The Austrian airline Niki has grounded its planes, stranding thousands of passengers, after filing for insolvency protection.The company has about 20 planes serving resorts in southern Europe and north Africa. It was founded by Niki Lauda, the Austrian ex-F1 racing champion, but he sold it to Air Berlin in 2011.Austria’s Der Standard daily says about 5,000 Niki passengers are stuck abroad and need to be brought back to Austria.Most of the stranded passengers are in Majorca. Austrian authorities are now trying to get them transferred to other flights.On Wednesday the German carrier Lufthansa scrapped plans to buy Niki. Earlier, the European Commission had expressed “deep competition concerns” about a possible Lufthansa purchase of 81 Air Berlin and Niki jets. It said that would limit choice for air travellers in Germany, Austria and Switzerland.Loss-making Air Berlin turned insolvent in August, after its biggest shareholder, Etihad Airways, withdrew financial support.According to Der Standard, about 40,000 people are booked to fly home on Niki during the next two weeks.The airline was planning to operate at least 145 flights per week during the winter holiday season, from Austria, Switzerland and Germany.The assets of Air Berlin, Germany’s second largest airline, are being sold off.The European Commission has approved EasyJet’s purchase of some Air Berlin assets, ruling that there is no breach of EU competition policy. They include slots at Berlin Tegel airport and at some destination airports, as well as some aircraft equipment.