Wizz A321 at Sofia on Feb 26th 2018, unreliable airspeed

Last Update: February 13, 2022 / 19:02:57 GMT/Zulu time

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Incident Facts

Date of incident
Feb 26, 2018

Classification
Incident

Airline
Wizz Air

Flight number
W6-4427

Departure
Sofia, Bulgaria

Destination
Tel Aviv, Israel

Aircraft Registration
HA-LXP

Aircraft Type
Airbus A321

ICAO Type Designator
A321

Airport
Sofia Airport, Sofia

Airport ICAO Code
LBSF

A Wizz Air Airbus A321-200, registration HA-LXP performing flight W6-4427 from Sofia (Bulgaria) to Tel Aviv (Israel) with 220 people on board, was climbing to FL230 out of Sofia when the crew requested to stop climb at 10,000 feet MSL, accepted a climb to 12,000 feet due to terrain and entered a hold. The crew subsequently declared PAN, PAN, PAN reporting unreliable airspeed. After checking weather conditions around the aircraft climbed to FL200 and diverted to Budapest (Hungary) for a safe landing on runway 31R about 105 minutes after departure from Sofia.

A replacement A321-200 registration HA-LXU reached Tel Aviv with a delay of 6 hours.

The airline reported the aircraft diverted to Budapest due to technical reasons. Another aircraft continued the flight.

A few hours later another A321 needed to divert from Sofia to Budapest for similiar reasons, see Incident: Wizz A321 at Sofia on Feb 26th 2018, unreliable airspeed indications, another company aircraft rejected takeoff, see Incident: Wizz A321 at Sofia on Feb 26th 2018, rejected takeoff due to unreliable airspeed.

On Mar 8th 2018 Bulgaria's AAIU reported the aircraft had been parked on the apron in Sofia for more than 6 hours in low temperatures and snow fall, then had been de-iced. The aircraft's airspeed indicators showed a wide range of unreliable values and variations during the initial climb prompting the crew to divert to Budapest. The occurrence was rated a serious incident and is being investigated by Bulgaria's AAIU.

On Feb 13th 2022 the Bulgarian AAIU released their final report into all three occurrences at Sofia of Feb 26th 2018 concluding the probable causes of the serious incidents were:

Based on the analysis performed, the Commission points out that the serious incident resulted from the following causes:

1. Non-compliance with the adverse weather conditions of snowfall by the ground crews performing and participating in the preparation of the aircraft for the flights related to the serious incident.

2. Admission to flight of the aircraft with the presence of ice after De-icing procedure with anti-icing fluid by the ground crew, organizing and performing the treatment.

The AAIU analysed that there were no deficiencies in the contracts regarding de-icing procedures carried out by the contracted ground handling operator. There also were no deficiencies in the task cards with respect to de-icing at Sofia Airport that day.

The AAIU also analysed that there were no deficiencies in the flight crews' flight preparations and actions. The AAIU wrote:

The Commission did not detect the weaknesses in the actions of the flight crews, errors and deviations from the prescribed procedures. Pre-flight preparation has been thoroughly conducted, severe weather and RWY conditions have been foreseen. It is a pilots responsibility to reject or to continue take-off and the decision to ensure safety has been taken in time deficit.-Thanks to the pilot’s skills, they have coped with the difficult situation and the flights have ended uneventfully.

The AAIU wrote however:

There is no evidence of snow removal and De-icing protective spraying on the front of the fuselage of the aircraft. The terms and responsibilities of the contract between the aviation operator and airport operators are set out in the ground handling agreement cover sheet in force at the time of the event.

The AAIU wrote:

The aircrafts of WIZZ AIR landed the day before at Sofia Airport and during their stay were covered with a significant amount of snow, and the fuselage of the aircraft has cooled to temperatures below 0°. According to the explanations of the ground crew during the preparation, the aircraft were treated with Type II liquid in the areas of the wing and tails, but not in the area of the front part of the cabin above the receivers for dynamic and static pressure.

In preparation for the flight, the cockpit and passenger cabin are heated, as a result of which the snow accumulated on the aircraft of the aircraft melted and running down, which in this area has a lower temperature for obvious reasons and the melted water freeze in the form of ice ridges. Such ice ridges are often observed on the lateral and lower surface of the fuselage of aircraft when the snow in the upper part of the fuselage is not removed and melted after warming the aircraft cabins during parking on apron, taxiing or waiting for De-icing procedure or other delay before takeoff. (Editorial note: watch the similiarities to this serious incident: Incident: S7 A21N at Magadan on Dec 2nd 2021, unreliable airspeed).

The AAIU continued analysis:

All aircraft of the "family" of Airbus A320 - (A318, A319, A320 and A321) are similar in appearance, with small differences in dimensions. The procedures for anti-icing and de-icing of the different operators are similar. Quality control of the completed activity is performed in the De-icing Pads by a ground crew who reports by the radio to the flight crew about the results. In all three cases, the inspections were carried out by three different personnel who did not detect the untreated lower nose fuselage of the aircraft.

The lack of adequate training of the staff performing the work directly and of the controlling personnel’s after that has significantly contributed to the poor quality and incomplete de-icing/anti-icing procedure. As a result, pilots are unprepared for the difficult situation of running take-off in bad weather and poor surface friction on RWY.

Metars:
LBSF 260800Z 05007KT 2200 SN FEW010 BKN031 OVC039 M07/M09 Q1008 WS ALL RWY R09/490293 TEMPO 1200 SN BKN008=
LBSF 260730Z 05007KT 2400 SN SCT010 OVC035 M07/M10 Q1008 WS ALL RWY R99/490093 TEMPO 1200 SN BKN008=
LBSF 260700Z 05007KT 2800 SN FEW009 SCT024 OVC030 M07/M09 Q1009 WS ALL RWY R99/490093 TEMPO 1200 SN BKN005=
LBSF 260630Z 04007KT 2000 SN FEW007 SCT009 OVC027 M07/M09 Q1009 WS ALL RWY R09/490093 TEMPO 1200 SN BKN005=
LBSF 260600Z 04006KT 1800 SN FEW009 BKN015 OVC025 M08/M09 Q1009 R09/490292 TEMPO 0600 +SN BKN003=
LBSF 260530Z 06007KT 030V090 1400 R09/1600U SN SCT011 OVC013 M07/M08 Q1009 R99/490392 TEMPO 0600 +SN BKN003=
LBSF 260500Z 06007KT 1600 SN FEW004 SCT008 OVC013 M07/M08 Q1009 R09/490392 TEMPO 0600 +SN BKN003=
LBSF 260430Z 06006KT 1600 SN VV009 M07/M07 Q1009 R99/490293 TEMPO 0600 +SN BKN003=
LBSF 260400Z 07007KT 1600 SN VV008 M06/M06 Q1009 R99/490293 TEMPO 0600 +SN BKN003=
LBSF 260330Z 08007KT 1700 SN VV010 M06/M06 Q1009 R99/490293 TEMPO 0600 +SN BKN003=
LBSF 260300Z 07005KT 1400 R09/1600U SN VV007 M06/M07 Q1009 R09/490293 TEMPO 0600 +SN BKN003=