I had an in-flight fire a few years ago in a CH-53E. We were about halfway through a 3-hour flight, and the January cold at 7000ft MSL had finally had it's way with us (crew of four - two pilots, a crewchief and an aerial observer {me}), so we decided to crank up the cabin heater.
A little background - the cabin heater on the -53E is well-known in the fleet for being a stubborn system. It's great when it works (600,000 BTU's will get you warmed-up quick, even in Norway in February), but those times are somewhat rare, so it doesn't get used often. Anyhoo, there's 2 blowers in the system - a vent blower which moves the heated air from the plenum chamber to the cabin vents, and a combustion-air blower which (as you may have guessed) accelerates the air/fuel mixture into the plenum chamber for a good, hot burn. The fuel nozzle is mounted in the combustion-air blower's outlet port, which is V-band clamped to the plenum chamber.
So there we were... cold. The crewchief turned the heater on, and about 10 seconds later the pilots say "Hey, we've got a fire light!". At the same time they're saying this, smoke starts pouring in through the cabin & cockpit vents. In just a few seconds, the cabin and cockpit were full of smoke, and I could barely see well enough to get to the gunner's window on the left side of the cabin to open it (step 1 of the Emergency Procedure for "smoke and fume elimination in-flight" in the 53 - open all cabin windows and doors). As the crewchief and I were opening the windows, the copilot turned off the heater and the pilot made a MAYDAY call. Once we opened the gunner's window and the upper crew door, the smoke was swirling around a lot, enough that I could see what I was doing again. Smoke was still slowly coming through the vents, and the fire light was still lit in the APP/Heater T-handle (meaning the system was still detecting flames in the APP/heater compartment), so I pulled all the circuit breakers for the heater while the crewchief pulled the T-handle (the T-handle closes the firewall fuel valves for the APP and heater and discharges the fire bottle into the compartment). In about 10 seconds, the fire light went out. While I was pulling C/B's, the pilots dumped the collective to get us on the ground ASAP, and we basically auto-rotated through the descent even though we still had all 3 engines online. It's the fastest way down in a -53; at one point, I saw almost 6000FPM descent rate on the VSI... We landed in a farmer's field somewhere in MD and had to shut-down with no electrics (without the APP running, the generators drop off-line around 92-90% Nr, and there's currently no battery back-up for the electrical system in the -53) and no hydraulics (the pumps stop producing 3000psi for the systems at about 20% Nr). Finally safe-on-deck, but we've got no way to tell anyone - no battery means the aircraft radio's won't work, so we're NORDO. Hey, wait - we're all wearing our survival vests with an emergency radio - we've got FOUR comms! Well, that was the theory... reality was that 3 of the 4 had dead batteries, and the 4th wouldn't talk to anyone. But that's another story... One of the pilots had a cellphone with him, so he dug-up a number for whatever agency he had been talking to when he MAYDAY'd and called them to let them know we were safe. Meanwhile the crewchief and I opened up the "doghouse" (the cowling over the APP/heater compartment) to assess the damage, and the Coast Guard buzzed us a coule times in a Dauphin before they landed near us to talk to the pilots. About 10 minutes later, the farmer showed up. He was a little pissed that we landed in his stubble field, but as soon as I told him we'd had a fire in-flight and it was an emergency landing, his attitude did a 180. He brought us coffee & sandwiches, and offered us just about anything we wanted or needed. We assessed the damage and got another 53 headed our way with the parts we needed to "one-shot" our aircraft home. Swapped out an APP wiring harness and a fuel line, and we were on our way home. The aircraft was down for almost a month for investigation and repairs.
The cause of the fire was determined to be the combustion-air blower. The impeller shaft was seized in it's bearings, which caused the motor wiring to overheat and catch fire when we turned the heater on. The motor's windings also caught fire. I don't like to think too much about the proximity of the flames to the fuel nozzle and it's supply line, which had fuel being pumped through it at the time.