Wizz A320 at Sofia on Jan 3rd 2016, inadvertently retracted flaps on final approach

Last Update: March 23, 2021 / 08:19:39 GMT/Zulu time

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Incident Facts

Date of incident
Jan 3, 2016


Wizz Air

Flight number

Sofia, Bulgaria

Aircraft Registration

Aircraft Type
Airbus A320

ICAO Type Designator

Airport ICAO Code

A Wizz Air Airbus A320-200, registration HA-LYP performing flight W6-4326 from Eindhoven (Netherlands) to Sofia (Bulgaria), was on an ILS approach to Sofia's runway 09, gear down and flaps in position 2, about 4nm before touchdown, when the pilot flying commanded flaps 3 and full, the pilot monitoring however inadvertently selected flaps 0 (flaps up). The pilot flying noticed that the lowest selectable airspeed began to increase, selected full power and initiated a go around. After the loss of energy had been compensated, the pilot flying selected CLIMB power, however, soon after noticed that the airspeed was decaying again and the aircraft began to descend again, the GPWS issued aural alert "SINK RATE!". The pilot flying therefore selected TOGA, disconnected the flight directors, stabilized the aircraft and climbed out. The aircraft positioned for another approach to Sofia's runway 09 and landed safely about 15 minutes after going around.

Hungary's KBSZ reported that initial investigation results suggest, that due to the selection of flaps 0 the Go Around Mode was no longer armed, the Flight Directors therefore still were following the LOC and Glidepath indications with the active mode still being APPROACH. This had resulted in automatic thrust setting at IDLE thrust. The occurrence was rated a serious incident, Hungary's KBSZ is investigating the occurrence (editorial note: no explanation as to why Bulgaria's AAIB isn't investigating, the investigation was probably delegated to Hungary).

Some time in past (around Feb 2020) the KBSZ released their final report explaining: "As the occurrence was not treated as a serious incident at the beginning, the IC was only informed on it several days later. The Bulgarian investigating organisation stated they did not intend to investigate into the occurrence or to send a representative to take part in it. Subsequently, it was TSB that started to investigate into the occurrence" and concluding the probable causes of the serious incident were:

The IC concluded during the investigation that the cause of the occurrence was:

− that the PM retracted the slats/flaps in contrast to the PF’s instruction which was to further extend them.

In addition to the above, the IC presumes the following probable causes:

− the flight crew did not follow the company standard operating procedure in regards of go-around flow, callouts and check/review of SLAT/FLAP movement when setting the flap lever to a different position;

− late disconnection of automation and reverting to “basic modes” when its operation causes confusion to flight crew or things do not go as expected;

− the automated systems did not help the pilots in performing the go-around manoeuvre.

The KBSZ analysed:

Flight Control

After the instruction to deploy flaps, when the PM set the control lever to CONF0, the PF saw on his instrument that the lowest selectable speed (VLS) value was rapidly growing and reaching a value above the current airspeed. The immediate danger of stalling was eliminated by the action of the protective system of the aircraft which did not allow full retraction of the slats immediately because of low airspeed and high angle of attack. In order to eliminate the problem, the PF disconnected the auto pilots, initiated steeper descent, and then set the thrust levers to maximum thrust (TOGA). Despite the fact that the thrust levers were now in TOGA position, the aircraft control did not switch from approach mode to go-around mode because the CONF0 position of the flaps lever inhibited that process according the AFS logic.

At the cost of a loss of altitude of ca. 500 ft, airspeed reached a safe value of 220 kt, after which the slats were retracted fully. The aircraft started to climb, and PF pulled the thrust back to CLIMB position. The landing gears were retracted. In response to the thrust lever position, the A/THR engaged, and, in accordance with the still active approach mode, it tried to reduce airspeed to Green Dot speed (198 kt). As at this time the current aircraft speed was 230 kt, the A/THR reduced engine thrust (to a level near idle).

After normalising the speed and descent situation, the PF tried again to activate automatic modes. He reconnected AP1 which had still been working in approach mode and trying to follow the ILS glide path and localiser. As the aircraft was already above the glide path, the auto pilot initiated steep descent and then six seconds after engagement was disconnected by the flight crew. As both speed and altitude were decreasing, the PF pushed the thrust levers to TOGA position again. As a combined effect of increased thrust and steep descent, airspeed also increased quickly to exceed 300 knots, although the position of the thrust levers decreased to MCT value for a short time. The flight crew was able to stop descent, by pulling the sidestick backward, only at an altitude of 1133 ft above ground level. Climb started, the PF pulled thrust levers back to CLIMB position, and as an effect, the auto thrust set the engines to idle in order to reduce airspeed, because it had still been in approach mode.

With the engine thrust close to IDLE, the speed of the aircraft in steep climb was decreasing fast. The automatic control finally quitted approach modes (most likely due to the loss of one of the ILS signals, LOC or G/S signal) and then about 45 seconds after both flight directors were disengaged by the flight crew. Stopping climb at 5000 ft, the PF initiated a left turn upon instruction from the air traffic control. The PF, who was overloaded by the unusual, unexpected and frightening situation with series of information that he did not understand, initiated a left turn that finally reached a roll angle of 42.9° value. The PF pushed the thrust levers to MCT position again. The IC assumes that the sudden acceleration with longitudinal direction caused by thrust increase might have caused a sensory illusion to the pilot who might have had the false perception of steep climb, as an effect of which he initiated a steep descent. The sink rate of the aircraft reached the -5000 ft/min value, which made the ground proximity warning system give the audio warning SINK RATE for a short time.

Descent was stopped at 1750 ft (533 m) thanks to nose up inputs ordered on the captain sidestick. Then, relying on the flight control system, AP1 and the auto-thrust, the flight crew normalized the flight gradually, managing to reach and maintain the direction of 270° and flight altitude of 6000 ft as permitted by the Air Traffic Control. The flight crew might have realized at that point the cause of the unusual behaviour of the aircraft, i.e. the fully retracted position of the slats/flats system. After that, the aircraft finished the traffic circuit and landed safely on Runway 09.

Protection of the slats/flaps mechanism

At the moment the slats and flaps were inadvertently retracted, the airspeed (141 knots) was lower than the minimum speed required by the protection of the slats (148 knots). As a result, the retraction process halted at 18°, and was finished only after the sufficient speed was reached. The system worked in accordance with its design purpose, i.e. it managed to prevent a dangerously high angle of attack.

Switch to go-around mode

When the thrust levers were set in TOGA position, the slats/flaps configuration was already CONF0, so the automatic systems did not switch from approach mode to go-around mode. As the flight directors remained engaged after disconnection of the auto pilots, the ILS approach mode also remained active when the PF performed go-around. When reconnected, the auto pilot began to work in ILS approach (LOC+G/S) mode which was still active and, contrary to the PF’s intention, attempted to perform the landing procedure, which resulted in intensive descent.

In the opinion of the IC, the risk of the irregular flight situation occurring due to retraction of slats and flaps was worsened by the fact that the automatic system did not support the flight crew’s intention to go-around, and the flight crew were not aware of this fact for minutes.

Engine control

During the event, the engine powers varied between extreme values. Indeed, when the PF found current airspeed dangerously low or dangerously decreasing and set the thrust levers to higher position than the functional range of the auto-thrust, then the engines began to work at a high thrust level corresponding to the thrust lever position. But when the PF pulled the thrust levers back to the functional range of the A/THR, then the A/THR set an engine thrust to the adequate level to reach and maintain the speed target (Green Dot speed). During the event, it happened that the automatic system lowered the engine thrust close to IDLE in order to reach such target speed (because at that time the actual aircraft speed was well above this target).

In the opinion of the IC, the fact that on this aircraft type the auto-thrust system changes engine thrust without changing the position of the thrust levers might have effect on flight crew’s awareness concerning the set engine thrust during unexpected and unusual situation, because crew were able to primarily rely on indirect in-formation from the instruments relating to actual engine function.

Trigger of the GPWS aural warnings

The purpose of the ground proximity warning system (GPWS) is to prevent dangerous approach of terrain (or controlled flight into terrain in extreme cases) by warning the flight crew on time. In the case involved, the “SINK RATE!” warning of the system triggered when the sink rate reached 5000 ft/min (25.4m/s) at only 2200 ft (670 m) RA, which sink rate, if maintained, would have resulted in ground impact in 26 seconds. The system worked in accordance with its design purpose, i.e. it managed to prevent dangerous approach of terrain, although the PF had already initiated a climbing manoeuvre before hearing the GPWS warning alert, but deflection of the sidestick significantly increased after the warning.

Human factor evaluation using the SHELL model

The SHELL model may be of help for the analysis of the PF’s activity. The model classifies the factors influencing the acts of the individual into four major groups:

Liveware (co-operation of the flight crew):

Under the influence of a week off before the occurrence and a night of disturbed rest, the 29-year-old PM who had 2600 hours of flight experience, set the slats/flaps control lever to a position which resulted in full retraction of the flaps and slats instead of deploying them according to the instruction received. The IC could not clearly identify the specific cause of the error.


Visibility conditions were adversely influenced by dusk and snowing. The geomorphology conditions dominated by high mountains around Sofia Airport increased the load of the PF. As it was his base airport, he had to be aware of the disadvantageous terrain, which might have increased his psychological stress.


Some of the features of the controls of the A320 Airbus model (Sections 1.6.5, 2.1, 2.3) did not make it easy for the PF to realise and manage the unexpected and unnatural flight situation. After the thrust levers were pushed to the TOGA position, the automatic systems still did not switch to the go-around mode. When the thrust levers were moved just a few centimetres, the engine power altered between near-maximum and near-minimum values.

Software (rules):

In addition to inadvertent retraction of the slats and flaps, the flight crew did not follow the company standard operating procedure (disregarding E/WD and FMA displays, failing to disengage the flight director, etc.), which made it even more difficult for them to identify and manage the situation. Their failing to disengage the flight director kept the ILS (G/S and LOC) approach mode active. When the auto pilot was reconnected in a hurry, it took the aircraft into a manoeuvre which made the situation even worse.

General analysis of the PF’s reactions

Continuous, detailed overviewing and full understanding of the operation of such heavily automated systems like an Airbus A320 aircraft typically exceeds the possibilities of an average operator. Practical operation of such systems is based on practising those procedures which are used on a daily basis. An unexpected, substantial difference from the usual procedures may mean a serious challenge to the person operating the system. It is particularly typical of the situation of the flight crew whose mental load is further increased by strong time constraint and their awareness of potential serious consequences of a possible mistake.

Right before the serious incident occurred, the PF might have reasonably expected that a routine approach and landing procedure was to follow. But to the contrary, the unusual error made by the PM who retracted the slats and flaps instead of fully deploying them changed the situation fundamentally, situation which was difficult for the PF to understand at that time. In the first moments, the most important information for him was that the airspeed had become lower than the lowest selectable airspeed (VLS), which implied the danger of getting too close to the stalling speed, and might require activation of further automatic protective functions in order to maintain safe controllability of the aircraft. The PF eliminated immediate danger by available means (descending manoeuvre and increasing of engine thrust) but he was not aware of the origin of the problem.

The next decision was the abortion of the approach procedure and performing go-around, as stabilization of the flight of the aircraft within acceptable period of time did not seem possible. In general practice, setting the thrust levers to the TOGA position will switch the systems of the aircraft to go-around mode, but in this case it did not happen because the slats/flaps lever was in CONF0 position. Safe management of the situation was hindered largely by the fact that, for several minutes, the flight crew’s intention was to abort the approach while the automatic systems of the aircraft were still in approach modes.

Training for pilots for managing extraordinary situations

During the investigation, the IC found that, although pilots learn during their training that, in CONF0 position of the slats/flaps lever the automatic systems do not switch to goaround mode despite the throttle levers are set to TOGA position in aircraft of the A320 family of Airbus, but that knowledge fades out with time, because pilots do not face that situation during their work or recurrent training. This deficiency can be eliminated by changing this feature of the aircraft or by further development of training.

LBSF 031830Z 09014KT 3500 -SN FEW012 SCT026 OVC040 M07/M09 Q1011 R99/490195 WS ALL RWY NOSIG=
LBSF 031800Z 09014KT 3500 -SN SCT028 OVC040 M09/M11 Q1011 R99/490195 WS ALL RWY NOSIG=
LBSF 031730Z 09015KT 3500 -SN SCT027 SCT039 OVC048 M09/M10 Q1011 R09/490195 WS ALL RWY NOSIG=
LBSF 031700Z 09016KT 2600 -SN FEW018 SCT022 OVC025 M09/M10 Q1011 R09/490195 WS ALL RWY NOSIG=
LBSF 031630Z 09016KT 3500 -SN FEW026 OVC034 M08/M10 Q1011 R99/41//95 NOSIG=
LBSF 031600Z 09016KT 8000 -SN FEW025 BKN040 M08/M10 Q1011 R99/19//95 NOSIG=
LBSF 031530Z 09015KT 7000 -SN FEW028 BKN039 M08/M09 Q1011 R99/19//95 NOSIG=
LBSF 031500Z 09014KT 2500 -SN BKN025 OVC060 M08/M09 Q1011 WS ALL RWY R99/19//95 NOSIG=
LBSF 031430Z 11016KT 2000 -SN FEW019 BKN024 OVC035 M08/M09 Q1012 RESN R99/19//95 NOSIG=
LBSF 031400Z 11017KT 2300 SN FEW020 BKN025 OVC033 M05/M08 Q1012 R99/19//95 NOSIG=
LBSF 031330Z 11016KT 8000 -SN BKN028 OVC047 M05/M09 Q1012 R99/19//95 NOSIG=
LBSF 031300Z 11017KT 7000 -SN FEW022 BKN032 OVC040 M06/M09 Q1013 R99/19//95 NOSIG=
LBSF 031230Z 11018KT 8000 BKN030 OVC047 M06/M09 Q1013 R99/19//95 NOSIG=
LBSF 031200Z 11017KT 8000 SCT035 OVC046 M05/M08 Q1014 R99/19//95 NOSIG=
LBSF 031130Z 10016KT 3000 BR SCT031 SCT037 OVC044 M06/M08 Q1014 R99/19//95 TEMPO 2000 SN=
Incident Facts

Date of incident
Jan 3, 2016


Wizz Air

Flight number

Sofia, Bulgaria

Aircraft Registration

Aircraft Type
Airbus A320

ICAO Type Designator

Airport ICAO Code

This article is published under license from Avherald.com. © of text by Avherald.com.
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