Stockbroker IG has raised concerns with Thomas Cook's ongoing turnaround plan following the news that chief executive Harriet Green has stepped down with immediate effect.The revelation, alongside with a broadly in-line set of annual results, sent shares in the travel group plummeting over 20% on Wednesday.Analyst Chris Beauchamp from IG said Green's departure will be "the big news in these Thomas Cook figures"."Having arrived when the company was in the departure lounge and trading at 14p, she leaves it in a much sunnier disposition," he said, with the shares having closed at 137.9p on Tuesday.However, Beauchamp said that investors will be right to ask questions over how her departure will affect Thomas Cook's turnaround plan."The other issue is the turnaround plan - the warning that growth will be more moderate (i.e. slower) in the coming year is a sign that the easy bit has been done," he said."Expectations of further improvement are now baked into the price, although the signs of growth at the newly-improved web platform should provide a useful growth engine."