If you're tuning in for the airplane stuff, I hope the last couple weeks' jobsearch travelogue isn't boring you. I promise to go back to pretending to know all about airplanes soon. And here's a nice technical post to tide you over: the inside story on the twin otter fire detection system.
The twin otter fire detection system is a marvel of space age technology. And whe I say space age, I'm talking about the 1960s, the era of the first man in space, and the American moon landing. A twin otter pilot I know was alerted to the presence of a fire in his engine because he saw the reflection of the flames in the dashboard. But back up systems are everywhere on the twin otter, so in addition to the well-polished dashboard, there are four thermal switches located in each engine nacelle.
The thermal switches are composed of two different sorts of metal that expand at different rates when heated. The forward switches are constructed such that they will bend enough to close a circuit at 450 degrees F, while the rear switches will close at 300 degrees F. The original design had all 300 degree F switches, but the forward ones kept triggering by accident in hot climates when reverse thrust was selected. The switches are connected in parallel so that if any one of them closes, it will activate a cockpit warning light and paralysingly loud fire bell. The number one checklist item to be completed in the event of an engine fire is Fire Bell MUTE, so the pilots can hear themselves think. The fire bell mute switch is located either above the captain's left knee or behind the captain's head, depending on the serial number of the airplane, so the captain has to be able to think a little bit in order to silence the alarm. That's why he gets paid more.
The pilots have to be able to test the system, so there is another switch sort of in parallel with the four thermal switches, a normal switch that closes when someone presses the test button in the cockpit. Holding the test switch illuminates the lights and rings the bell, if everything is working. This is only the beginnings of the workings of the fire detection circuitry.
Rather than being a simple connection to each side of the thermal switches, de Havilland has engineered two loops, which they call Loop A and Loop B. That way if there is a break in either loop, there is still a path for electricity to follow to ground, and detection will work. The test switch bypasses the loop, so that fire detection is still available, but the test switch will indicate the fault, so it can be repaired. There's more.
Loop A is normally powered, waiting for a thermal switch to trip, so that power can get through loop B to the warning light and bell. If Loop A should develop a short circuit, then a magnetic circuit breaker will pop. That magnetic CB is cunningly configured so that as well as cutting power to Loop A, it closes the circuit in Loop B, so that Loop B becomes powered. In this configuration, if one of the thermal switches reaches its trigger heat and closes, electricity will flow through Loop B, through the switch, through Loop A, through the warning lights and to ground, through the short circuit. Thus even with a short circuit in A, the fire warning will work.
The fire detection system is powered off the left bus bar, so that if you were unfortunate enough to have your right engine catch fire while the left generator was offline and the bus tie open, you'd have to depend on the shiny dashboard method of fire detection.