What I Thought I Saw

Another beautiful morning, this time in Ironwood, Michigan in Part Two, the eastern segment, of my week-long aviation saga. The bags were packed in the back of the Warrior early, and I was ready for departure an hour ahead of my infinitely flexible plan. After the mandatory pre-flight and start-up, the only thing left to do was the engine run up just prior to take-off. Pushing the throttle forward to reach the prescribed 2000 r.p.m., smoke suddenly started pouring out of the defrost vents over the dash. It smelled like burning leaves or paper, and was pushed along by the defroster I had turned on earlier, or so I thought. Smoke in the cockpit is never a good thing, whether in the air or on the ground.

Quickly shutting down the defroster, heat, and fan I had used to manage the moisture that was forming on the inside of the windscreen, I turned and scooted back to the tarmac. Along the way, as I opened the side window and door to vent the cabin, the smoke ceased but not the smell. After parking the plane some distance away from the FBO, the investigation started on the premise that something in the path of the defroster had burned. Surely it was turned on for the first time in a long time, since the 42 degree temperature had not been seen for some time. Perhaps something in the hot box, the chamber that draws warmed air from around the exhaust system, had just been waiting for someone to move that control lever to release itself onto a hot surface for combustion.

That analysis stemmed from the assumption that the smoke came from the defrosting vents on top of the dash. That's what I saw, and that's what I shared with the very kind and generous airport manager and former Aircraft and Powerplant mechanic that came to see if he could help. We traced all the cabin heat hoses from the engine, and looked into the by-pass controller for any evidence of combustible foreign matter like birds nest components or leaves or anything that might have made it into the air scoop. We agreed that it didn't make much sense that something like that might choose just now, rather than over the preceding four days, to present itself. But if I was sure the smoke came from those vents, then it must have come from that chamber, even though there was no evidence to support that theory. In any case, two run ups of the engine could not reproduce the smoke or the smell Since everything looked in tact and operational, I should be able to continue my voyage under that assumption.

The plan was to taxi to the run-up area, run-up the engine with a little longer dwell time at 2000 r.p.m., and if everything was normal, take off and climb above the airport to a cruise elevation of 6,000 feet. If no issue by that point, continue on the flight plan.

Starting the engine, I opened the overhead vents for air to combine with the defrosting heat to keep the windows clear. That is when the acrid smell returned, but without smoke. That's not consistent with the cause assumption. After shutting down again, I pulled out my luggage to clear the tail cone access door so I could see what was going on in the back of the plane, the source of cabin air for the ventilation system. The smell was fresh and intense, with an identifiable electrical source. Looking at the circuit panel, as I should have much earlier at the start of my investigation, I saw the AC/Blower circuit breaker had been tripped. The blower fan that draws air from a scoop in the vertical stabilizer was fried. The smoke had not come from the defrosters at all, but rather from the overhead vents It must have been forced down to the windshield, on to the dash, and back in my direction. What I thought I saw was not what I saw, providing a flawed assumption that caused me to look in the wrong direction.

Fortunately, the cause was now clear, and would have no impact on my trip. The fan was not really necessary, and I could leave the breaker tripped to keep it from any other unwanted consequences.

Go/No Go decisions, especially related to mechanical issues, are tricky, and it's wise to err on the side of over-caution. Knowing the exact cause, with the ability to isolate a non-critical failures, allows the continuation of a flight. The rest of Michigan was now still available for exploration in the remaining three days of the trip.

Here are links to the other segments of this series following my (some would say Don Quixote-ish) barnstorming escapade.

Part 1 - A Highschool Daydream
Part 2 -  Pilot Amenities

Part 3 - A Study in Topography

Part 5 - S.U.M.M.E.R.