"As a freshly minted instrument pilot back in 1983, I was keen to make the trip from Watsonville, California, up to Beaverton, Oregon, in my Piper Lance II. Portland was forecast to be IFR, with fairly high ceilings (1,200 ft.) for our arrival.
Our trip continued uneventfully until we reached the Portland area, where it was IFR and icing was not forecast. I remember being on the gauges at 10,000 ft. and watching the airspeed slowly start to decay. Call it fixation, but I hesitated to wander too far from the primary instruments for fear of losing control in IMC. As the airspeed decayed further, I lowered the nose, trying to keep the plane flying.
ATC suddenly became concerned and said, "Lance 2096G, I cleared you to 10,000 ft., say intentions." I said that I was in a downdraft and doing what I could to get back to the assigned altitude. But the indications on the instruments (and the increasing sound of air rushing by the fuselage) didn't make sense.
Suddenly it dawned on me: The pitot heat switch is there for a reason. Use it! Almost within seconds of flicking the switch, the airspeed flipped from an indicated 45 knots to redline. I was in a pronounced dive and my task was to level out without pulling the wings off. In time, I got the plane under control and was glad that my passenger was oblivious to the actual events.
So what did I learn? I learned from the icing experience that no matter what the forecast says, if you're IFR and in the clouds, turn on your pitot heat. It's good insurance.
The key to flying is to live and learn - but the real key is to live long enough to do that".