Tornado F-3 's scrambled from Leuchars to intercept the Rssian Bears

Would somebody identify all that smoky black discolouration on the base of the rudder of the Tornado - or is it just showing it's age ?
UPDATE 16/9/07
Anon tells us that an engineer is required to clean aircraft - Extraordinary, aren't mops and buckets of soapy water all that is required and little vigorous application ?
Assumed thedesignation is correctly F3 and corrected.