S7 A321 Unreliable airspeed event, Magadan, 02 Dec 2021
Given all the problems caused by pitot tubes over the years (AF447,2xMAX) and more, surely there must be better technology that perhaps an air-computer could compare pitot readings to, see if it's likely the aircraft is in icing conditions or the pitots are just plain likely to be giving duff info, then suggest to the pilots to switch to an alternate system until the disagree is resolved?
Keeping pitot probes clear of ice isn't rocket science - but it takes lots of electrical power (engine inlet probes are around 500 watts, the aircraft probes are even more). Problem starts when designers try and reduce that electrical load to better optimize the aircraft as a whole.
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Here is a Safety First unreliable speed article:
https://safetyfirst.airbus.com/app/t...able-speed.pdf
https://safetyfirst.airbus.com/app/t...able-speed.pdf
Given all the problems caused by pitot tubes over the years (AF447,2xMAX) and more, surely there must be better technology that perhaps an air-computer could compare pitot readings to, see if it's likely the aircraft is in icing conditions or the pitots are just plain likely to be giving duff info, then suggest to the pilots to switch to an alternate system until the disagree is resolved?
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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.
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.
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.
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.
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If you have snow and ice on the windshields and switch on the probe/window heat (that is the procedure in cold weather), it will take just a few minutes for this to melt and run down the sides of the aircraft and then refreeze. As in you switch it on and walk out for the external inspection and see a nice layer of rippled clear ice that will make your day really bad if you don*t remove it. The system will come on automatically after the first engine start. If the crew fails to switch on the window heat and just use the wipers to clear the snow, the melt and refreeze will take place after first engine start and with both pilots inside the aircraft.
And the fun starts at lift off if you don*t catch any airspeed issues during takeoff.
And the fun starts at lift off if you don*t catch any airspeed issues during takeoff.
