mad-jock - each engine is in its own firewalled enclosure - apart from the cowlings of course, but I guess they are dispensible! However it you look at the nature of the firewalls, they never look that substantial to me, and just behind them is the main gearbox / flight control jacks and all hydraulic systems. Just underneath them is the passenger cabin.
Of course the firewall is to an approved standard, and will deal with the "standard" fire whatever that is, but trouble is the fire doesn't know what the standard is!
I recall a rotorbrake fire back in the early 80s, caused by carrying out a groundrun with the rotor brake on. The carbon brake disc became incandescent white, the caliper softened so the pistons popped out and sprayed hydraulic fluid over the white-hot brake disc. The resulting fire caused moulten titanium to drip into the cabin. Fortunately all this was on the ground, but the pilot knew nothing about it until he was walking away from the aircraft wondering why the rotor brake didn't work when he shut down, and what the smell and fuss was. (no transmission area fire detection in the UK reg AS332L).
Edited to add that the melting point of titanium is around 1650 deg C, so the 1100 deg C "standard" fire was clearly well exceeded.
Last edited by HeliComparator; 18th Dec 2012 at 20:46.