The clinical efficacy of noninvasive ventilation (NIV) has now been demonstrated in the management of acute-on-chronic respiratory failure (ACRF) of various etiologies. Endotracheal mechanical ventilation (ETMV) can lead to numerous complications and weaning difficulties increasing the risk of prolonged ETMV, morbidity and mortality as well as excess cost of intensive care. Therefore, it could be interesting to use NIV for delivering effective ventilatory support and reduce the length of ETMV in ACRF patients still not capable to maintain spontaneous breathing. From the interesting but discordant results of two recent randomised controlled trial, a working group from the Société de Réanimation de Langue Française (SRLF) decided to perform a new prospective randomised controlled and multicenter trial. The aim of the study is to assess the usefulness of NIV as an extubation and weaning technique in ventilated ACRF patients. The methodology used compares three weaning strategies in parallel in ACRF patients considered difficult to wean : invasive conventional weaning (group A), extubation relayed by nasal oxygentherapy (group B), and extubation relayed by NIV (group C). Based on the main end-point defined as the weaning success/failure rate, 208 patients from17 investigator centers are planned to be included. Results of the study will also allow to assess the respective impact of the three weaning strategies on the length of ETMV and weaning, the mechanical ventilation-related morbidity, the patients lengths of stay and mortality.