S7 A21N at Magadan on Dec 2nd 2021, unreliable airspeed

Last Update: February 10, 2022 / 20:53:28 GMT/Zulu time

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Incident Facts

Date of incident
Dec 2, 2021

Classification
Incident

Flight number
S7-5220

Aircraft Registration
VQ-BGU

Aircraft Type
Airbus A321-Neo

ICAO Type Designator
A21N

A S7 Sibir Airlines Airbus A321-200N, registration VQ-BGU performing flight S7-5220 from Magadan to Novosibirsk (Russia) with 202 passengers and 7 crew, was climbing out of Magadan's runway 10 when the crew declared Mayday due to unreliable airspeed and related problems with autopilot and autothrust. The aircraft attempted to return to Magadan but had to abort the approach at about 5000 feet, attempted a second time to return to Magadan and again needed to go around due to severe icing. The crew subsequently decided to divert to Irkutsk (Russia) where the aircraft landed safely about 4.5 hours after departure.

On Dec 4th 2021 Rosaviatsia reported following takeoff the aircraft flew into clouds and into a zone of severe turbulence accompanied by strong icing. As result the aircraft got into a difficult spatial position, according to FDR the roll angles varied between 49.8 degrees and -91.1 degrees, pitch angles from 43.8 to -23.9 degrees. The crew declared Mayday. The crew was able to resolve the situation and divert to an alternate airfield. No injuries occurred.

On Dec 8th 2021 Rosaviatsia released a first safety information stating the aircraft had been de-iced prior to departure in heavy snow, temperature -9 degrees C, dew point -12 degrees C, visibility about 750 meters horizontal (550 meters to the south). After takeoff all three air data systems malfunctioned resulting in unreliable airspeed (different airspeeds on each system), the flight controls went into minimum operation mode (editorial note: unclear whether this means direct law). The crew decided to return to Magadan, the aircraft however stalled during the return to Magadan prompting the crew to divert to Irkutsk, where the aircraft landed without further incident. The flight crew found frozen fluids in the nose section of the airframe disrupting the pressure sensors ("Barrier Ice") as well as deposits of rough ice on the leading edges of the wings and radome.

Rosaviatsia mentioned a number of similiar occurrences, amongst them a crash of an Antonov 148, see Crash: Saratov A148 at Moscow on Feb 11th 2018, lost height after departure, pitot heatings off, and the crash of an ATR 72 see Crash: UTAir AT72 near Tyumen on Apr 2nd 2012, lost height in initial climb. Rosaviatsia recommended to re-iterate the procedures for de-icing, protection against icing and keeping aircraft surfaces and control surfaces clean with ground support staff and flight crews, monitor ground support crews, re-iterate flight planning procedures to take into account such hazards associated with natural anomalies as well as to take into account protective properties of de-icing fluids as well as taking into account additional properties of the aircraft's de-icing systems and power plants.

On Dec 9th 2021 Rosaviatsia informed other accident investigation units that the aircraft suffered an ADR #1, #2 and #3 fault after takeoff, Large rolls and pitches occurred, the aircraft however was not damaged. The occurrence was rated a serious incident and is being investigated by the Russian Federation.

On Dec 9th 2021 Rosaviatsia reported in a safety brief, that the aircraft had been de-iced in two stages, first with fluid type I and second with fluid type IV. However, only wings and stabilizer surfaces were de-iced. The fuselage, which had accumulated a large amount of snow in the heavy snowfall during 2.5 hours on the ground, had not been de-iced. After the windshield heating had been turned on, the snow melted and water flowed down the front part of the fuselage. Continuous snow fall and melting during taxi in ambient temperatures below freezing caused a layer of frozen ice at the front part of the fuselage disrupting the airflow of the pitot tubes. Therefore, takeoff with the fuselage surface and engine hoods covered with a thick layer of snow in icing and turbulence conditions posed a real danger to the flight safety.

Rosaviatsia released their final report in Russian only dated Dec 24th 2021 (Editorial note: to serve the purpose of global prevention of the repeat of causes leading to an occurrence an additional timely release of all occurrence reports in the only world spanning aviation language English would be necessary, a Russian only release does not achieve this purpose as set by ICAO annex 13 and just forces many aviators to waste much more time and effort each in trying to understand the circumstances leading to the occurrence. Aviators operating internationally are required to read/speak English besides their local language, investigators need to be able to read/write/speak English to communicate with their counterparts all around the globe).

The report concludes the probable causes of the serious incident were:

a combination of following factors:

- non compliance with the clean aircraft concept by the aircraft crew and ground personnel in organizing and performing de-icing; as result snow was not removed from the forward fuselage (on and under the cockpit glazing) the melting during taxi and takeoff of which caused "barrier ice" in front of the pitot tubes, distortion of air flow and switching the control system's roll channel to DIRECT and the pitch channel to "ALTERNATE" mode.

- while flying the aircraft in degradated control mode the pilot made disporportionate control inputs which caused the aircraft to stall. When the aircraft stalled and got into a difficult spatial position, both captain and first officer made control inputs (dual control) which are prohibited by the FCOM and made it difficult to return the aircraft into operational flight modes and thus increased risk to safety.

- the decision to initiate de-icing by flight crew and ground personnel were done in heavy snowfall conditions which leave no time for protective action by the de-icer

- only 99 liters of type IV deicing fluid were used despite the recommended 230 liters for the A321 Neo

- the dynamic viscosity of the type IV de-icing fluid deviated at 20 degrees C (1860 mPas instead of 2000 mPas). Reduced dynamic viscosity results in lesser retention on the aerodynamic surfaces of the aircraft and may increase the risk of ice on critical surfaces.

The aircraft had been parked in conditions of heavy downpour and temperatures between -9 and -12 degrees Centigrade. The flight crew initiated de-icing of wing and stabilizer upper surfaces, but did not request the fuselage to be de-iced. Type I fluid was first applied followed by type IV fluid. The aircraft departed at a takeoff weight of 89315kg (MTOW 93500 kg) and a CG at 25.3% MAC well within all limits. In the initial climb as result of disagreeing airspeeds the aircraft control systems switched from normal mode to direct mode for the roll channel and alternate mode for the pitch channel, the autopilot disengaged. A second after the autopilot's disengagment the stall warning sounded. The crew took manual control of the aircraft. The differences in the speed readings increased further. 5 minutes after takeoff the crew declared Mayday reporting unreliable airspeed, decided to return to Magadan and stopped the climb at 8900 feet MSL. Descending through 4800 feet MSL on approach to Magadan the indicated airspeed increased to 370 KIAS causing an overspeed warning prompting the pilot flying to pull back on the side stick resulting in a climb at a rate of 17300 feet/minuate. The aircraft reached 13800 feet MSL and stalled. After regaining control the crew attempted an approach to Magadan but reported they were not in a position to land and went around. The crew subsequently decided to divert to Yakutsk. While enroute to Yakutsk S7's control center sent a message recommending to divert to Irkutsk where the aircraft ultimately landed.

Rosaviatsia analysed that the snow condition was "fluffy", not wet. The crew, not contradicted by ground crews, thus considered the snow on top of the fuselage would be just blown off during takeoff. Magadan Airport has no possibility to blow snow off with compressed air. The crews did not consider that the snow on the windshield would melt and run down as water the sides of the fuselage where the water would freeze again and form a "barrier" in front of the pitot tubes. The remains of barrier ice however were discovered after landing in Irkutsk. The FCOM warns of snow melting on the windscreen when the window heating if forced on rather than set to automatic - the window heat switch was on throughout the flight, it could not be established, when it was moved into the on position.

Rosaviatsia analysed that after descending to about 5300 feet (standby altimeter showed 4090 feet MSL) while attempting to return to Magadan the aircraft's pitch stabilized at +10 degrees, the aircraft experienced a vertical acceleration of +2G and the aircraft began to climb again. Thrust was set to 90%. Subsequently there were synchronous control inputs by both pilots deflecting their side sticks to the maximum left/right positions attempting to limit the roll of the aircraft between +/- 42 degrees of bank angle. There were no rudder movements, this however may indication disconnection of the yaw control channel. In the 42 seconds following the thrust being set to 90% the aircraft climbed rapidly and encountered roll oscillations between -90 degrees and +47 degrees with vertical accelerations between -0.5 and +1.9G. The angle of attack remained fairly stable throughout that time at 10 degrees +/- 5 degrees nose up.

After the first officer stopped providing control inputs the captain was able to stop the roll oscillations and stabilized the heading about a minute later, pitch oscillations remained however. 4 minutes after the onset of the upset the crew finally moves the stabilizer, which had remained at 9.3 degrees at that time, to 7.5 degrees, the pitch angle begins to decrease and the stall warning ceases. About 12 minutes after the onset of the upset the flight crew regains full control of the aircraft and begins to control the aircraft with the assistance of the FMGS again.

Metars Magadan:
UHMM 020700Z 36005G11MPS 0750 0200W R10/0750N +SHSN BLSN VV002 M12/M14 Q0975 R10/490535 TEMPO 0400 +SHSN BLSN RMK QBB060 QFE717 R28/10005G11MPS=
UHMM 020630Z 36007G12MPS 1100 0350W R10/1000D +SHSN BLSN VV002 M11/M14 Q0975 R10/490535 TEMPO 0400 +SHSN BLSN RMK QBB080 QFE717 R28/08004G10MPS 020V130=
UHMM 020600Z 01007MPS 1000 0350W R10/1000D +SHSN BLSN VV002 M10/M13 Q0976 R10/490535 TEMPO 0400 +SHSN RMK QBB080 QFE717 R28/11005MPS=
UHMM 020530Z 03005G10MPS 350V070 1200 0550W R10/1000N +SHSN VV003 M10/M13 Q0977 WS R10 R10/490535 TEMPO 0400 +SHSN RMK QBB090 QFE718 R28/15001MPS=
UHMM 020500Z 02005MPS 1800 R10/1500D SHSN DRSN VV005 M09/M13 Q0977 R10/490535 TEMPO 1000 SHSN RMK MT OBSC QFE719 R28/12003MPS 080V170=
UHMM 020430Z 04005G10MPS 340V070 9999 2500W -SHSN SCT010 OVC018CB M08/M13 Q0978 R10/490535 TEMPO 1000 SHSN RMK MT OBSC QFE719 R28/10004MPS=
UHMM 020400Z 03003MPS 9999 -SHSN SCT010 OVC019CB M07/M13 Q0980 R10/490535 TEMPO 3100 -SHSN RMK MT OBSC QFE720 R28/14002MPS=
UHMM 020330Z 06006MPS 9999 -SHSN SCT010 OVC030CB M07/M13 Q0981 R10/490535 TEMPO 3100 -SHSN RMK MT OBSC QFE721 R28/02003MPS 340V080=
UHMM 020300Z 05006G11MPS 020V100 9999 -SHSN SCT010 OVC032CB M07/M13 Q0981 R10/490535 TEMPO 3100 -SHSN RMK MT OBSC QFE721 R28/04002MPS=
UHMM 020230Z 03004MPS 9999 -SHSN SCT010 OVC031CB M08/M13 Q0982 R10/490535 TEMPO 3100 -SHSN RMK MT OBSC QFE722 R28/23001MPS=
UHMM 020200Z 04005MPS 9000 -SHSN SCT010 OVC035CB M08/M13 Q0983 R10/490535 TEMPO 3100 -SHSN RMK MT OBSC QFE723 R28/26001MPS=
UHMM 020130Z 03005G10MPS 7000 2800S -SHSN BKN008 OVC024CB M08/M13 Q0985 R10/490535 TEMPO 3100 -SHSN RMK MT OBSC QFE724 R28/26003MPS 210V290=
UHMM 020100Z 00000MPS 2800 1100E SHSN BKN008 OVC017CB M09/M12 Q0985 R10/490535 NOSIG RMK MT OBSC QFE724 R28/01002MPS 330V050=
UHMM 020030Z 04004MPS 010V110 0750 0550S +SHSN VV006 M09/M12 Q0987 R10/490535 NOSIG RMK MT OBSC QFE725 R28/28002MPS=
UHMM 020000Z 00000MPS 0350 0300S R10/1000U +SHSN VV004 M10/M12 Q0988 R10/490535 NOSIG RMK MT OBSC QFE726 R28/10001MPS=
UHMM 012330Z 04003MPS 0300 0250E R10/0550N +SHSN VV002 M11/M13 Q0989 R10/490535 NOSIG RMK QBB060 MT OBSC QFE727 R28/14003MPS=
UHMM 012300Z 07004MPS 030V100 0300 0200E R10/0700D +SHSN VV002 M11/M13 Q0990 R10/490535 NOSIG RMK QBB060 MT OBSC QFE728 R28/11004MPS=
UHMM 012230Z 05006G12MPS 0400 0350E R10/0900D +SHSN VV003 M11/M13 Q0991 R10/490240 TEMPO 0300 +SHSN BLSN RMK QBB090 MT OBSC QFE729 R28/13002MPS=
UHMM 012200Z 05006MPS 1500 1000E SHSN BKN006 OVC024CB M11/M15 Q0992 R10/490240 TEMPO 1000 SHSN RMK MT OBSC QFE729 R28/27002MPS=
Incident Facts

Date of incident
Dec 2, 2021

Classification
Incident

Flight number
S7-5220

Aircraft Registration
VQ-BGU

Aircraft Type
Airbus A321-Neo

ICAO Type Designator
A21N

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