Our pilot had justflown through some light snow showers on his way to Kingston, Ontario thatmorning, so he turned on the pitot heat just before joining the circuit to makesure the pitot blade on his Warrior was clear. At the end of the downwind leg heslowed the engine, reduced power, dropped flaps, verified 70-80 knot airspeed,turned a tight base over the icy water of Lake Ontario, then looked again atthe airspeed indicator (ASI). 35 knots! Well below stall speed! 

“But the plane was flying fine” says our pilot. “The nose wasn’t high,the controls weren’t mushy, the stall buzzer wasn’t blaring, the wings weren’tbuffeting, and most importantly, the ice floes weren’t spinning and gettinglarger in the windshield.”

Our pilot gently pushed the nose down enough to speed up 5-10 knots, butstill the needle didn’t move. He checked the altimeter and it was behavingproperly, showing a slow descent towards field elevation. “That meant a pitotfailure” deduced our pilot.

“The trickiest part was the turn to final. After that, it was prettymuch a normal approach and landing (no point declaring an emergency when therunway is less than a minute away).” Our pilot continues, “The ASI flickeredback to life on short final to show that I was 5-10 knots above my normalapproach speed. It froze again at some point during the flare and landing (Idon’t look at the panel once I’m past the airport fence), then graduallyclimbed to 90 knots as I taxied in to park the plane.”

The aftermath and resolution

Now on the ground, our pilot called an AME (mechanic) at the airport,tested the pitot system by blowing gently into it (no joy), then went out forlunch so that he wouldn’t stay around fretting. Three hours later, the AMEhadn’t had time to get to the plane yet, and the ASI still wasn’t responding tothe blow test, so he decided to try something else (with the AME’s blessing, ofcourse): he started the plane, turned on the pitot heat, then did a high-speedtaxi down the 5,000 ft runway.

The needle climbed again during slow taxi, then dropped at the start of hishigh-speed run, then climbed up again — then, suddenly, at the very end, itstarted responding normally. Since there was no other traffic, our pilot turnedaround and did the same thing the other way, and this time, the needleresponded normally the whole way. Our pilot taxied around, did pre-takeoffchecks, then went back to the runway for a real takeoff roll, prepared to aborthalfway if the ASI wasn’t behaving — no problem at all, all the way home – thoughhis mode C encoder started acting up, “because there’s a law of physics that atleast one thing always has to be broken on an airplane” jokes our pilot.

The analysis

There must have been some snow or ice near the opening of the Warrior’spitot blade. Turning on the heat partly melted it and let it get into thepin-sized hole, and the water blocked the pitot line, possibly as slush or evenas tiny ice crystals. Our pilot’s high-speed taxis, combined with the pitotheat, forced the blockage the rest of the way through the line and cleared it.

“Pitot heat on was a good idea, but turning it on just before joiningthe circuit wasn’t” concludes our pilot. “Lesson: make as few configurationchanges as possible when you’re close to landing — if something’s alreadyworking, why mess with it? If I’d turned on the pitot heat 10 or 15 minutesearlier, I would have had the ASI failure at 5,500 ft, where it was no risk atall, instead of in the most dangerous possible phase of flight, and it wouldhave worked itself out before I had time to land anywhere. Since I hadn’tturned it on earlier, I shouldn’t have turned it on at all.”

In the end, no harm, no cost, and a little bit of extra confidence that hecould handle his plane by feel when the ASI fails, at least in visualconditions.