What happened
On February 9, 2021, a Boeing 737-800, registration SP-RSY, was prepared for departure at Kraków Airport (EPGD) following a four-day period of stationary parking under snowy and windy conditions. During pre-flight inspections, the pilot noted that the wings, tail, engines, and fuselage were covered in a thick layer of snow. While the engine inlets and blades were cleared using hot air and a subsequent inspection confirmed no frost or ice remained in the engines, the aircraft required de-icing for the lower wing surfaces.
Following standard procedures, the crew taxied to a de-icing station, performed takeoff configuration tests, and departed from runway 11. The takeoff proceeded normally; however, after the landing gear was retracted, the crew began to feel vibrations throughout the airframe. These vibrations persisted until the flaps were retracted.
During the noise abatement climb, at approximately 4,000 ft, the crew executed a 160-degree left turn toward waypoint SABAK. During this maneuver, the aircraft entered a temperature inversion zone, where the temperature rose sharply from -13°C to -6°C. While the aircraft was climbing at a rate exceeding 5,000 fpm with a bank angle of 29°, the autopilot (LNAV mode) attempted to correct the flight path, triggering an excessive bank angle warning. During this period, the crew was also retracting the flaps from position 1 to 0. The aircraft's speed dropped to 182 knots, which was 9 knots below the minimum clean configuration speed of 191 knots.
The crew leveled off at FL 100, re-engaged the autopilot, and noted that the vibrations had ceased. After discussing the event, they continued the flight to their destination. Upon landing, a post-flight inspection revealed that pieces of ice had accumulated in the engine inlets.
The investigation
Investigators analyzed Objective Flight Data Monitoring (OFDM) records and examined the aircraft's flight control systems. The analysis of the Flight Control Computer B (FCCB) revealed errors related to sensor delays, which caused a momentary asymmetry in flap deployment. However, the cockpit indicators did not show any errors or asymmetry during the flight. The investigation concluded that this transient asymmetry was not the cause of the excessive bank angle.
Findings
- The primary cause of the event was the entry into a temperature inversion zone under icing conditions while performing a high-rate climb and a turn during flap retraction.
- The lack of information regarding icing and temperature inversion in the ATIS (Automatic Terminal Information Service) contributed to the event, as the crew was not alerted to the specific atmospheric conditions.