This is all i could find
http://www.caa.co.uk/default.aspx?ca...=90&pageid=533
Auditory Standards: there shall be no hearing loss in either ear, when tested separately, of more than 20 dB (HL) at any of the frequencies 500, 1000 and 2000 Hz, or of more than 35 dB (HL) at 3000 Hz.
Im sure there are more knowledgeble people out there about the specific requirements
Hope this helps