Thomas Cook shares dropped following an announcement that three of the firm's non-executive directors are to leave in February.It is the latest worrying episode in what has been a traumatic period for the company. In November last year the travel firm had to negotiate a new £200m banking facility after warning investors it was close to running out of cash.In December the company reported a £398m annual pre-tax loss.The departures of David Allvey, Bo Lerenius and Peter Middleton will be seen by City observers as an attempt to draw a line under recent mishaps.Thomas Cook stresses that Allvey's retirement is in line with corporate governance best practice in that he has already served nine years with the firm.In December, Frank Meysman, a 59 year old Belgian, took over as chairman at Thomas Cook following the retirement of Michael Beckett. It looks as if he is beginning to mark out his territory.At 10:04am shares in Thomas Cook were down 1%. Over the last 12 months the firm's stock value has dropped an eye watering 92%.BS