Burnt rubber sucked in by the airconditioning packs. Not an uncommon occurence on landing. In my 32 years of operating into Kai Tak airport, there was quite a different smell permeating the cabin after landing. The runway was parallel to an open sewer culvert, called a nullah. We had many first time passengers asking about the origin of this particular odour of the "Fragrant Harbour", which is the English translation of Hong Kong.