The great PPL Instructor debate was done to the death recently in the private flying and instructor forums. There was the obvious argument about whether someone with ATPL knowledge was better placed to teach PPL's than an 'experienced' PPL holder. The most important and essential issue that came from the argument was that the Instructor(irrespective of level of theoretical knowledge) needed to have the ability to teach! You could be the greatest 'boffin' in the world but you might not be able to impart that knowledge worth a damn.