JSC Yakovlev Outlines Measures to Prevent Incidents with SSJ100 Angle of Attack Sensors

Photo by © Russian Aviation

Following the crash of an SSJ100 aircraft, registration RA-89049, on 14th July 2014 near Kolomna, which was attributed to the incorrect installation of angle of attack (AoA) sensors, Yakovlev has developed a service bulletin for maintenance personnel. The document defines procedures for checking sensor installation, securing AoA sensor fairings in fuselage bays and the application of indelible markings indicating the correct device orientation.

An angle of attack sensor is a device featuring a vane, situated on the exterior of the fuselage. There are four such sensors on the ‘Superjet 100’ aircraft, two on each side. They are designed to determine the angle between the longitudinal axis of the aircraft and the velocity vector – the angle of attack.

The bulletin provides additional clarification to the existing installation procedures for the Thales manufactured production standard sensors; however, no changes to the sensor design are anticipated. Drawings illustrating the principle of applying control markings have been added to the procedures for installing and removing the AoA sensors, according to Izvestia reports.

Analysis of the situation has shown that the cause of the accident was the incorrect positioning of two out of four AoA sensors, with a deviation of around five degrees, incurred during maintenance. This inaccuracy in installation led to the malfunction of the stall protection system, which, having received erroneous data, triggered the aircraft to pitch nose-down.

The manufacturer has implemented measures to prevent a recurrence of the incident. Now, after sensor removal, the covers must be replaced and secured to the fuselage, preventing any confusion in their orientation during subsequent installation. When reinstalling two or more sensors, mandatory checks on their installation angle must be carried out.

The service bulletin defines procedures for the installation and operation of SSJ100 aircraft with imported avionics and engines. Furthermore, for the new, domestically produced SJ-100, angle of attack sensors with a different attachment design are provided, and their compliance with airworthiness regulations and rules is being verified during the certification testing process.

Alongside the technical bulletin, an information document has been issued to airlines, detailing the aircraft’s behaviour in the event of incorrect operation of two or more AoA sensors. If identical, excessive values are detected, it is possible that the angle of attack limiting system will activate. This will result in symmetrical deployment of the flight spoilers, causing a significant reduction in lift.

Such a situation, during flight at a constant angle of attack, could trigger a steady descent of the aircraft, which cannot be compensated by applying full back stick input. Based on preliminary findings by the IAC (Interstate Aviation Committee), this combination of factors was the reason that the crew of the crashed SSJ100 were unable to recover the aircraft from its dive.

Due to this situation, Rosaviatsia (the Russian Federal Agency for Air Transport) has issued a directive obliging SSJ100 operators to adopt and implement all measures to prevent similar accidents. Also, on the initiative of Rosaviatsia, Yakovlev has developed and issued a bulletin with recommendations for crews on actions to take in situations where information from the AoA sensors is incorrect at various stages of flight. The directive has been issued to airlines for flight crew familiarisation.

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