ECJ, 17 April 2018, Case C‑195/17, Helga Krüsemann and Others v. TUIfly Gmbh: The spontaneous absence of a significant part of the flight crew staff (‘wildcat strikes’), which stems from the surprise announcement by an operating air carrier of a restructuring of the undertaking, following a call echoed not by the staff representatives of the company but spontaneously by the workers themselves who placed themselves on sick leave, is not covered by the concept of ‘extraordinary circumstances’.