing airspeed was 0.3 º below the recorded AOA values and a corrective pitch down was applied by the protection logic contained in the Elevator Aileron Computer (ELAC). After the minute or so of descent, at a point coincidental with the transfer of control to the Commander, the ELAC rejected ADR 3 data because its AOA 3 input data (it was not frozen) differed from that being supplied to both ADR 1 and 2 and it became possible to fly level manually with two-thirds continuous backward sidestick input.
"Turning off ADR 2 resulted in the flight control system switching into Alternate Law and subsequently the high angle-of-attack protection had also switched off because ADR 3 had already been rejected. After the FAC had been reset, the values of the AOA sensors no longer deviated so strongly that the limits for rejection of one ADR were reached. After ADR 2 had been turned off the FAC had two ADRs available. Once the high angle-of-attack protection had deactivated the autopilot could be engaged again."
Safety Action taken so far as a result of the findings of the Investigation are as follows:
- Airbus communicated with all A320 family operators on 8 December 2014 to advise that if two or three AOA probes are blocked at the same angle, an increase in Mach Number may activate high angle-of-attack protection which would result in a continuous pitch down which backward sidestick inputs may not be able to stop. The interim and immediately-applicable recommended response was to turn off two of the three ADRs in order to put the flight control system into Alternate Law and thus de-activate high angle-of-attack protection. Corresponding FCOM amendments, a preliminary AFM revision and a QRH amendment were also issued with the caveat that the new procedure is provisional pending a final solution for the problem encountered. Similar action was also taken for the A330/A340.
- The EASA issued, on 9 December 2014, an AD to mandate the amended AFM/QRH crew response procedure issued by Airbus and the communication of the information contained to crews prior to their next flight. A similar AD was also issued in respect of the A330/A340.
An Interim Report was published on 17 March 2015. This notes that due to the above Safety Actions, "the BFU has, for the time being, abstained from issuing a Safety Recommendation".