Flashing Lights

We've just leveled off at 17'000 feet and I'm starting to get comfortable in my sheep skin seat for the two hour trip north. Looking out the window into gray nothingness thinking about grabbing a hot cup of coffee to warm up the dreary day, when red lights start flashing across the glare shield. The "left engine fire" light is brightly glowing on the warning panel. A short breath hops out of my lungs. My captain swears and then looks out his window to check out the engine for any sign of fire. There is none. A quick scan over the engine instruments shows normal and matched indications. No low torque or high temp on the left engine that would indicate a fire was happening.

We call up ATC and ask for a heading change to the left. A common problem with the fire sensors on the King Air is that they are susceptible to other elements than just fire to set them off. The King Air has three photo sensors placed inside the cowling of each engine nacelle. They don't sense temperature but instead they work on detecting inferred rays that fires produce. Therefore if a ray of sunlight somehow sneaks through the cowling and rests it's beams on the photo cell it will set off the engine fire sensor. Standard procedure is to steer 45 to 90 degrees off of heading to change the angle that the sun is contacting the plane.

However, today we're under a thick layer of cloud and when we turn off our heading the light does not go off. Now we start to suspect that some moisture has sneaked on board and triggered the fire sensor, another susceptible spot on the photo cells. It had been raining all morning and the plane had sat outside for sometime, so we figured it was just a moisture problem. But because we couldn't get the engine fire light to go out, we decided to return to Saskatoon as we had only gotten about 60 miles to the north enroute. We asked for clearance back to the airport and quickly explained to the passengers why we were returning to Saskatoon. One passenger figured his business that he was going to was more important than a chance with his life and argued with us to continue on. He lost the argument needless to say.