gnow, your quote from CS29 indicates that para 1191 can apply - ie the need for a firewall. 1191 doesn't give much info about what a firewall must be (maybe its in the AMC?) but surely the sorts of firewalls on helicopters are only superficially fireproof, and if the fire gets hot enough (and bearing in mind you are still pumping fuel into the area) I am sure it could eventually melt, or certainly conduct enough heat to ignite something on the other side. I suspect that the presence of fire detection and protection is used as a means to comply with the rules, but that presumes they will actually be used in a timely manner.